title: Tamagawa numbers of polarized algebraic varieties

abstract:
  Let ${\cal L} = (L, \| \cdot \|_v)$ be an ample metrized invertible sheaf on
a smooth quasi-projective algebraic variety $V$ defined over a number field.
Denote by $N(V,{\cal L},B)$ the number of rational points in $V$ having ${\cal
L}$-height $\leq B$. We consider the problem of a geometric and arithmetic
interpretation of the asymptotic for $N(V,{\cal L},B)$ as $B \to \infty$ in
connection with recent conjectures of Fujita concerning the Minimal Model
Program for polarized algebraic varieties.
  We introduce the notions of ${\cal L}$-primitive varieties and ${\cal
L}$-primitive fibrations. For ${\cal L}$-primitive varieties $V$ over $F$ we
propose a method to define an adelic Tamagawa number $\tau_{\cal L}(V)$ which
is a generalization of the Tamagawa number $\tau(V)$ introduced by Peyre for
smooth Fano varieties. Our method allows us to construct Tamagawa numbers for
$Q$-Fano varieties with at worst canonical singularities. In a series of
examples of smooth polarized varieties and singular Fano varieties we show that
our Tamagawa numbers express the dependence of the asymptotic of $N(V,{\cal
L},B)$ on the choice of $v$-adic metrics on ${\cal L}$.


title: Real Algebraic Surfaces

abstract:
  These are the notes for my lectures at the Trento summer school held
September 1997. The aim of the lectures is to provide an introduction to real
algebraic surfaces using the minimal model program. This leads to a fairly
complete understanding of real rational surfaces and to a complete topological
classification of real Del Pezzo surfaces. Almost all the results are contained
in the works of Comessatti and Silhol.


title: Real Algebraic Threefolds I: Terminal Singularities

abstract:
  This is the first of a series of papers studying real algebraic threefolds
using the minimal model program. The main results are outlined in Part II. The
present part I. contains the necessary preliminary work concerning terminal
singularities. First I give standard forms for 3-dimensional terminal
singularities over arbitrary fields. This is then used to develop a fairly
complete topological classification over the reals.


title: Real Algebraic Threefolds II: Minimal Model Program

abstract:
  This is the second of a series of papers studying real algebraic threefolds
using the minimal model program. The main result is the following. Let $X$ be a
smooth projective real algebraic 3-fold. Assume that the set of real points is
an orientable 3-manifold (this assumption can be weakened considerably). Then
there is a fairly simple description on how the topology of real points changes
under the minimal model program. The first application is to study the topology
of real projective varieties which are birational to projective 3-space (Nash
conjecture). The second application is a factorization theorem for birational
maps.


title: Homological infiniteness of Torelli groups

abstract:
  We prove that rational homology of the Torelli group of genus g is infinite
dimensional, provided g>6. This means that rational homology of the Torelli
space of genus g>6 is infinite dimensional. The Torelli groups with marked
points are also considered. In addition, we prove that rational homology of the
subgroup of the Torelli group of genus g generated by all the Dehn twists along
separating simple closed curves is infinite dimensional for g>2.


title: Toric varieties and minimal complexes

abstract:
  We translate the equivariant decomposition theorem (in the case of a proper
morphism of toric varieties) in to the language of combinatorially defined
``shifted minimal complexes''.


title: Quantum Hyperplane Section Theorem For Homogeneous Spaces

abstract:
  We formulated a mirror-free approach to the mirror conjecture, namely,
quantum hyperplane section conjecture, and proved it in the case of nonnegative
complete intersections in homogeneous manifolds. For the proof we followed the
scheme of Givental's proof of a mirror theorem for toric complete
intersections.


title: The Character of the Infinite Wedge Representation

abstract:
  We study the character of the infinite wedge projective representation of the
algebra of differential operators on the circle. We prove quasi-modularity of
this character and also compute certain generating functions for traces of
differential operators which we call correlation functions. These correlation
functions are sums of determinants built from genus 1 theta functions and their
derivatives.


title: Structures du cube et fibres d'intersection

abstract:
  We define the notion of a hypercube structure on a functor between two
strictly commutative Picard categories which generalizes the notion of a cube
structure on a $G_m$-torsor over an abelian scheme. We use this notion to
define the intersection bundle of $n+1$ line bundles on a relative scheme $X/S$
of relative dimension $n$ and to construct an additive structure on the functor
$I_{X/S}:PIC(X/S)^{n+1}\F PIC(S)$. Finally, we study a section of
$I_{X/S}(L_1,...,L_{n+1})$ which generalizes the resultant of $n+1$ polynomials
in $n$ variables and we interprete some classical formulas with this formalism.


title: Mirror Principle I

abstract:
  We propose and study the following Mirror Principle: certain sequences of
multiplicative equivariant characteristic classes on Kontsevich's stable map
moduli spaces can be computed in terms of certain hypergeometric type classes.
As applications, we compute the equivariant Euler classes of obstruction
bundles induced by any concavex bundles -- including any direct sum of line
bundles -- on $\P^n$. This includes proving the formula of Candelas-de la
Ossa-Green-Parkes hence completing the program of Candelas et al, Kontesevich,
Manin, and Givental, to compute rigorously the instanton prepotential function
for the quintic in $\P^4$. We derive, among many other examples, the multiple
cover formula for Gromov-Witten invariants of $\P^1$, computed earlier by
Morrison-Aspinwall and by Manin in different approaches. We also prove a
formula for enumerating Euler classes which arise in the so-called local mirror
symmetry for some noncompact Calabi-Yau manifolds. At the end we interprete an
infinite dimensional transformation group, called the mirror group, acting on
Euler data, as a certain duality group of the linear sigma model.


title: Hyperholomorphic sheaves and new examples of hyperkaehler manifolds

abstract:
  Given a compact hyperkaehler manifold $M$ and a holomorphic bundle B over
$M$, we consider a Hermitian connection $\nabla$ on B which is compatible with
all complex structures on $M$ induced by the hyperkaehler structure. Such a
connection is unique, because it is Yang-Mills. We call the bundles admitting
such connections hyperholomorphic bundles. A stable bundle is hyperholomorphic
if and only if its Chern classes $c_1$, $c_2$ are SU(2)-invariant, with respect
to the natural SU(2)-action on the cohomology. For several years, it was known
that the moduli space of stable hyperholomorphic bundles is singular
hyperkaehler. More recently, it was proven that singular hyperkaehler varieties
admit a canonical hyperkaehler desingularization. In the present paper, we show
that a moduli space of stable hyperholomorphic bundles is compact, given some
assumptions on Chern classes of B and hyperkaehler geometry of $M$ (we also
require $dim_C M>2$). Conjecturally, this leads to new examples of hyperkaehler
manifolds. We develop the theory of hyperholomorphic sheaves, which are
(intuitively speaking) coherent sheaves compatible with hyperkaehler structure.
We show that hyperholomorphic sheaves with isolated singularities can be
canonically desingularized by a blow-up. This theory is used to study
degenerations of hyperholomorphic bundles.


title:
Eigenvalues of products of unitary matrices and quantum Schubert
  calculus

abstract:
  We describe the inequalities on the possible eigenvalues of products of
unitary matrices in terms of quantum Schubert calculus. Related problems are
the existence of flat connections on the punctured two-sphere with prescribed
holonomies, and the decomposition of fusion product of representations of
SU(n), in the large level limit. In the second part of the paper we investigate
how various aspects of the problem (symmetry, factorization) relate to
properties of the Gromov-Witten invariants.


title: Subadjunction of log canonical divisors II

abstract:
  We extend a subadjunction formula of log canonical divisors as in [K3] to the
case when the codimension of the minimal center is arbitrary by using the
positivity of the Hodge bundles.


title:
The universal regular quotient of the Chow group of 0-cycles on a
  singular projective variety

abstract:
  We show the existence of a regular universal quotient as a smooth commutative
algebraic group of the Chow group of 0-cycles on a projective reduced variety,
and give over the field of complex numbers an analytic description of it. This
generalizes the classical theory of the Albanese. The Chow group of 0-cycles is
then isomorphic to this smooth algebraic group if and only if it is finite
dimensional in the sense of Mumford. This generalizes the classical theorem of
Roitman.


title: Invariance of Plurigenera

abstract:
  The following conjecture on the deformation invariance of plurigenera is
proved. For a smooth projective holomorphic family of compact complex manifolds
over the open unit 1-disk such that all the fibers are of general type, every
plurigenus of the fiber is independent of the fiber.
  The proof uses Nadel's multiplier ideal sheaves, Skoda's result on the
generation of ideals with L2 estimates with respect to a plurisubharmonic
weight, and the extension theorem of Ohsawa-Takegoshi-Manivel for holomorphic
top-degree forms which are L2 with respect to a plurisubharmonic weight.


title:
Derived categories of coherent sheaves on abelian varieties and
  equivalences between them

abstract:
  We study derived categories of coherent sheaves on abelian varieties. We give
a criterion for the equivalence of the derived categories on two abelian
varieties. We describe the autoequivalence group for the derived category of
coherent sheaves of an abelian variety.


title: Deformations of canonical singularities

abstract: We prove that small deformations of canonical singularities are canonical.


title: On the cone of curves of an abelian variety

abstract:
  Let $X$ be a smooth projective variety over the complex numbers. One knows by
the Cone Theorem that the closed cone of curves of $X$ is rational polyhedral
whenever $c_1(X)$ is ample. For varieties $X$ such that $c_1(X)$ is not ample,
however, it is in general difficult to determine the structure of $\bar NE(X)$.
The purpose of this paper is to study the cone of curves of abelian varieties.
Specifically, the abelian varieties $X$ are determined such that the closed
cone $\bar NE(X)$ is rational polyhedral. The result can also be formulated in
terms of the nef cone of $X$ or in terms of the semi-group of effective classes
in the N\'eron-Severi group of $X$.


title: Secondary Kodaira-Spencer classes and nonabelian Dolbeault cohomology

abstract:
  If $X$ is a smooth projective variety moving in a family, we define a
secondary Kodaira-Spencer class for nonabelian Dolbeault cohomology
$Hom(X_{Dol}, T)$ of $X$ with coefficients in the complexified 2-sphere
$T=S^2\otimes \cc$ (which is a 3-stack on $Sch /\cc$). Let $Z$ be a simply
connected projective surface with $h^{2,0}\neq 0$, and let $X$ be the blow-up
of $Z$ at a point $P$. As $P$ moves in $Z$, the blow-up $X$ moves in a family
and we show that the secondary Kodaira-Spencer class is nontrivial. This
contrasts with the fact that the variations of mixed Hodge structures on the
homotopy groups of $X$ are constant. We discuss various surrounding notions,
including two appendices where we give some details about the Breen
calculations in characteristic zero and representability of simply connected
complex shapes.


title: Analogue of Weil representation for abelian schemes

abstract:
  In this paper we construct a projective action of certain arithmetic group on
the derived category of coherent sheaves on an abelian scheme $A$, which is
analogous to Weil representation of the symplectic group. More precisely, the
arithmetic group in question is a congruence subgroup in the group of
"symplectic" automorphisms of $A\times\hat{A}$ where $\hat{A}$ is the dual
abelian scheme. The "projectivity" of this action refers to shifts in the
derived category and tensorings with line bundles pulled from the base. In
particular, if $A$ is an abelian scheme over $S$ equipped with an ample line
bundle $L$ of degree 1 then we construct an action of a central extension of
$Sp_{2n}(\Bbb Z)$ by $\Bbb Z\times Pic(S)$ on the derived category of coherent
sheaves on $A^n$ (the $n$-th fibered power of $A$ over $S$). We describe the
corresponding central extension explicitly using the the canonical torsion line
bundle on $S$ associated with $L$. As a main technical result we prove the
existence of a representation of rank $d$ for a symmetric finite Heisenberg
group scheme of odd order $d^2$.


title:
Poisson structures and birational morphisms associated with bundles on
  elliptic curves

abstract:
  In this paper we define a Poisson structure on some moduli spaces related to
principal G-bundles on elliptic curves, the simplest example being the moduli
space of stable pairs: a vector bundle and its global section. We also study
birational morphisms between projective spaces appearing as such moduli spaces.


title: A canonical lift of Chern-Mather classes

abstract:
  There are several ways to generalize characteristic classes for singular
algebraic varieties. The simplest ones to describe are Chern-Mather classes
obtained by Nash blow up. They serve as an ingredient to construct
Chern-MacPherson-Schwartz classes. Unfortunately, they all are defined in
homology. There are examples showing, that they do not lie in the image of
Poincar\'e morphism. On the other hand they are represented by an algebraic
cycles. Barthel, Brasselet, Fiesler, Kaup and Gabber have shown that, any
algebraic cycle can be lifted to intersection homology. Nevertheless, a lift is
not unique. The Chern-Mather classes are represented by polar varieties. We
show how to define a canonical lift of Chern-Mather classes to intersection
homology. Instead of the polar variety alone, we consider it as a term in the
whole sequence of inclusions of polar varieties. The inclusions are of
codimension one. In this case the lifts are unique.


title: Symplectic cutting of Kaehler manifolds

abstract:
  We obtain estimates on the character of the cohomology of an
  $S^1$-equivariant holomorphic vector bundle over a Kaehler manifold $M$ in
terms of the cohomology of the Lerman symplectic cuts and the symplectic
reduction of $M$. In particular, we prove and extend inequalities conjectured
by Wu and Zhang.
  The proof is based on constructing a flat family of complex spaces $M_t$ such
that $M_t$ is isomorphic to $M$ for $t\not=0$, while $M_0$ is a singular
reducible complex space, whose irreducible components are the Lerman symplectic
cuts.


title: Heisenberg-invariant Kummer surfaces

abstract:
  We study the geometry of Nieto's quintic threefold (Barth & Nieto, J. Alg.
Geom. 3, 1994) and the Kummer and abelian surfaces that correspond to special
loci.


title: Degenerations of (1,3) abelian surfaces and Kummer surfaces

abstract:
  We continue our study of the geometry of Nieto's quintic threefold, looking
at degenerate surfaces that correspond to certain loci and showing how they
arise from a toroidal compactification of a suitable moduli space.


title: The arithmetic-geometric mean and isogenies for curves of higher genus

abstract:
  Computation of Gauss's arithmetic-geometric mean involves iteration of a
simple step, whose algebro-geometric interpretation is the construction of an
elliptic curve isogenous to a given one, specifically one whose period is
double the original period. A higher genus analogue should involve the explicit
construction of a curve whose jacobian is isogenous to the jacobian of a given
curve. The doubling of the period matrix means that the kernel of the isogeny
should be a lagrangian subgroup of the group of points of order 2 in the
jacobian. In genus 2 such a construction was given classically by Humbert and
was studied more recently by Bost and Mestre. In this article we give such a
construction for general curves of genus 3. We also give a similar but simpler
construction for hyperelliptic curves of genus 3. We show that the
hyperelliptic construction is a degeneration of the general one, and we prove
that the kernel of the induced isogeny on jacobians is a lagrangian subgroup of
the points of order 2. We show that for g at least 4 no similar construction
exists, and we also reinterpret the genus 2 case in our setup. Our construction
of these correspondences uses the bigonal and the trigonal constructions,
familiar in the theory of Prym varieties.


title:
Zero-pole interpolation for matrix meromorphic functions on a compact
  Riemann surface, and a matrix Fay trisecant identity

abstract:
  This paper presents a new approach to constructing a meromorphic bundle map
between flat vector bundles over a compact Riemann surface having a prescribed
Weil divisor (i.e., having prescribed zeros and poles with directional as well
as multiplicity information included in the vector case). This new formalism
unifies the earlier approach of Ball-Clancey (in the setting of trivial bundles
over an abstract Riemann surface) with an earlier approach of the authors
(where the Riemann surface was assumed to be the normalizing Riemann surface
for an algebraic curve embedded in ${\bold C}^2$ with determinantal
representation, and the vector bundles were assumed to be presented as the
kernels of linear matrix pencils). The main tool is a version of the Cauchy
kernel appropriate for flat vector bundles over the Riemann surface. Our
formula for the interpolating bundle map (in the special case of a single zero
and a single pole) can be viewed as a generalization of the Fay trisecant
identity from the usual line bundle case to the vector bundle case in terms of
Cauchy kernels. In particular we obtain a new proof of the Fay trisecant
identity.


title:
Reconstruction of a variety from the derived category and groups of
  autoequivalences

abstract:
  We consider smooth algebraic varieties with ample either canonical or
anticanonical sheaf. We prove that such a variety is uniquely determined by its
derived category of coherent sheaves. We also calculate the group of exact
autoequivalences for these categories. The technics of ample sequences in
abelian categories is used.


title:
The L^2 dbar method, weak Lefschetz theorems, and the topology of Kahler
  manifolds

abstract:
  A new approach to Nori's weak Lefschetz theorem is described. The new
approach, which involves the dbar-method, avoids moving arguments and gives
much stronger results. In particular, it is proved that if X and Y are
connected smooth projective varieties of positive dimension and f is a
holomorphic immersion of Y into X with ample normal bundle, then the image of
the fundamental group of Y in that of X is of finite index. This result is
obtained as a consequence of a direct generalization of Nori's theorem. The
second part concerns a new approach to the theorem of Burns which states that a
quotient of the unit ball in complex Euclidean space (of dimension at least 3)
by a discrete group of automorphisms which has a strongly pseudoconvex boundary
component has only finitely many ends. The following generalization is
obtained. If a complete Hermitian manifold X of dimension at least 3 has a
strongly pseudoconvex end E and the Ricci curvature of X is bounded above by a
negative constant, then, away from E, X has finite volume.


title: Descent for Shimura Varieties

abstract:
  We verify that the descent maps provided by Langlands's Conjugacy Conjecture
do satisfy the continuity condition necessary for them to be effective. Thus
Langlands's conjecture does imply the existence of canonical models.
  This replaces an earlier version of the paper --- the proof in this version
is simpler, and the exposition more detailed.


title: A morphism of intersection homology induced by an algebraic map

abstract:
  Let $f:X-->Y$ be a map of algebraic varieties. Barthel, Brasselet, Fieseler,
Gabber and Kaup have shown that there exists a homomorphism of intersection
homology groups $f^*:IH^*(Y)-->IH^*(X)$ compatible with the induced
homomorphism on cohomology. The crucial point in the argument is reduction to
the finite characteristic. We give an alternative and short proof of the
existence of a homomorphism $f^*$. Our construction is an easy application of
the Decomposition Theorem.


title:
On the classification of hyperbolic root systems of the rank three. Part
  II

abstract:
  Here we prove classification results announced in Part I (alg-geom/9711032).
We classify maximal hyperbolic root systems of the rank 3 having restricted
arithmetic type and a generalized lattice Weyl vector $\rho$ with $\rho^2\ge 0$
(i.e. of elliptic or parabolic type). We give classification of all reflective
of elliptic or parabolic type elementary hyperbolic lattices of the rank three.
  We apply the same method (narrow places of polyhedra) which was developed to
prove finiteness results on reflective hyperbolic lattices. We also use some
additional arithmetic arguments: studying of class numbers of central
symmetries.
  The same methods permit to get similar results for hyperbolic type. We will
consider hyperbolic type in Part III.
  These results are important for Theory of Lorentzian Kac--Moody algebras and
some aspects of Mirror Symmetry.


title: Toric Degenerations of Fano Varieties and Constructing Mirror Manifolds

abstract:
  For an arbitrary smooth n-dimensional Fano variety $X$ we introduce the
notion of a small toric degeneration. Using small toric degenerations of Fano
n-folds $X$, we propose a general method for constructing mirrors of Calabi-Yau
complete intersections in $X$. Our mirror construction is based on a
generalized monomial-divisor mirror correspondence which can be used for
computing Gromov-Witten invariants of rational curves via specializations of
GKZ-hypergeometric series.


title: Comparison of the algebraic and the symplectic Gromov-Witten invariants

abstract:
  We show that the algebraic and the symplectic GW-inivariants of smooth
projective varieties are equivalent.


title:
North Atlantic thermohaline circulation predictability in a coupled
  ocean-atmosphere model

abstract:
  Predictability of the North Atlantic thermohaline circulation (THC)
variability as simulated in the GFDL coupled ocean-atmosphere general
circulation model is established for a set of ensemble experiments. The
ensembles consist of identical oceanic initial conditions underneath a model
atmosphere chosen randomly from the model climatology. This experimental design
is based on the separation in time scales present in the model which motivates
the assumption that the predictability deduced from these ensembles provides an
upper limit to the model's THC predictability. The climatology is taken from a
multi-century model integration whose THC variability has power concentrated at
the 40-60 year time scale. A linear stochastic perspective is shown to be
generally consistent with the ensemble statistics. The linear theory suggests a
natural measure of ensemble predictability as the time at which the ensemble
variance becomes a subjectively defined fraction (0.5 used here) of the
climatological variance. It is furthermore of interest to distinguish
predictability of the rapidly de-correlating portion of the model's THC from
the longer time correlated portion. The rapidly de-correlating portion shows
predictability for $\approx 1.5$ years. The slower portion shows predictability
for $\approx 5-7$ years. The linear stochastic framework provides for the
straightforward construction of an optimal forecast of the model's THC
variability and an understanding of why the optimal forecast is useless beyond
a particular limit.


title:
A linear thermohaline oscillator driven by stochastic atmospheric
  forcing

abstract:
  The interdecadal variability of a stochastically forced four-box model of the
oceanic meridional thermohaline circulation (THC) is described and compared to
the THC variability in the coupled ocean-atmosphere GCM of Delworth, Manabe,
and Stouffer (1993). The box model is placed in a linearly stable thermally
dominant mean state under mixed boundary conditions. A linear stability
analysis of this state reveals one damped oscillatory THC mode in addition to
purely damped modes. The variability of the model under a moderate amount of
stochastic forcing, meant to emulate the random variability of the atmosphere
affecting the coupled model's interdecadal THC variability, is studied. A
linear interpretation, in which the damped oscillatory mode is of primary
importance, is sufficient for understanding the mechanism accounting for the
stochastically forced variability. Direct comparison of the variability in the
box model and coupled GCM reveals common qualitative aspects. Such a comparison
supports, although does not verify, the hypothesis that the coupled model's THC
variability can be interpreted as the result of atmospheric weather exciting a
linear damped oscillatory THC mode.


title:
A Scaling Theory for Horizontally Homogeneous, Baroclinically Unstable
  Flow on a Beta-Plane

abstract:
  The scaling argument developed by Larichev and Held (1995) for eddy
amplitudes and fluxes in a horizontally homogeneous, two-layer model on an
f-plane is extended to a beta-plane. In terms of the non-dimensional number x =
U/(beta*lambda^2), where lambda is the deformation radius and U is the mean
thermal wind, the result for the RMS eddy velocity V, the characteristic
wavenumber of the energy-containing eddies and of the eddy-driven jets k, and
the magnitude of the eddy diffusivity for potential vorticity D, in the limit
x>>1, are as follows: V/U ~ x ; k*lambda ~ 1/x ; D/(U*lambda) ~ x^2. Numerical
simulations provide qualitative support for this scaling, but suggest that it
underestimates the sensitivity of these eddy statistics to the value of x. A
generalization that is applicable to continuous stratification is suggested
which leads to the estimates: V ~ 1/(beta T^2); k ~ beta*T; D ~
1/{[beta^2][T^3]} where T is a time-scale determined by the environment; in
particular, it equals lambda/U in the two-layer model and N/(fS) in a
continuous flow with uniform shear S and stratification N. This same scaling
has also been suggested as relevant to a continuously stratified fluid in the
opposite limit, x<<1 (Held, 1980). Therefore, we suggest that it may be of
general relevance in planetary atmospheres and in the oceans.


title: The stability of a zonally averaged thermohaline circulation model

abstract:
  A combination of analytical and numerical techniques are used to efficiently
determine the qualitative and quantitative behaviour of a one-basin zonally
averaged thermohaline circulation ocean model. In contrast to earlier studies
which use time stepping to find the steady solutions, the steady state
equations are first solved directly to obtain the multiple equilibria under
identical mixed boundary conditions. This approach is based on the
differentiability of the governing equations and especially the convection
scheme. A linear stability analysis is then performed, in which the normal
modes and corresponding eigenvalues are found for the various equilibrium
states. Resonant periodic solutions superimposed on these states are predicted
for various types of forcing. The results are used to gain insight into the
solutions obtained by Mysak, Stocker and Huang in a previous numerical study in
which the eddy diffusivities were varied in a randomly forced one-basin zonally
averaged model. Resonant stable oscillations with century scale periods are
predicted with structures that compare favorably with those found in the
previous study. In cases with large horizontal diffusivities, the two-cell
pattern is also stable, which parallels cases in the previous study where large
vacillations were seen between the three stable steady states. Further, in
cases with large horizontal {\it and} large vertical diffusivities, no one-cell
pattern can be realised and the only asymptotic behaviour found is the two-cell
pattern.


title:
Interpretation of TOVS Water Vapor Radiances Using a Random Strong Line
  Model

abstract:
  This study illustrates the application of a random strong line (RSL) model of
radiative transfer to the interpretation of satellite observations of the
upwelling radiation in the 6.3 micron water vapor absorption band. The model,
based upon an assemblage of randomly overlapped, strongly absorbing, pressure
broadened lines, is compared to detailed radiative transfer calculations of the
upper (6.7 micron) tropospheric water vapor radiance and demonstrated to be
accurate to within ~ 1.2 K. Similar levels of accuracy are found when the model
is compared to detailed calculations of the middle (7.3 micron) and lower (8.3
micron) tropospheric water vapor radiance, provided that the emission from the
underlying surface is taken into account. Based upon these results, the RSL
model is used to interpret TOVS-observed water vapor radiances in terms of the
relative humidity averaged over deep layers of the upper, middle, and lower
troposphere. We then present near-global maps of the geographic distribution
and climatological variations of upper, middle and lower tropospheric humidity
from TOVS for the period 1981-1991. These maps clearly depict the role of
large-scale circulation in regulating the location and temporal variation of
tropospheric water vapor.


title: Mechanisms of Seasonal - ENSO interaction

abstract:
  The mechanisms of interaction between the seasonal cycle and ENSO are
investigated using the Zebiak and Cane ENSO prediction model. The most dominant
seasonal effect is found to be due to the wind divergence field, as determined
by the seasonal motion of the ITCZ, through its effect on the atmospheric
heating. The next order seasonal effects are due to the seasonality of the
background SST and ocean upwelling velocity, and the corresponding mechanisms
are analyzed. It is suggested that the seasonal forcing has a first order
effect on ENSO's dynamics. Important aspects of the seasonal forcing may be
included in idealized delayed oscillator ENSO models by making the model
background seasonally shift from stable to unstable states.


title:
Hadley circulations and large scale motions of moist convection in the
  two dimensional numerical model

abstract:
  As a tool for understanding the meridional circulation of the atmosphere, a
two-dimensional ( latitude -- height ) numerical model is used to clarify the
relationship between the Hadley circulation and large-scale motions associated
with moist convection. The model is based on the primitive equations including
the moist process, and two kinds of coordinates are used: the spherical
coordinate and the Cartesian coordinate with a uniform rotation. The surface
temperature is externally fixed and the troposphere is cooled by the radiation;
unstable stratification generates large-scale convective motions.
  Dependencies on the surface temperature difference from north to south Delta
T_s are investigated. The numerical results show that a systematic multi-cell
structure exists in every experiment. If the surface temperature is constant
(Delta T_s = 0 ), convective motions are organized in the scale of the Rossby
deformation radius and their precipitation patterns have a periodicity of the
advective time tau_D. As Delta T_s becomes larger, the organized convective
system tends to propagate toward warmer regions. The convective cells
calculated in the Cartesian coordinate model is very similar to those of the
mid-latitudes in the spherical coordinate model. In particular, the Hadley cell
can be regarded as the limit of the convective cells in the equatorial
latitudes.


title: A Stochastic Diffusion Model of Climate Change

abstract:
  We present a model for variations in atmospheric temperature from time scales
of one day to one million years based on a stochastic diffusion (random walk)
model of the turbulent transport of heat energy vertically in a coupled
atmosphere-ocean model. The predictions of the model are supported by station
records and paleoclimatic proxy data of temperature variations.


title:
Kardar-Parisi-Zhang model for the fractal structure of cumulus cloud
  fields

abstract:
  We model the ascent of warm, moist air in the Earth's atmosphere by turbulent
convection and expansion with the KPZ equation, familiar in the physics
literature on surface growth. Clouds form in domains where the interface
between the rising air and its surrounding air achieves an elevation higher
than that necessary for condensation. The model predictions are consistent with
the perimeter fractal dimension and the cumulative frequency-size distribution
of cumulus cloud fields observed from space.


title: Analysis and modeling of scale-invariance in plankton abundance

abstract:
  The power spectrum, $S$, of horizontal transects of plankton abundance are
often observed to have a power-law dependence on wavenumber, $k$, with exponent
close to -2: $S(k)\propto k^{-2}$ over a wide range of scales. I present power
spectral analyses of aircraft lidar measurements of phytoplankton abundance
from scales of 1 to 100 km. A power spectrum $S(k)\propto k^{-2}$ is obtained.
As a model for this observation, I consider a stochastic growth equation where
the rate of change of plankton abundance is determined by turbulent mixing,
modeled as a diffusion process in two dimensions, and exponential growth with a
stochastically variable net growth rate representing a fluctuating environment.
The model predicts a lognormal distribution of abundance and a power spectrum
of horizontal transects $S(k)\propto k^{-1.8}$, close to the observed spectrum.
The model equation predicts that the power spectrum of variations in abundance
in time at a point in space is $S(f)\propto f^{-1.5}$ (where $f$ is the
frequency). Time series analysis of local variations of phytoplankton and
zooplankton yield a power-law power spectrum with exponents -1.3 and -1.2,
respectively from time scales of one hour to one year. These values are roughly
consistent with the model prediction of -1.5. The distribution of abundances is
nearly lognormal as predicted. The model may be more generally applicable than
for the spatial distribution of plankton. I relate the model predictions to
observations of spatial patchiness in vegetation.


title: Entropic "sound" in the atmosphere

abstract:
  It is shown that small, local disturbances of entropy in the atmosphere may
give rise to "sound" waves propagating with a velocity which depends on the
amplitude ratio of the local relative variations of temperature and volume.
This velocity is much smaller than the mean molecular velocity and the usual,
adiabatic sound velocity.


title: Interdecadal variability and oceanic thermohaline adjustment

abstract:
  Changes in the strength of the thermohaline overturning circulation are
associated, by geostrophy, with changes in the east-west pressure difference
across an ocean basin. The tropical-polar density contrast and the east-west
pressure difference are connected by an adjustment process. In flat-bottomed
ocean models the adjustment is associated with viscous, baroclinic Kelvin wave
propagation. Weak-high latitude stratification leads to the adjustment having
an interdecadal timescale. We reexamine model interdecadal oscillations in the
context of the adjustment process, for both constant flux and mixed surface
boundary conditions. Under constant surface flux, interdecadal oscillations are
associated with the passage of a viscous Kelvin wave around the model domain.
Our results suggest the oscillations can be self-sustained by perturbations to
the western boundary current arising from the southward boundary wave
propagation. Mixed boundary condition oscillations are characterized by the
eastward, cross-basin movement of salinity-dominated density anomalies, and the
westward return of these anomalies along the northern boundary. We suggest the
latter is associated with viscous Kelvin wave propagation. Under both types of
boundary conditions, the strength of the thermohaline overturning and the
tropical-polar density contrast vary out of phase. We show how the phase
relationship is related to the boundary wave propagation. The importance of
boundary regions indicates an urgent need to examine the robustness of
interdecadal variability in models as the resolution is increased, and as the
representation of the coastal, shelf/slope wave guide is improved. (Abriged
abstract)


title: Sub-Suns and Low Reynolds Number Flow

abstract:
  The phenomenon called the ``sub-Sun'' is the specular reflection of sunlight
by horizontally oriented plates of ice. Although well-known in meteorological
optics, the hydrodynamics of the orientation is not quantitatively understood.
I review the theory of torques on objects at low Reynolds numbers, define
coefficients $C_o$, $C_p$, and $C_\psi$ which describe the orienting torques on
discs, rods, and hexagonal prisms, and report here the results of experiments
to measure $C_o$ and $C_p$.


title: Statistics of Turbulence from Spectral-Line Data Cubes

abstract:
  Emission in spectral lines can provide unique information on interstellar
turbulence. Doppler shifts due to supersonic motions contain information on
turbulent velocity field which is otherwise difficult to measure. However, the
problem of separation of velocity and density fluctuations is far from being
trivial. Using atomic hydrogen (HI) as a test case, I review techniques
applicable to emission line studies with the emphasis on those that can provide
information on the underlying power spectra of velocity and density. I show
that recently developed mathematical machinery is promising for the purpose.
Its application to HI shows that in cold neutral hydrogen the velocity
fluctuations dominate the small scale structures observed in spectral-line data
cubes and this result is very important for the interpretation of observational
data, including the identification of clouds. Velocity fluctuations are shown
to dominate the formation of small scale structures that can be erroneously
identified as diffuse clouds. One may argue that the HI data is consistent with
the Goldreich-Shridhar picture of magnetohydrodynamic turbulence, but the
cascade from the scales of several kpc that this interpretation involves does
not fit well in the current paradigm of energy injection. The issue whether
magnetic field does make the turbulence anisotropic is still open, but if this
is the case, I show that studies of emission lines can provide a reliable way
of determining magnetic field direction. I discuss various techniques for
studying interstellar turbulence using emission lines, e.g. spectral
correlation functions, genus statistics and principal component analysis.

categories: astro-ph cond-mat.stat-mech physics.ao-ph physics.flu-dyn physics.plasm-ph physics.space-ph


title: On the inverse Compton scattering model of radio pulsars

abstract:
  Some characteristics of the inverse Compton scattering (ICS) model are
reviewed. At least the following properties of radio pulsars can be reproduced
in the model: core or central emission beam, one or two hollow emission cones,
different emission heights of these components, diverse pulse profiles at
various frequencies, linear and circular polarization features of core and
cones.


title: Dark Halo and Disk Galaxy Scaling Laws in Hierarchical Universes

abstract:
  We use cosmological N-body/gasdynamical simulations that include star
formation and feedback to examine the proposal that scaling laws between the
total luminosity, rotation speed, and angular momentum of disk galaxies reflect
analogous correlations between the structural parameters of their surrounding
dark matter halos. The numerical experiments follow the formation of
galaxy-sized halos in two Cold Dark Matter dominated universes: the standard
Omega=1 CDM scenario and the currently popular LCDM model. We find that the
slope and scatter of the I-band Tully-Fisher relation are well reproduced in
the simulations, although not, as proposed in recent work, as a result of the
cosmological equivalence between halo mass and circular velocity: large
systematic variations in the fraction of baryons that collapse to form galaxies
and in the ratio between halo and disk circular velocities are observed in our
numerical experiments. The Tully-Fisher slope and scatter are recovered in this
model as a direct result of the dynamical response of the halo to the assembly
of the luminous component of the galaxy. We conclude that models that neglect
the self-gravity of the disk and its influence on the detailed structure of the
halo cannot be used to derive meaningful estimates of the scatter or slope of
the Tully-Fisher relation. Our models fail, however, to match the zero-point of
the Tully-Fisher relation, as well as that of the relation linking disk
rotation speed and angular momentum. These failures can be traced,
respectively, to the excessive central concentration of dark halos formed in
the Cold Dark Matter cosmogonies we explore and to the formation of galaxy
disks as the final outcome of a sequence of merger events. (abridged)


title:
Theory of the Interaction of Planetary Nebulae with the Interstellar
  Medium

abstract:
  The theory of the interaction of planetary nebulae with the interstellar
medium is important for the interpretation of nebular morphologies that deviate
from point symmetry. It can be used to probe the interstellar medium and its
magnetic field. We emphasize in this review the role of hydrodynamical
instabilities in the interaction.


title:
Restrictions on parameters of power-law magnetic field decay for
  accreting isolated neutron stars

abstract:
  In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
  We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.


title: On the Clustering of GRBs on the Sky

abstract:
  The two-point correlation of the 4th (current) BATSE catalog (2494 objects)
is calculated. It is shown to be consistent with zero at nearly all angular
scales of interest. Assuming that GRBs trace the large scale structure in the
universe we calculate the angular correlation function for the standard CDM
(sCDM) model. It is shown to be $\le 10^{-4}$ at $\theta \simeq 5^\circ$ if the
BATSE catalog is assumed to be a volume-limited sample up to $z \simeq 1$.
Combined with the error analysis on the BATSE catalog this suggests that nearly
$10^5$ GRBs will be needed to make a positive detection of the two-point
angular correlation function at this angular scale.


title:
Wide Field CCD Surface Photometry of the Giant Elliptical Galaxy NGC
  4472 in the Virgo Cluster

abstract:
  We present deep wide field (16'.4 x 16'.4) Washington CT1 CCD surface
photometry of the giant elliptical galaxy NGC 4472, the brightest member in the
Virgo cluster. Our data cover a wider field than any previous CCD photometry as
well as going deeper. Surface brightness profiles of NGC 4472 are not well fit
by a single King model, but they can be fit approximately by two King models:
with separate models for the inner and outer regions. Surface brightness
profiles for the outer region can also be fit approximately by a deVaucouleurs
law. There is clearly a negative color gradient within 3' of NGC 4472, in the
sense that the color gets bluer with increasing radius. The slope of the color
gradient for this region is derived to be $\Delta \mu (C-T_1)$ = -0.08 mag
arcsec$^{-2}$ for $\Delta \log r =1$, which corresponds to a metallicity
gradient of $\Delta$ [Fe/H] $= -0.2$ dex. However, the surface color gets
redder slowly with increasing radius beyond 3'. A comparison of the structural
parameters of NGC 4472 in C and T1 images has shown that there is little
difference in the ellipse shapes between isochromes and isophotes. In addition,
photometric and structural parameters of NGC 4472 have been determined.


title:
Gamma-Ray Bursts in the 1990's - a Multi-wavelengths Scientific
  Adventure

abstract:
  In 1997 the first optical afterglow of a cosmic Gamma-Ray Burst was
discovered, and substantial progress has been achieved since then. Here we
present a short review of some recent developments in this field, with emphasis
on observational aspects of the GRB phenomenon.


title:
The Nature of Ultra-Luminous Compact X-Ray Sources in Nearby Spiral
  Galaxies

abstract:
  Studies were made of ASCA spectra of seven ultra-luminous compact X-ray
sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6
(Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.
1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &
Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.
1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they
are thought to represent a class of enigmatic X-ray sources often found in
spiral galaxies. For some of them, the ASCA data are newly processed, or the
published spectra are reanalyzed. For others, the published results are quoted.
The ASCA spectra of all these seven sources have been described successfully
with so called multi-color disk blackbody (MCD) emission arising from
optically-thick standard accretion disks around black holes. Except the case of
M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not
to exceed the Eddington limits, the black holes are inferred to have rather
high masses, up to ~100 solar masses. However, the observed innermost disk
temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be
compatible with the required high black-hole masses, as long as the standard
accretion disks around Schwarzschild black holes are assumed. Similarly high
disk temperatures are also observed from two Galactic transients with
superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high
disk temperature may be explained by the black hole rotation, which makes the
disk get closer to the black hole, and hence hotter.


title: Feedback Processes in the Early Universe

abstract:
  Feedback effects due to massive stars and supernovae in the first objects are
shown to strongly regulate both galaxy formation/evolution and the reionization
process. Here we review the most important ones in some detail. We discuss how
Type II supernovae can be used as tracers of the first objects and detected
with NGST, for which we predict supernova number counts including the effects
of gravitational lensing. Preliminary results on the formation of dust in the
ejecta of supernovae of primordial composition are also presented. We finally
turn to the consideration of the process of inhomogeneous reionization due to
primordial stellar sources by means of high resolution numerical simulations,
allowing for a self-consistent treatment of the above feedback processes. These
simulations allow us to draw conclusions on the evolution and epoch of
reionization and about the fate of reionizing objects. We conclude that a large
fraction (~ 99%) of collapsed objects must be dark at redshift around eight.


title:
What can BeppoSAX tell us about short GRBs: An update from the Subsecond
  GRB Project

abstract:
  We present some statistical considerations on the BeppoSAX hunt for subsecond
GRBs at the Scientific Operation Center. Archive analysis of a BATSE/SAX
sub-sample of bursts indicates that the GRB Monitor is sensitive to short (< 2
sec) events, that are in fact about 22% of the total. The non-detection of
corresponding prompt X-ray counterparts to short bursts in the Wide Field
Cameras, in about 3 years of operations, is discussed: with present data no
implications on the X-to-gamma-ray spectra of short vs long GRBs may be
inferred. Finally, the status of searching procedures at SOC is reviewed.


title: Observed Properties of Mass Loss in Symbiotic Binaries

abstract:
  Both the red giants and the Mira variables in symbiotic systems have
systematically higher mass-loss rates than do typical galactic giants and
Miras, which suggests that only very evolved giants, and so those with highest
mass-loss rates, can support symbiotic behaviour in widely separated binary
systems. They often show a flattened mass-loss geometry due to an intrinsically
inhomogeneous mass loss and/or tidal interactions between the binary
components. The main body of a symbiotic nebula is thus formed from material
lost in the giant wind, while the hot component is responsible for its
ionization and excitation. In addition, the fast wind and/or jet ejection from
the hot component, whenever occur, give rise to the complex, often bipolar,
shape of symbiotic nebulae. Observations of resolved nebulae also suggest that
the binary geometry and nebular structure are aligned but the bipolar outflow
may be not orthogonal to the orbital plane in all cases.


title:
Formation of Millisecond Pulsars with Heavy White Dwarf Companions -
  Extreme Mass Transfer on Sub-Thermal Timescales

abstract:
  We have performed detailed numerical calculations of the non-conservative
evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M_sun)
donor stars and a 1.3 M_sun accreting neutron star. We calculated the thermal
response of the donor star to mass loss, in order to determine its stability
and follow the evolution of the mass transfer. Under the assumption of the
"isotropic re-emission model" we demonstrate that in many cases it is possible
for the binary to prevent a spiral-in and survive a highly super-Eddington
mass-transfer phase (1 << M_dot/M_Edd < 10^5) on a sub-thermal timescale, if
the convective envelope of the donor star is not too deep. These systems thus
provide a new formation channel for binary millisecond pulsars with heavy CO
white dwarfs and relatively short orbital periods (3-50 days). However, we
conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or P_orb ~1
day (e.g. PSR B0655+64) the above scenario does not work, and a spiral-in phase
is still considered the most plausible scenario for the formation of such a
system.


title: Spin-Orbit Couplings in X-ray Binaries

abstract:
  We discuss the influence of tidal spin-orbit interactions on the orbital
dynamics of close intermediate-mass X-ray binaries. In particular we consider
here a process in which spin angular momentum of a contracting RLO donor star,
in a synchronous orbit, is converted into orbital angular momentum and thus
helps to stabilize the mass transfer by widening the orbit. Binaries which
would otherwise suffer from dynamically unstable mass transfer (leading to the
formation of a common envelope and spiral-in evolution) are thus shown to
survive a phase of extreme mass transfer on a sub-thermal timescale.
Furthermore, we discuss the orbital evolution prior to RLO in X-ray binaries
with low-mass donors, caused by the competing effects of wind mass loss and
tidal effects due to expansion of the (sub)giant.


title: New Direct Observational Evidence for Kicks in SNe

abstract:
  We present an updated list of direct strong evidence in favour of kicks being
imparted to newborn neutron stars. In particular we discuss the new cases of
evidence resulting from recent observations of the X-ray binary Circinus X-1
and the newly discovered binary radio pulsar PSR J1141-6545. We conclude that
the assumption that neutron stars receive a kick velocity at their formation is
unavoidable (van den Heuvel & van Paradijs 1997).


title:
The Space Density of Primordial Gas Clouds near Galaxies and Groups and
  their Relation to Galactic HVCs

abstract:
  The Arecibo HI Strip Survey probed the halos of ~300 cataloged galaxies and
the environments of ~14 groups with sensitivity to neutral hydrogen masses >
10^7 M_sun. The survey detected no objects with properties resembling the High
Velocity Clouds (HVCs) associated with the Milky Way or Local Group. If the
HVCs were typically M_HI=10^{7.5} M_sun objects distributed throughout groups
and galaxy halos at distances of 1 Mpc, the survey should have made ~70 HVC
detections in groups and ~250 detections around galaxies. The null detection
implies that HVCs are deployed at typical distances of <200 kpc from the
galaxies or group barycenters. If the clouds are in virial equilibrium, their
average dark matter fraction must be 98% or higher.


title: H2 emission from CRL 618

abstract:
  We present a complete study of the H2 infrared emission, including the pure
rotational lines, of the proto Planetary Nebulae CRL 618 with the ISO SWS. A
large number of lines are detected. The analysis of our observations shows: (i)
an OTP ratio very different from the classical value of 3, probably around
1.76-1.87; (ii) a stratification of the emitting region, and more precisely
different regions of emission, plausibly located in the lobes, in an
intermediate zone, and close to the torus; (iii) different excitation
mechanisms, collisions and fluorescence.


title: Do AGB Stars Differ Chemically from RGB Stars in Globular Clusters?

abstract:
  Recent improvements in globular cluster colour-magnitude diagrams, coupled
with an increase in large-sample spectroscopic abundance studies of cluster
giants, finally allow some attempts at a statistically meaningful comparison of
the chemical compositions of red giant branch (RGB) and asymptotic branch (AGB)
cluster stars. We review some of the extant data here, concluding that in a few
clusters the AGB stars show on average smaller amounts of high-temperature
proton-capture synthesis products (low oxygen, high sodium and aluminum) at
their surfaces than do the first-ascent RGB stars. This suggests that those RGB
stars with envelopes that have been enriched with proton-capture material also
have high helium contents. Such stars after the He-flash then take up residence
on the bluest parts of the horizontal branch (as a consequence of their high
envelope helium), probably never to return to the AGB during subsequent
evolutionary stages.


title:
The Ulysses Supplement to the GRANAT/WATCH Catalog of Cosmic Gamma-Ray
  Bursts

abstract:
  We present 3rd interplanetary network (IPN) localization data for 56
gamma-ray bursts in the GRANAT/ WATCH catalog which occurred between 1990
November and 1994 September. These localizations are obtained by triangulation
using various combinations of spacecraft in the IPN, which consisted of
Ulysses, BATSE, Pioneer Venus Orbiter (PVO), Mars Observer (MO), WATCH, and
PHEBUS. The intersections of the triangulation annuli with the WATCH error
circles produce error boxes with areas as small as 16 sq. arcmin., reducing the
sizes of the error circles by up to a factor of 800.


title: HST/WFPC2 observations of the core of KjPn 8

abstract:
  Narrow-band images of the core of the extraordinary poly-polar planetary
nebula KjPn 8 have been obtained with the WFPC2 camera on board the Hubble
Space Telescope. Spasmodic bipolar ejections, in changing directions have
occurred over thousands of years to create KjPn 8. The central star is finally
revealed in these observations and its compact nebular core is resolved into a
remarkably young, aprox. 500 years old, elliptical ring. The highest speed
bipolar outflows are perpendicular to this central ring which is identified as
the latest event in the creation of this nebula. The formation history of KjPn
8 has involved two distinct planetary nebula-like events, probably originating
froma a binary core evolution with components of similar mass.


title:
Testing envelope models of young stellar objects with submillimeter
  continuum and molecular-line observation

abstract:
  This paper examines the density and velocity structure of envelopes around
young stellar objects through submillimeter continuum imaging of four objects
in Taurus and previously obtained molecular-line data. Observations carried out
with the SCUBA on the JCMT at 850 and 450 micron of L1489 IRS, L1535 IRS, L1527
IRS, and TMC 1 reveal ~2000 AU elongated structures embedded in extended
envelopes. The density distribution in these envelopes is equally well fit by a
radial power-law of index p=1.0-2.0 or with a collapse model such as that of
Shu (1997: ApJ, 214, 488). This inside-out collapse model predicts 13CO, C18O,
HCO+, and H13CO+ line profiles which closely match observed spectra toward
three of our four sources. This shows that the inside-out collapse model offers
a good description of YSO envelopes, but also that reliable constraints on its
parameters require independent measurements of the density and the velocity
structure, e.g., through continuum and line observations. For the remaining
source, L1489 IRS, we find that a model consisting of a 2000 AU radius,
rotating, disk-like structure better describes the data. Possibly, this source
is in transition between the embedded Class I and the optically revealed T
Tauri phases. Two apparently starless cores are found at ~10,000 AU from L1489
IRS and L1535 IRS. They are cold, 10-15 K, contain 0.5-3.0 M_sol, and have flat
density distributions characterized by a Gaussian of ~10,000 AU FWHM. The
proximity of these cores shows that star formation in truly isolated cores is
rare even in Taurus.


title:
A multipole-Taylor expansion for the potential of gravitational lens MG
  J0414+0534

abstract:
  We employ a multipole-Taylor expansion to investigate how tightly the
gravitational potential of the quadruple-image lens MG J0414+0534 is
constrained by recent VLBI observations. These observations revealed that each
of the four images of the background radio source contains four distinct
components, thereby providing more numerous and more precise constraints on the
lens potential than were previously available. We expand the two-dimensional
lens potential using multipoles for the angular coordinate and a modified
Taylor series for the radial coordinate. After discussing the physical
significance of each term, we compute models of MG J0414+0534 using only VLBI
positions as constraints. The best-fit model has both interior and exterior
quadrupole moments as well as exterior m=3 and m=4 multipole moments. The
deflector centroid in the models matches the optical galaxy position, and the
quadrupoles are aligned with the optical isophotes. The radial distribution of
mass could not be well constrained. We discuss the implications of these models
for the deflector mass distribution and for the predicted time delays between
lensed components.


title: Studies of Low-Mass Star Formation with ALMA

abstract:
  ALMA will revolutionize the study of star formation by providing a
combination of angular resolution and sensitivity that far exceeds that of
present instruments. I will focus on studies of relatively isolated cores that
are forming low-mass stars. There is a general paradigm for the formation of
such stars, and there are detailed theoretical predictions for the evolution of
the density and velocity fields for different assumptions about the initial
conditions. Because the theory is well developed, observational tests are
particularly revealing. The primary probes of physical conditions in these
regions are discussed and the sensitivity of ALMA to these probes is shown and
compared to the current state of the art. The consequences for the ALMA
requirements are discussed.


title:
Physical Conditions, Grain Temperatures, and Enhanced Very Small Grains
  in the Barnard Loop

abstract:
  We derive the radio spectral index of the Barnard Loop (BL) from large-scale
radio surveys at four frequencies and find it to be a thermal source. We use
the radio data together with H-alpha data to determine the electron temperature
in BL, the lambda-Ori HII region, and a high-latitude filament; all of these
regions are somewhat cooler than typical HII regions.
  We perform least squares fits of the DIRBE diffuse IR intensities to the
21-cm line and radio continuum intensities. After the resolution of a
``geometrical conundrum'', this allows us to derive the electron density n_e;
we find n_e ~ 2.0 cm^{-3} and pressure (P/k) ~ 24000 cm^{-3} K.
  Grains within BL are warmer than in HI regions. Trapped L-alpha accounts for
the extra heating that is required. This is a general effect that needs to be
accounted for in all analyses that examine IR emission from H^+ regions. Very
small grains that emit 60 micron radiation are enhanced in BL relative to HI by
a factor of 2-3, while PAH's that emit 12 micron are probably deficient by a
factor ~2.


title: Resonant capture, counter-rotating disks, and polar rings

abstract:
  We suggest that polar rings and/or counter-rotating disks in flattened
galaxies can be formed from stars captured at the Binney resonance, where the
rate of precession of the angular momentum vector of a disk star equals the
pattern speed of a triaxial halo. If the halo pattern speed is initially
retrograde and slowly decays to zero, stars can be trapped as the Binney
resonance sweeps past them, and levitated into polar orbits. If the halo
pattern speed is initially retrograde and slowly changes to prograde, trapped
stars can evolve from prograde to retrograde disk orbits. The stellar
components of polar rings formed by this process should consist of two equal,
counter-rotating star streams.


title: The Central X-Ray Point Source in Cassiopeia A

abstract:
  The spectacular first light observation by the Chandra X-Ray Observatory
revealed an X-ray point source near the center of the 300 yr old Cas A
supernova remnant. We present an analysis of the public X-ray spectral and
timing data. No coherent pulsations were detected in the Chandra/HRC data. The
3-sigma upper limit on the pulsed fraction is <35% for P>20 ms. The
Chandra/ACIS spectrum of the point source may be fit with an ideal blackbody
(kT=0.5 keV), or with BB models modified by the presence of a NS atmosphere
(kT=0.25-0.35 keV), but the temperature is higher and the inferred emitting
area lower than expected for a 300 yr old NS according to standard cooling
models. The spectrum may also be fit with a power law model (photon index
2.8-3.6). Both the spectral properties and the timing limits of the point
source are inconsistent with a young Crab-like pulsar, but are quite similar to
the properties of the anomalous X-ray pulsars. The spectral parameters are also
very similar to those of the other radio-quiet X-ray point sources in the
supernova remnants Pup A, RCW 103, and PKS 1209-52. Current limits on an
optical counterpart for the Cas A point source rule out models that invoke
fallback accretion onto a compact object if fallback disk properties are
similar to those in quiescent low-mass X-ray binaries. However, the optical
limits are marginally consistent with plausible alternative assumptions for a
fallback disk. In this case, accreting NS models can explain the X-ray data,
but an accreting BH model is not promising.


title: Is Thermal Instability Significant in Turbulent Galactic Gas?

abstract:
  We investigate numerically the role of thermal instability (TI) as a
generator of density structures in the interstellar medium (ISM), both by
itself and in the context of a globally turbulent medium. Simulations of the
instability alone show that the condenstion process which forms a dense phase
(``clouds'') is highly dynamical, and that the boundaries of the clouds are
accretion shocks, rather than static density discontinuities. The density
histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak
at low densities plus a slope change at high densities. Final static situations
may be established, but the equilibrium is very fragile: small density
fluctuations in the warm phase require large variations in the density of the
cold phase, probably inducing shocks into the clouds. This result suggests that
such configurations are highly unlikely. Simulations including turbulent
forcing show that large- scale forcing is incapable of erasing the signature of
the TI in the density PDFs, but small-scale, stellar-like forcing causes
erasure of the signature of the instability. However, these simulations do not
reach stationary regimes, TI driving an ever-increasing star formation rate.
Simulations including magnetic fields, self-gravity and the Coriolis force show
no significant difference between the PDFs of stable and unstable cases, and
reach stationary regimes, suggesting that the combination of the stellar
forcing and the extra effective pressure provided by the magnetic field and the
Coriolis force overwhelm TI as a density-structure generator in the ISM. We
emphasize that a multi-modal temperature PDF is not necessarily an indication
of a multi-phase medium, which must contain clearly distinct thermal
equilibrium phases.


title: The variability analysis of PKS 2155-304

abstract:
  In this paper, the post-1977 photometric observations of PKS 2155-304 are
compiled and used to discuss the variation periodicity. Largest amplitude
variations ($\Delta U = 1^{m}.5$; $\Delta B = 1^{m}.65$;
  $\Delta V = 1^{m}.85 $; $\Delta R = 1^{m}.25$; $\Delta I = 1^{m}.14 $) and
color indices ($(B-V) = 0.30\pm 0.06$; $(U-B) = -0.72\pm 0.08$;
  $(B-R) = 0.62\pm 0.07$; $(V-R) = 0.32\pm 0.04$) are found.
  The Jurkevich's method and DCF (Discrete Correlation Function) method
indicate possible periods of 4.16-year and 7.0-year in the V light curve.


title: Long-term Variability Properties and Periodicity Analysis for Blazars

abstract:
  In this paper, the compiled long-term optical and infrared measurements of
some blazars are used to analyze the variation properties and the optical data
are used to search for periodicity evidence in the lightcurve by means of the
Jurkevich technique and the discrete correlation function (DCF) method.
Following periods are found: 4.52-year for 3C 66A; 1.56 and 2.95 years for AO
0235+164;
  14.4, 18.6 years for PKS 0735+178; 17.85 and 24.7 years for PKS 0754+100;
5.53 and 11.75 for OJ 287. 4.45, and 6.89 years for PKS 1215; 9 and 14.84 years
for PKS 1219+285;
  2.0, 13.5 and 22.5 for 3C273; 7.1 year for 3C279;
  6.07 for PKS 1308+326; 3.0 and 16.5 years for PKS 1418+546;
  2.0 and 9.35 years for PKS 1514-241; 18.18 for PKS 1807+698;
  4.16 and 7.0 for 2155-304; 14 and 20 years for BL Lacertae. Some explanations
have been discussed.


title:
A Hard Medium Survey with ASCA. III.: a Type 2 AGN revealed from X-ray
  Spectroscopy

abstract:
  In this paper we report the discovery of an hard X-ray selected Type 2
Seyfert galaxy and we present and discuss its X-ray and optical spectrum
together with the radio to X-ray energy distribution. The X-ray source -
AXJ2254+1146 - is part of the ASCA Hard Serendipitous Survey (HSS). What makes
this discovery particularly noteworthy is the fact that the Type 2
classification of this Seyfert galaxy has resulted directly from the X-ray data
and has been confirmed by optical spectroscopy only subsequently. The X-ray
spectrum of AXJ2254+1146 is best described by a model consisting of an
unresolved Gaussian line at $6.43\pm 0.1$ keV plus the so called
"leaky-absorber" continua having an intrinsic power law photon index of
$\Gamma$ = $2.51^{2.76}_{2.17}$ (1 $\sigma$ confidence interval). The best fit
values of the absorbing column density ($N_H$ = $1.85^{2.24}_{1.47} \times
10^{23}$ cm$^{-2}$), of the line equivalent width ($0.6^{0.84}_{0.36}$ keV) and
of the scattering fraction ($0.7^{1.4}_{0.1} %$), lead us to classify it as a
Type 2 AGN from an X-ray point of view. Inspection of the POSS II image reveals
the presence, within the ASCA X-ray error circle, of the nearby Sbc spiral
galaxy UGC 12237 ($m_{B_o}=14.26$) that, even on positional ground
considerations alone, is the most likely optical counterpart of AXJ2254+1146.
Subsequent optical spectroscopy of UGC 12237 has confirmed its Seyfert 2
optical nature.


title: Multiple Outflows in AFGL 2688

abstract:
  We present high resolution (1.1x0.9'') imaging of the proto-planetary nebula
AFGL 2688 in the CO (J=2-1) line using the IRAM interferometer. The
observations reveal with unprecedented detail the structure and the kinematics
of the gas ejected by the star over the past few hundred years and exemplify
the mechanism by which point symmetries are imprinted on the structure of
planetary nebulae at early stages of their formation.


title:
Limits on the Spatial Extent of AGN Measured with the Fine Guidance
  Sensors of the HST

abstract:
  The optical structure of several AGN has been studied using the Fine Guidance
Sensors (FGS) on the Hubble Space Telescope (HST). The FGSs are interferometric
devices which can resolve structure on scales of 20 milliarcsecs or less and
hence have the potential to improve on the resolution attainable by HST's
cameras. The FGSs produce interferometric fringes known as S-curves which are
related to the intensity profile of the object on the sky. These have been
analyzed using a simple model for the radial intensity distribution and
strength of the underlying background illumination of the observed objects.
Eight different observations of six different AGN have been analyzed. No
statistically significant differences from point sources are detected but
significant upper limits of order 20 milliarcseconds are placed on any spatial
extent. Systematic effects limiting the resolution are discussed and some
simple conclusions about the physical size and luminosity densities of the
emitting regions of the AGN implied by the data are given.


title: Radio emission of the Galactic X-rays binaries with relativistic jets

abstract:
  Variable non-thermal radio emission from Galactic X-ray binaries is a trace
of relativistic jets, created near accretion disks. The spectral
characteristics of a lot of radio flares in the X-ray binaries with jets (RJXB)
is discussed in this report. We carried out several long daily monitoring
programs with the RATAN-600 radio telescope of the sources: SS433, Cyg X-3,
LSI+61o303, GRS 1915+10 and some others. We also reviewed some data from the
GBI monitoring program at two frequencies and hard X-ray BATSE (20-100 keV) and
soft X-ray RTXE (2-12 keV) ASM data. We confirmed that flaring radio emission
of Cyg X-3 correlated with hard and anti-correlated with soft X-ray emission
during the strong flare (>$ Jy) in May 1997. During two orbital periods we
investigated radio light curves of the remarkable X-binary LSI+61o303. Two
flaring events near a phase 0.6 of the 26.5-day orbital period have been
detected for first time at four frequencies simultaneously. Powerful flaring
events of SS433 were detected at six frequencies in May 1996 and in May 1999.
The decay of the flare is exactly fitted by an exponential law and the rate of
the decay $\tau$ depends upon frequency as tau \propto \nu^{-0.4} in the first
flare and does not depend upon frequency in the second flare, and is equal to
\tau=6+-1 days at frequencies from 0.96 to 21.7 GHz in the last flare in May
1999. Many flaring RJXB show two, exponential and power, laws of flare decay.
Moreover, these different laws could be present in one or several flares and
commonly flare decays are faster at a higher frequency. The decay law seems to
change because of geometric form of the conical hollow jets. The synchrotron
and inverse Compton losses could explain general frequency dependences in flare
evolution. In conclusion we summarized the general radio properties of RJXB.


title:
First Light Measurements of Capella with the Low Energy Transmission
  Grating Spectrometer aboard the Chandra X-ray Observatory

abstract:
  We present the first X-ray spectrum obtained by the Low Energy Transmission
Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum
is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The
measured wavelength resolution, which is in good agreement with ground
calibration, is $\Delta \lambda \simeq$ 0.06 A (FWHM). Although in-flight
calibration of the LETGS is in progress, the high spectral resolution and
unique wavelength coverage of the LETGS are well demonstrated by the results
from Capella, a coronal source rich in spectral emission lines. While the
primary purpose of this letter is to demonstrate the spectroscopic potential of
the LETGS, we also briefly present some preliminary astrophysical results. We
discuss plasma parameters derived from line ratios in narrow spectral bands,
such as the electron density diagnostics of the He-like triplets of carbon,
nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII
line at 15.014 A.


title: O-bearing Molecules in Carbon-rich Proto-Planetary Objects

abstract:
  We present ISO LWS observations of the proto-planetary nebula CRL 618, a star
evolving very fast to the planetary nebula stage. In addition to the lines of
12CO, 13CO, HCN and HNC, we report on the detection of H2O and OH emission
together with the fine structure lines of [OI] at 63 and 145 um. The abundance
of the latter three species relative to 12CO are 4 10^{-2}, 8 10^{-4} and 4.5
(approximate value) in the regions where they are produced. We suggest that
O-bearing species other than CO are produced in the innermost region of the
circumstellar envelope. The UV photons from the central star photodissociate
most of the molecular species produced in the AGB phase and allow a chemistry
dominated by standard ion-neutral reactions. Not only allow these reactions the
formation of O-bearing species, but they also modify the abundances of C-rich
molecules like HCN and HNC for which we found an abundance ratio of roughly 1,
much lower than in AGB stars. The molecular abundances in the different regions
of the circumstellar envelope have been derived from radiative transfer models
and our knowledge of its physical structure.


title:
On Luminosity and morphology segregation in the Southern Sky Redshift
  Survey 2

abstract:
  The results are still valid, however this letter is superceeded by a
significantly extended version which is available at astro-ph/0003358 and is
scheduled for publication in the ApJ Dec. 10, 2000. We have withdrawn this
letter to avoid confusion.


title: The BeppoSAX High Energy Large Area Survey (HELLAS): a progress Report

abstract:
  The integrated emission of highly obscured AGN is expected to provide a major
contribution to the X-ray energy density in the Universe: the X-ray background
(XRB). The study of these objects is possible only at energies where the
effects of absorption are less severe. For this reason we have carried out the
BeppoSAX High Energy LLarge Area Survey in the hardest band (5-10 keV)
accessible so far with imaging X-ray instruments. The source surface density at
the survey limiting flux accounts for a significant fraction (20-30%) of the
hard XRB. The X-ray data complemented by multiwavelength follow--up
observations suggest that a large fraction of the hard sources are AGN and that
X-ray absorption with column densities in the range 10^22-23.5 cm-2 is common
among them. The great diversity in their optical-near-IR properties suggests
that the optical appearance of obscured sources is a function of the X-ray
luminosity. We briefly discuss the implications of these findings for the XRB
models.


title:
Morphological evolution of X-ray clusters using hydrodynamical
  simulations

abstract:
  A large set of TREESPH simulations is used to test the global morphology of
galaxy clusters and its evolution against X-ray data. A powerful method to
investigate substructures in galaxy clusters are the power ratios introduced by
Buote & Tsai. We consider three flat cosmological models: CDM,
LCDM($\Omega_\Lambda = 0.7$) and CHDM($\Omega_h = 0.2$, 1 massive $\nu$), all
normalized so to fit the observed number of clusters. For each model we built
40 clusters, using a TREESPH code, and performed a statistical comparison with
a data sample including nearby clusters observed with ROSAT PSPC instrument.
The comparison disfavors the LCDM model, as clusters appear too relaxed, while
CDM and CHDM clusters, in which a higher degree of complexity occurs, seem to
be closer to observations. A better fit of data can be expected for some
different DM mix. If DM distributions are used instead of baryons, we find
substructures more pronounced than in gas and models have a different score.
Using hydrodynamical simulations is therefore essential to our aims.


title: Antikaon condensation in neutron stars

abstract:
  We investigate the condensation of charged $K^-$ meson and neutral $\bar K^0$
meson in dense neutron star matter. Calculations are performed in relativistic
mean field models in which both the baryon-baryon and (anti)kaon-baryon
interactions are mediated by meson exchange. It is found that $\bar K^0$
condensation is quite sensitive to the antikaon optical potential and depends
more strongly on the nucleonic equation of state. For moderate values of
antikaon potential and a rather stiff equation of state, a significant region
of maximum mass star will contain $\bar K^0$ meson. The critical density of
$\bar K^0$ condensation is always higher than that of $K^-$ condensation. With
the appearance of $K^-$ and $\bar K^0$ condensates, pairs of $p-K^-$ and
$n-\bar K^0$ are produced with equal proportion leading to a perfectly
symmetric matter of nucleons and antikaons in neutron stars. Along with $K^-$
condensate, $\bar K^0$ condensate makes the equation of state much softer
resulting in smaller maximum mass stars compared to the case without any
condensate.


title: Updated Information on the Local Group

abstract:
  The present note updates the information published in my recent monograph on
\underline{The Galaxies of the Local Group}. Highlights include (1) the
addition of the newly discovered Cetus dwarf spheroidal as a certain member of
the Local Group, (2) an improved distance for SagDIG, which now places this
object very close to the edge of the Local Group zero-velocity surface, (3)
more information on the evolutionary histories of some individual Local Group
members, and (4) improved distance determinations to, and luminosities for, a
number of Local Group members. These data increase the number of certain (or
probable) Local Group members to 36. The spatial distribution of these galaxies
supports Hubble's claim that the Local Group ``is isolated in the general
field.'' Presently available evidence suggests that star formation continued
much longer in many dwarf spheroidals than it did in the main body of the
Galactic halo. It is suggested that ``young'' globular clusters, such as
Ruprecht 106, might have formed in now defunct dwarf spheroidals. Assuming
SagDIG, which is the most remote Local Group galaxy, to lie on, or just inside,
the zero-velocity surface of the Local Group yields a dynamical age \gtrsim
17.9 \pm 2.7 Gyr.


title:
Evidence for a low-density Universe from the relative velocities of
  galaxies

abstract:
  The motions of galaxies can be used to constrain the cosmological density
parameter Omega and the clustering amplitude of matter on large scales. The
mean relative velocity of galaxy pairs, estimated from the Mark III survey,
indicates that Omega = 0.35 +0.35/-0.25. If the clustering of galaxies is
unbiased on large scales, Omega = 0.35 +/- 0.15, so that an unbiased
Einstein-de Sitter model (Omega = 1) is inconsistent with the data.


title:
Eccentric Stellar Discs with Strong Density Cusps and Separable
  Potentials

abstract:
  We introduce a class of eccentric discs with "strong" density cusps whose
potentials are of St\"ackel form in elliptic coordinates. Our models exhibit
some striking features: sufficiently close to the location of the cusp, the
potential and surface density distribution diverge as $\Phi \propto r^{-1}$ and
$\Sigma \propto r^{-2}$, respectively. As we move outward from the centre, the
model takes a non-axisymmetric, lopsided structure. In the limit, when $r$
tends to infinity, the isocontours of $\Phi$ and $\Sigma$ become spherically
symmetric. It is shown that the configuration space is occupied by three
families of regular orbits: {\it eccentric butterfly}, {\it aligned loop} and
{\it horseshoe} orbits. These orbits are properly aligned with the surface
density distribution and can be used to construct self-consistent equilibrium
states.


title:
Photometric Properties of 47 Clusters of Galaxies: I. The Butcher-Oemler
  Effect

abstract:
  We present gri CCD photometry of 44 Abell clusters and 4 cluster candidates.
Twenty one clusters in our sample have spectroscopic redshifts. Fitting a
relation between mean g, r and i magnitudes, and redshift for this subsample,
we have calculated photometric redshifts for the remainder with an estimated
accuracy of 0.03. The resulting redshift range for the sample is 0.03<z<0.38.
Color-magnitude diagrams are presented for the complete sample and used to
study evolution of the galaxy population in the cluster environment. Our
observations show a strong Butcher-Oemler effect (Butcher & Oemler 1978, 1984),
with an increase in the fraction of blue galaxies (f_B) with redshift that
seems more consistent with the steeper relation estimated by Rakos and
Schombert (1995) than with the original one by Butcher & Oemler (1984).
However, in the redshift range between ~ 0.08 and 0.2, where most of our
clusters lie, there is a wide range of f_B values, consistent with no redshift
evolution of the cluster galaxy population. A large range of f_B values is also
seen between ~ 0.2 and 0.3, when Smail at al. (1998) x-ray clusters are added
to our sample. The discrepancies between samples underscore the need for an
unbiased sample to understand how much of the Butcher-Oemler effect is due to
evolution, and how much to selection effects. We also tested the idea proposed
by Garilli et al. (1996) that there is a population of unusually red galaxies
which could be associated either with the field or clusters, but we find that
these objects are all near the limiting magnitude of the images (20.5<r<22) and
have colors that are consistent with those expected for stars or field galaxies
at z ~ 0.7.


title:
Interferometric 12CO Observations of the Central Disk of NGC 4631: An
  Energetic Molecular Outflow

abstract:
  We present interferometric observations of CO J=1-0 emission in the central
regions of the edge-on galaxy NGC 4631, known for its extended gaseous halo and
its tidal interactions. Previous single-dish observations revealed that almost
all of the CO emission arises from a central ring or bar-like structure of
length $\sim$ 4 kpc. We confirm this structure at higher resolution, and find
that it is bent at the center, reflecting the overall bend in this galaxy
apparent from optical images. The kinematic evidence favors a rigidly rotating
ring over a bar. The gaseous halo emission in several tracers is concentrated
above and below this molecular structure. To the north of an emission peak at
the eastern end of the structure is an extraplanar feature showing filamentary
and shell-like properties which we interpret as an energetic molecular outflow.
The energies involved are difficult to estimate, but are probably of order
10$^{54}$ ergs or more. The CO concentration in the disk below this structure
coincides with a bright HII region complex, a peak of radio emission, and the
brightest X-ray feature in the inner disk of the galaxy seen in a ROSAT HRI
map, all suggesting intense star formation. A filament of radio continuum
emission may also have a footprint in this region of the disk. The origin of
the outflow is unclear.


title:
The Second Peak: The Dark-Energy Density and the Cosmic Microwave
  Background

abstract:
  Supernova evidence for a negative-pressure dark energy (e.g., cosmological
constant or quintessence) that contributes a fraction $\Omega_\Lambda\simeq0.7$
of closure density has been bolstered by the discrepancy between the total
density, $\Omega_{\rm tot}\simeq1$, suggested by the location of the first peak
in the cosmic microwave background (CMB) power spectrum and the
nonrelativistic-matter density $\Omega_m\simeq0.3$ obtained from dynamical
measurements. Here we show that the impending identification of the location of
the {\it second} peak in the CMB power spectrum will provide an immediate and
independent probe of the dark-energy density. As an aside, we show how the
measured height of the first peak probably already points toward a low matter
density and places upper limits to the reionization optical depth and
gravitational-wave amplitude.


title:
Non-Thermal Emission from Relativistic Electrons in Clusters of
  Galaxies: A Merger Shock Acceleration Model

abstract:
  We have investigated evolution of non-thermal emission from relativistic
electrons accelerated at around the shock fronts during merger of clusters of
galaxies. We estimate synchrotron radio emission and inverse Compton scattering
of cosmic microwave background photons from extreme ultraviolet (EUV) to hard
X-ray range. The hard X-ray emission is most luminous in the later stage of
merger. Both hard X-ray and radio emissions are luminous only while signatures
of merging events are clearly seen in thermal intracluster medium (ICM). On the
other hand, EUV radiation is still luminous after the system has relaxed.
Propagation of shock waves and bulk-flow motion of ICM play crucial roles to
extend radio halos. In the contracting phase, radio halos are located at the
hot region of ICM, or between two substructures. In the expanding phase, on the
other hand, radio halos are located between two ICM hot regions and shows
rather diffuse distribution.


title:
Evidence for TeV gamma-ray emission from the shell type SNR
  RXJ1713.7-3946

abstract:
  We report the results of TeV gamma-ray observations of the shell type SNR
RXJ1713.7-3946 (G347.3-0.5). The discovery of strong non-thermal X-ray emission
from the northwest part of the remnant strongly suggests the existence of
electrons with energies up to 100 TeV in the remnant, making the SNR a good
candidate TeV gamma-ray source. We observed RXJ1713.7-3946 from May to August
1998 with the CANGAROO 3.8m atmospheric imaging Cerenkov telescope and obtained
evidence for TeV gamma-ray emission from the NW rim of the remnant with the
significance of 5.6 sigma. The observed TeV gamma-ray flux from the NW rim
region was estimated to be (5.3 +/- 0.9[statistical] +/- 1.6[systematic]) *
10^{-12} photons cm^{-2} s^{-1} at energies >= 1.8 +/- 0.9 TeV. The data
indicate that the emitting region is much broader than the point spread
function of our telescope. The extent of the emission is consistent with that
of hard X-rays observed by ASCA. This TeV gamma-ray emission can be attributed
to the Inverse Compton scattering of the Cosmic Microwave Background Radiation
by shock accelerated ultra-relativistic electrons. Under this assumption, a
rather low magnetic field of 11 micro gauss is deduced for the remnant from our
observation.


title: The 24-Hour Night Shift: Astronomy from Microlensing Monitoring Networks

abstract:
  Scores of on-going microlensing events are now announced yearly by the
microlensing discovery teams OGLE, MACHO and EROS. These early warning systems
have allowed other international microlensing networks to focus considerable
resources on intense photometric - and occasionally spectroscopic - monitoring
of microlensing events. Early results include: metallicity measurements of main
sequence Galactic bulge stars; limb darkening determinations for stars in the
Bulge and Small Magellanic Cloud; proper motion measurements that constrain
microlens identity; and constraints on Jovian-mass planets orbiting (presumably
stellar) lenses. These results and auxiliary science such as variable star
studies and optical identification of gamma ray bursts are reviewed.


title:
The Rise Times of High and Low Redshift Type Ia Supernovae are
  Consistent

abstract:
  We present a self-consistent comparison of the rise times for low- and
high-redshift Type Ia supernovae. Following previous studies, the early light
curve is modeled using a t-squared law, which is then mated with a modified
Leibundgut template light curve. The best-fit t-squared law is determined for
ensemble samples of low- and high-redshift supernovae by fitting simultaneously
for all light curve parameters for all supernovae in each sample. Our method
fully accounts for the non-negligible covariance amongst the light curve
fitting parameters, which previous analyses have neglected. Contrary to Riess
et al. (1999), we find fair to good agreement between the rise times of the
low- and high-redshift Type Ia supernovae. The uncertainty in the rise time of
the high-redshift Type Ia supernovae is presently quite large (roughly +/- 1.2
days statistical), making any search for evidence of evolution based on a
comparison of rise times premature. Furthermore, systematic effects on rise
time determinations from the high-redshift observations, due to the form of the
late-time light curve and the manner in which the light curves of these
supernovae were sampled, can bias the high-redshift rise time determinations by
up to +3.6/-1.9 days under extreme situations. The peak brightnesses - used for
cosmology - do not suffer any significant bias, nor any significant increase in
uncertainty.


title: Near-infrared emission-line galaxies in the Hubble Deep Field North

abstract:
  We present the 2.12~$\mu$m narrow-band image of the Hubble Deep Field North
taken with the near-infrared camera (CISCO) on the Subaru telescope. Among five
targets whose H$\alpha$ or [O~{\sc iii}] emission lines are redshifted into our
narrow-band range expected from their spectroscopic redshift, four of them have
strong emission lines, especially for the two [O~{\sc iii}] emission-line
objects. The remaining one target shows no H$\alpha$ emission in spite of its
bright rest-UV luminosity, indicating that this object is already under the
post-starburst phase. The volume-averaged $SFR$ derived from the detected two
H$\alpha$ emission is roughly consistent with that evaluated from the rest-UV
continuum.


title:
Infrared Imaging of the Gravitational Lens PG 1115+080 with the Subaru
  Telescope

abstract:
  We present high spatial resolution images of the gravitational-lens system PG
1115+080 taken with the near-infrared camera (CISCO) on the Subaru telescope.
The FWHM of the combined image is $0.''32$ in the $K'$-band, yielding spatial
resolution of $0.''14$ after a deconvolution procedure. This is a first
detection of an extended emission adjacent to the A1/A2 components, indicating
the presence of a fairly bright emission region with a characteristic angular
radius of $\sim$ 5 mas (40 pc). The near-infrared image of the Einstein ring
was extracted in both the $J$ and $K'$ bands. The $J-K'$ color is found to be
significantly redder than that of a synthetic model galaxy with an age of 3
Gyr, the age of the universe at the quasar redshift.


title: Near infrared imaging spectroscopy of NGC1275

abstract:
  We present H and K band imaging spectroscopy of the core regions of the
cD/AGN galaxy NGC1275. The spectra, including lines from H2, H, 12CO bandheads,
[FeII], and [FeIII], are exploited to constrain the star formation and
excitation mechanisms in the galaxy's nucleus. The near-infrared properties can
largely be accounted for by ionized gas in the NLR, dense molecular gas, and
hot dust concentrated near the active nucleus of NGC1275. The strong and
compact H2 emission is mostly from circumnuclear gas excited by the AGN and not
from the cooling flow. The extended emission of latetype stars is diluted in
the center by the thermal emission of hot dust.


title: On the FIR emission from intracluster dust

abstract:
  We make predictions for the diffuse far-infrared (FIR) emission from dust in
the intracluster medium (ICM) of the Virgo cluster using detailed information
on potential dust sources, grain heating and sputtering rates available for
this cluster from recent optical and X-ray studies. We show that the infrared
emission from dust injected by sources inside the core region of the Virgo
cluster is a factor of 10 below the detection limit of currently available
telescopes. The outer regions of dynamically young clusters like the Virgo
cluster have a further potential source of intracluster grains since they are
still accreting freshly infalling spiral galaxies which are presumably
contained in an accreting intergalactic medium (IGM). We show that cosmic ray
driven winds from the infalling spirals can inject grains into a subvirial IGM
that is external to the observed X-ray-emitting ICM. Sputtering during the
injection process and in the IGM is weak, so that the injected grains should
accumulate in the IGM until the infall brings them into contact with the hot
ICM. This effect dominates the dust injection rate from known sources embedded
in the hot Virgo ICM. Thus, any detection of diffuse IR emission would probe
the current dust accretion rate for the cluster, acting as an indicator of the
youth and the dynamical state of the cluster. The predictions for the Virgo
cluster are generalised to other clusters and the possibility of detection of
dynamically young clusters at cosmological distances is discussed.


title:
The Hamburg/SAO Survey for Emission-Line Galaxies. III. The Third List
  of 81 Galaxies

abstract:
  We present the third list with results {Tables 2 to 6 are available only in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html. Figures A1 to
A9 will be made available only in the electronic version of the journal.} of
the Hamburg/SAO Survey for Emission-Line Galaxies (HSS therein, SAO - Special
Astrophysical Observatory, Russia). This survey is based on the digitized
objective-prism photoplate database of the Hamburg Quasar Survey (HQS).
  Here, we present new spectroscopic results of candidates which were obtained
in 1998 with the 2.1 m KPNO and the 2.2 m Calar Alto telescopes. All candidates
are selected in the declination band +35$^{\circ}$ to +40$^{\circ}$.
  The follow-up spectroscopy with the 2 m class telescopes confirmed 85
emission-line objects out of 113 observed candidates and allowed their
quantitative spectral classification. For 80 of them, the redshifts are
determined for the first time. For 5 previously known ELGs, line ratios are
presented for the first time. We could classify 55 out of the 85 emission-line
objects as BCG/H{\sc ii} galaxies or probable BCGs, 4 - as QSOs, 6 - as Seyfert
galaxies, 1 - as super-association in a subluminous spiral galaxy, and 11 are
low-excitation objects - either starburst nuclear (SBN), or dwarf amorphous
nuclear starburst galaxies (DANS). We could not classify 8 ELGs. Further, for 8
more galaxies we did not detect any significant emission lines.


title: The oxygen abundance deficiency in irregular galaxies

abstract:
  The observed oxygen abundances in a number of irregular galaxies have been
compared with predictions of the closed-box model of chemical and photometric
evolution of galaxies. Oxygen is found to be deficient with respect to the
predicted abundances. This is an indicator in favor of loss of heavy elements
via galactic winds or/and of infall of low--abundance gas onto the galaxy.
  The oxygen abundance deficiency observed within the optical edge of a galaxy
cannot be explained by mixing with the gas envelope observed outside the
optical limit. We confirm the widespread idea that a significant part of the
heavy elements is ejected by irregular galaxies in the intergalactic medium.


title: On the Formation of Jets

abstract:
  The phenomenology of jets associated with a variety of black hole systems is
summarized, emphasizing the constraints imposed on their origin. Models of jet
formation are reviewed, focusing in particular on recent ideas concerning MHD
models. Finally, the potential for advancing our understanding of jets both
through future observations - especially forthcoming X-ray missions - and for
elucidating some crucial theoretical questions is highlighted.


title:
On the origin of the luminosity - metallicity relation for late-type
  galaxies: Spirals to irregulars transition

abstract:
  We consider the roles of two widely invoked mechanisms for the
metallicity-luminosity correlation among late-type galaxies: higher astration
level and decreasing efficiency of heavy-element loss with increasing
luminosity. We find that both mechanisms contribute about equally to the range
in oxygen abundance between low (logL_B = 8) and high (logL_B = 10.5)
luminosity galaxies.
  We also consider the transition from spirals to irregulars, finding that both
the oxygen abundance deficiency (indicating the degree of mass exchange between
a galaxy and its environment) and the gas fraction vary smoothly along the
sequence. Thus we find no "irregular versus spiral dichotomy".


title:
Electron densities, temperatures and ionization rates in two
  interstellar clouds in front of beta Canis Majoris, as revealed by UV
  absorption lines observed with IMAPS

abstract:
  The spectrum of beta CMa (Mirzam) between 1000 and 1200 A was recorded at a
wavelength resolving power ~ 60 000 by the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) during its orbital flight on the ORFEUS-SPAS II
mission in 1996. New information about interstellar absorption lines of CII,
CII*, NI and OI from this spectrum are combined with the HST results reported
by Dupin & Gry (1998) to arrive at new conclusions about the physical
properties of the absorbing gas in front of beta CMa. For two prominent
velocity components centered at heliocentric velocities of +20.0 and +30.5
km/s, designated by Dupin & Gry as Components C and D respectively, we use
determinations of N(CII*)/N(CII) and N(MgI)/N(MgII) to solve for temperatures
and electron densities. From our knowledge that oxygen and nitrogen have their
ionizations coupled to that of hydrogen through charge exchange reactions, we
can derive the hydrogen ionizations by comparing these elements to sulfur,
which is likely not to be depleted onto dust grains. For Component C with an
approximate column density of neutral and ionized hydrogen N(Htotal)=6 10^{18}
cm^-2, we find that the neutral fraction n(HI)/n(Htotal)=0.25, 400<T<6500 K,
and 0.08<n(e)<0.6 cm^-3, while for Component D with N(Htotal)=1.2 10^{19}cm^-2,
we arrive at n(HI)/n(Htotal)=0.035, 8000<T<14000 K, and 0.09<n(e)<0.2cm^-3. The
relatively large ionization fractions of H can arise if the clouds are about
130 pc away from us, so that they are exposed to the strong, ionizing radiation
fields from epsilon and beta CMa. The presence of SiIII indicates the existence
of additional gas with even higher levels of ionization.