 near-IR, specifically in the F110W (∼J) and F160W (∼H) filters. For example, a 3 Gyr old 10 M Jup planet can be observed out to 20 pc with an HST observation of ∼1200s.
Searching a subset of white dwarfs that harbor markers for substellar objects can maximize the return of a survey.
ISM accretion has a wealth of problems in predicting many aspects of DAZs such as the large accretion rates required for some objects and the distribution of these objects with respect to known clouds of dense material (Aannestad et al. 1993;Zuckerman & Reid 1998;Zuckerman et al. 2003). The quick atmospheric settling times of hydrogen atmospheres imply that the white dwarfs are in close proximity with accretionary material.
Of the ∼34 DAZs known, seven of them have dM companions, supporting the argument that DAZs could have unseen companions that place material onto the WD surface through winds (Zuckerman et al. 2003). In order to accrete enough material, companions must be in extremely close orbits, bringing into question why these objects have yet to be discovered through transits, radial velocity surveys of compact objects, or observable excesses in near-IR flux. In most cases the reflex motion from such objects would be easily detectable (Zuckerman & Becklin 1992). The idea of the presence of unseen companions also cannot explain objects like WD 2326+049 (G 29-38) which has an infrared excess due to a dust disk at roughly the tidal disruption radius (Graham et al. 1990;Patterson et al. 1991).
The invocation of cometary or asteroidal material as a method of polluting WD atmospheres was developed to explain photospheric absorption lines due to metals in the DAZ WD 0208+395 (G 74-7) (Alcock et al. 1986). However, the rates predicted by these original studies could not satisfactorily explain the highest accretion rates inferred for some objects and could not easily reproduce the distribution of DAZs based on their effective temperatures (Zuckerman et al. 2003). However, mixing length theory predicts a drop-off of observability for accretion as a function of effective temperature which may swamp out the earlier prediction of Alcock et al. (1986) (Althaus & Benvenuto 1998). Also unclear is the effect non-axisymmetric mass loss could have on the fraction of comet clouds lost by their hosts during post main sequence evolution (Parriott & Alcock 1998). By hypothesis, cometary clouds are the result of planet formation, so the long term evolution of planetary systems and their interaction with these comet clouds needs to be investigated (Tremaine 1993).
The problems of the Alcock et al. (1986) model can be overcome by studying the stability of planetary systems during evolution of the central star as it undergoes mass loss, leaving the main sequence and evolving into a white dwarf. Most planetary systems are stable on timescales comparable to their current age. During adiabatic mass loss, companions expand their orbits in a homologous way, increasing their orbital semi-major axes by a factor M i /M f (Jeans 1924). This change in the central stellar mass affects the dynamics of the planetary system.
The change in stellar mass specifically affects the stability planetary systems, typified by the Hill stability criterion against close approaches for two comparable mass planets. The stability criterion is roughly described as ∆ c = (a 1 − a 2 )/a 1 = 3µ 1/3, where a is the semi-major axis, µ is the mass ratio of the planets to the host star, and ∆ c represents the critical separation at which the two planets become unstable to close approaches (Hill 1886;Gladman 1993). The critical separation grows as the relative separation of the two planets stays the same, resulting in marginally stable systems being tipped over the edge of stability. This instability can lead to orbital rearrangements, the ejection of one planet, and collisions (Ford et al. 2001). These three events dramatically change the dynamical state of the planetary system, leading to a fraction of systems that perturb the surviving comet cloud and sending a shower of comets into the inner system where they tidally disrupt, cause dust disks, and slowly settle onto the WD surface. This modification of the comet impact model can explain the accretion rates needed for the highest abundances of Ca observed and the presence of infrared excesses around WDs (Debes & Sigurdsson 2002).
For two of the three above explanations, unseen planetary or substellar objects lurk in the glare of nearby white dwarfs with metal lines in their atmospheres. DAZs represent a promising population for a search for cool objects in orbit around WDs. If such companions can be detected, this will open an exciting chapter in the study of extra-solar planets by presenting several objects that can be directly detected and characterized, constraining a host of theoretical issues, such as extra-solar planetary atmospheres and the long term evolution of Jovian planets. Such observations in the stellar graveyard can support future missions dedicated to the detection and characterization of terrestrial planets. White dwarfs represent an intermediate step between our current technology and what is needed for observations made with the James Webb Space Telescope (JWST) and the Terrestrial Planet Finder (TPF). Coupled with the possible marker of metal absorption, a sample of nearby stars easier to study than main sequence stars guaranteed to have some sort of planetary system could enhance the efficiency of such long term searches and may provide extra clues to the nature of planet formation.
To that end, we were motivated to search the seven brightest and closest DAZ white dwarfs with the NIC-2 coronograph on the NICMOS instrument of the Hubble Space Telescope (HST). This search was part of the the Cycle 12 program 9834, completed over the course of 2003 and 2004 with 14 orbits. The first results from this survey focused on WD 2326+049 (G 29-38), a DAZ with an infared excess (Debes et al. 2005b, hereafter DSW05). We present the observations we made in Section 2 and detail our data analysis in Section 3. We present candidate planetary and brown dwarf companions in Section 4 as well as place limits on the types of candidates we could have detected in Section 5. Finally, we discuss the implications of our work and lay out future possibilities in Section 6.

OBSERVATIONS
Only ∼34 DAZs are currently known to exist, since the detection of their weak metal lines are difficult without a high signal-to-noise, high resolution spectrograph (Zuckerman et al. 2003). Six of the most promising DAZ white dwarfs discovered or confirmed in the Zuckerman et al. (2003) survey were targeted for observation with NICMOS and are listed in Table 1. Our seventh target, WD 1620-391, was chosen for the presence of circumstellar gas absorption features as well as photospheric absorption due to Si and C (Holberg et al. 1995). We chose these targets based on the fact that these were the brightest and closest DAZs known. Each target was observed with the NIC-2 coronagraph in the F110W filter. The most promising targets, WD 2326+049, WD 1337+701, and WD 1620-391 were imaged in the F160W filter as well. With the exception of the newly discovered DAZ GD 362, both WD 2326+049 and WD 1337+701 have the highest [Ca/H] abundances measured (Gianninas et al. 2004). WD 1620-391 was chosen for extra observations due to the presence of its circumstellar material. These three targets were also observed without the coronagraph for shorter exposures in the F110W, F160W, and F205W filters in an attempt to resolve any smaller structure or companions at separations < 0.8 ′′. Acquisition images were used for the other targets. Following the prescription of Fraquelli et al. (2004), two coronagraphic exposures of ∼600 s were taken at two different spacecraft roll angles. Each exposure was separated by a differential roll angle of 10 •. The differential roll angle between images limits the angular separation at which one can detect a point source, requiring at least a two pixel separation between the centroids of the positive and negative conjugates to avoid the self subtraction of any point source companions. This requirement is tempered by the need to spend most of the HST orbit observing the target and not rolling the spacecraft. For our observations, we concentrated on integration time and chose a roll angle of 10 •, leading to an inner radius limit to extreme high contrast imaging with self subtraction of 0.86 ′′. Table 2 shows a log for all of the observations taken along with the total exposure times and the filter used. Each F110W observation was designed to be sensitive enough to detect an object with m F 110W ∼23 with a S/N of 10, which for a 1 Gyr substellar object at 10 pc would correspond to a ∼5 M Jup planet. For our seven targets, which range in age from 1 Gyr to 7 Gyr and 10 to 34 pc, we are sensitive to 7-18 M Jup objects.
In addition to the seven targets, three reference stars were imaged with the three WDs observed without the coronagraph. The goal was to use these to subtract out the point spread function (PSF) that can obscure fainter objects or dust disks. These targets were chosen to be close to the original target and have similar near-IR colors to aid in PSF subtraction. No close companions or structure were detected using the reference stars.
One group of observations taken of WD 0245+541 failed due to an incorrect calibration onboard the telescope, with the flight software (FSW). As a result, WD 0245+541