constraints on the velocities over which C II $\lambda$6580 is observed to be
present in SN 2011fe. They conclude that C II is present down to at least as
low as 8000 km s-1, which is 2000 km s-1 lower than that estimated by Parrent
et al. (2012), and is also $\sim$ 4000$-$6000 km s-1 (or more) lower than what
is predicted by some past and presently favored SN Ia abundance models (e.g.,
W7; Nomoto et al. 1984, and the delayed detonations of Höflich 2006 and Röpke
et al. 2012).

Hsiao et al. (2013) presented and discussed NIR time series spectra of SN
2011fe that span between day $-$15 and day $+$17\. In particular, they report
a detection of C I $\lambda$10693 on the blue-most side of a blended Mg II
$\lambda$10927 absorption feature at roughly the same velocities and epochs as
C II $\lambda$6580 found by Parrent et al. (2011) and Pereira et al. (2013),
which itself is blended on its _blue-most_ side with Si II $\lambda$6355\.
While searches and studies of C I $\lambda$10693 are extremely useful for
understanding the significance of C-rich material from normal to cooler sub-
luminous SN Ia within the greater context of all C I, C II, C III, O I
absorption features (C III for “hotter” SN 1991T-likes), blended C I
$\lambda$10693 absorption shoulders are certainly no more (nor no less) useful
for probing lower velocity boundaries than C II $\lambda$6580 absorption
notches. This is especially true given that C I $\lambda$10693 absorption
features are blended from the _red-most_ side (lower velocities) by the
neighboring Mg II line, which will only serve to obscure the lower velocity
information of the C I profile for the non-extreme cases (e.g., SN 1999by,
Höflich et al. 2002).

Hsiao et al. (2013) used the observed temporal behavior, and later velocity-
plateau, of Mg II $\lambda$10927 to estimate a lower extent of $\sim$ 11,200
km s-1 for carbon-burning products within SN 2011fe. Given that this in
contrast to the refined lower extent of C II at $\sim$ 8000 km s-1 by Pereira
et al. (2013), this _could_ imply (i.e. assuming negligible temperature
differences and/or non-LTE effects) that either some unburned material has
been churned below the boundary of carbon-burning products via turbulent
instabilities (Gamezo et al., 1999, 1999, 2004) and/or the distribution of
emitting and absorbing carbon-rich material is truly globally lopsided (Kasen
et al., 2009; Maeda et al., 2010b; Blondin et al., 2011), and may indicate the
remains of a degenerate secondary star (Moll et al., 2013).

Detailed studies of this nearby, normal, and unreddenned SN 2011fe have given
strong _support_ for double-degenerate scenarios (assuming low environmental
abundances of hydrogen) and have placed strong _constraint_ on single-
degenerate scenarios, i.e. MS and RG companion stars have been strongly
constrained for SN 2011fe (see Shappee et al. 2013 and references therein, and
also Hayden et al. 2010a; Bianco et al. 2011). Nugent et al. (2011), Li et al.
(2011b) and Bloom et al. (2012) confirm that the primary star was a compact
star (R∗ $\lesssim$ 0.1 R⊙, c.f., Bloom et al. 2012; Piro & Nakar 2013;
Chomiuk 2013). From the lack of evidence for an early shock outbreak (Kasen,
2010; Nakar & Sari, 2012), non-detections of radio and X-ray emissions (Horesh
et al., 2012; Chomiuk et al., 2012; Margutti et al., 2012), non-detections of
narrow Ca II H&K or Na D lines or pre-existing dust that could be associated
with the event (Patat et al., 2013; Johansson et al., 2013), and low upper-
limits on hydrogen-rich gas (Lundqvist et al., 2013), the paucity of evidence
for an environment dusted in CSM from a non-degenerate secondary strongly
supports the double degenerate scenario for SN 2011fe. Plus, this inferred
ambient environment is consistent with that of recent merger simulations (Dan
et al., 2012), and could signal an avenue of interpretation for signatures of
carbon-rich material as well (Branch et al., 2005; Dan et al., 2013; Moll et
al., 2013). Specifically, the remaining amount of carbon-rich material
predicted by some explosion models may already be accounted for, and more so
than would be required by the existence of low velocity detections of C I and
C II. If this turns out to be the case, spectroscopic signatures of both C and
O could tap into understanding (i) the sizes of merger C$+$O common envelopes,
(ii) potential downward mixing effects between the envelope and the underlying
ejecta, and/or (iii) test theories on possible asymmetries of C$+$O material
within the post-explosion ejecta of the primary and secondary stars (Livio &
Pringle, 2011), which is expected to depend on the degree of coalescence (Moll
et al., 2013; Raskin & Kasen, 2013).

Of course, this all rests on the assumptions that (i) the surrounding
environment of a single degenerate scenario just prior to the explosion ought
to be contaminated with some amount of CSM, above which it would be detected
(Justham, 2011; Brown et al., 2012), and (ii) the surrounding environment of a
merger remains relatively “clean” (Shen et al., 2013; Raskin & Kasen, 2013).
In this instance, and assuming similarly above that current DDT-like models
roughly fit the outcome of the explosion, absorption signatures of C ($+$ HV O
I) may point to super-massive single-degenerate progenitors with variable
enclosed envelopes and/or disks of material (e.g., Yoon & Langer 2004, 2005;
Hachisu et al. 2012; Scalzo et al. 2012; Tornambé & Piersanti 2013; Dan et al.
2013) or sub-Chandrasekhar mass “peri-mergers” for resolve (see Moll et al.
2013 and references therein).

###  4.2 Other Early Discoveries

####  4.2.1 SN 2009ig in NGC 1015

Foley et al. (2012c) obtained well-sampled, early UV and optical spectra of SN
2009ig as it was discovered 17 hr after the event (Kleiser et al., 2009;
Navasardyan et al., 2009). SN 2009ig is found to be a normal SN Ia, rising to
_B_ -band maximum in $\sim$ 17.3 days. From the earliest spectra, Foley et al.
(2012c) find Si II $\lambda$6355 line velocities around 23,000 km s-1, which
is exceptionally high for such a spectroscopically normal SN Ia (see also
Blondin et al. 2012). SN 2009ig possess either an overall shallower density
profile than other CN SN Ia, or a buildup of IMEs is present at high
velocities.

Marion et al. (2013) recently analyzed the photospheric to post-maximum light
phase spectra of SN 2009ig, arguing for the presence of additional high-
velocity absorption signatures from not only Si II, Ca II, but also Si III, S
II and Fe II. Whether or not two separate but compositionally equal regions of
line formation is a ubiquitous property of similar SN Ia remains to be seen.
However, it should not be unlikely for primordial amounts of said atomic
species to be present (in addition to singly-ionized silicon and calcium) on
account of possible density and/or abundance enhancements within the outermost
layers (Thomas et al., 2004; Mazzali et al., 2005b, a). For example, simmering
effects during convective phases prior to the explosion may be responsible for
dredging up IMEs later seen as HVFs, which would give favorability to single-
degenerate progenitor scenarios (see Piro 2011; Zingale et al. 2011).
Similarly, it is worthwhile to access the versatility of mergers in producing
high-velocity features.

####  4.2.2 SN 2012fr in NGC 1365

Childress et al. (2013c) report on their time series spectroscopic
observations of SN 2012fr (Klotz et al., 2012; Childress et al., 2012; Buil,
2012), complete with 65 spectra that cover between $\sim$ 15 days before and
40 days after it reached a peak _B_ -band brightness of $-$19.3. In addition
to the simultaneous spectropolarimetric observations of Maund et al. (2013),
the early to maximum light phase spectra of SN 2012fr reveal one of the
clearest indications that SN Ia of similar type (e.g., SN 1994D, 2001el,
2009ig, 2011fe, and many others; Mazzali05a) tend to have two distinctly
separate regions of Si-, Ca-based material that differ by a range of
separation velocities (Childress et al., 2013b).

Childress et al. (2013c) and Maund et al. (2013) discussed the various
interpretations that have been presented in the past, however no firm
conclusions on the origin of HVFs could be realized given the uncertainties of
current explosion models. Despite this, the most recent advance toward
understanding HVFs is the continual detection of polarization signatures due
to the high-velocity Si II and Ca II absorption features, indicating a
departure from a radially stratified, spherically symmetric geometry at some
layer near or above the “photospheric region” of IMEs.

###  4.3 Super-Chandrasekhar Candidate SN Ia

####  4.3.1 Over-luminous SN 2003fg (SNLS-03D3bb)

SN 2003fg was discovered as part of the Supernova Legacy Survey (SNLS); z =
0.2440 (Howell et al., 2006). Its peak absolute magnitude was estimated to be
$-$19.94 in _V_ -band, placing SN 2003fg completely outside the
M${}_{\emph{V}}$-distribution of normal low-z SN Ia (2.2 times brighter).
Assuming Arnett’s rule, such a high luminosity corresponds to $\sim$ 1.3 M⊙ of
56Ni, which would be in conflict with SN 2003fg’s spectra since only $\sim$
60% of a Chandrasekhar pure detonation ends up as radioactive nickel
(Steinmetz et al. 1992, however see also Pfannes et al. 2010). Given also the
lower mean expansion velocities, this builds upon the picture of a super-
Chandrasekhar mass progenitor for SN 2003fg and others like it (Howell et al.,
2006; Jeffery et al., 2006).

Yoon & Langer (2005) proposed the formation of super-Chandrasekhar mass WD
stars as a result of rapid rotation. Pfannes et al. (2010) later reworked
these models and found that the “prompt” detonation of a super-Chandrasekhar
mass WD produces enough nickel, as well as a remainder of IMEs in the outer
layers (in contrast to Steinmetz et al. 1992), to explain over-luminous SN Ia.

Hachisu et al. (2012) added to this model by taking into account processes of
binary evolution. Namely, with the inclusion of mass-striping, optically thick
winds of a differentially rotating primary star, Hachisu et al. (2012) find
three critical mass ranges that are each separated according to the spin-down
time of the accreting WD. All three of these single-degenerate scenarios may
explain a majority of events from sub-luminous to over-luminous SN Ia. So far
no super-Chandrasekhar mass WD stars that would result in a SN Ia have been
found in the sample of known WD stars in our Galaxy (Saffer et al. 1998, see
also Kilic et al. 2012). However, this does not so much rule out super-
Chandrasekhar mass models as it suggests that these systems are rare in the
immediate vicinity within our own galaxy.

Hillebrandt et al. (2007) proposed an alternative scenario involving only a
Chandrasekhar-mass WD progenitor to explain the SN 2003fg event. They
demonstrated that an off-center explosion of a Chandrasekhar-mass WD could
explain the super-bright SN Ia. However, in this off-center explosion model it
is not easy to account for the high Ni mass in the outer layers, in addition
to the special viewing direction.

####  4.3.2 Over-luminous SN 2009dc in UGC 10064

Yamanaka et al. (2009a) presented early phase optical and NIR observations for
SN 2009dc (Puckett et al., 2009; Harutyunyan et al., 2009; Marion et al.,
2009b; Nicolas & Prosperi, 2009). From the peak _V_ -band absolute magnitude
they conclude that SN 2009dc belongs to the most luminous class of SN Ia
($\Delta$m15(_B_) = 0.65), and estimate the 56Ni mass to be 1.2 to 1.6 M⊙.
Based on the JHK photometry Yamanaka et al. (2009a) also find SN 2009dc had an
unusually high NIR luminosity with enhanced fading after $\sim$ day $+$200
(Maeda et al., 2009; Silverman et al., 2011; Taubenberger et al., 2011). The
spectra of SN 2009dc also show strong, long lasting 6300 Å absorption features
(until $\sim$ two weeks post-maximum light) Based on the observed
spectropolarimetric indicators, in combination with photometric and
spectroscopic properties, Tanaka et al. (2010) similarly conclude that the
progenitor mass of SN 2009dc was of super-Chandrasekhar origin and that the
explosion geometry was globally spherically symmetric, with a clumpy
distribution of IMEs.

Silverman et al. (2011) presented an analysis of 14 months of observations of
SN 2009dc and estimate a rise-time of $\sim$ 23 days and $\Delta$m15(_B_) =
0.72. They find a lower limit of the peak bolometric luminosity $\sim$ 2.4 x
1043 erg s-1 and caution that the actual value is likely almost 40% larger.
Based on the high luminosity and low mean expansion velocities of SN 2009dc,
Silverman et al. (2011) derive a mass of more than 2M⊙ for the white dwarf
progenitor and a 56Ni mass of $\sim$ 1.4 to 1.7 M⊙. Taubenberger et al. (2011)
find the minimum 56Ni mass to be 1.8 M⊙, assuming the smallest possible rise-
time of 22 days, and the ejecta mass to be 2.8 M⊙.

Taubenberger et al. (2013) compared photometric and spectroscopic observations
of normal and SCC SN Ia at late epochs, including SN 2009dc, and find a large
diversity of properties spanning through normal, SS, and SCC SN Ia. In
particular the decline in the light curve “radioactive tail” for SCC SN Ia is
larger than normal, along with weaker than normal [Fe III] emission in the
nebular phase spectra. Taubenberger et al. (2013) argue that the weak [Fe III]
emission is indicative of an ejecta environment with higher than normal
densities. Previously, Hachinger et al. (2012) carried out spectroscopic
modeling for SN 2009dc and discussed the model alternatives, such as a 2 M⊙
rotating WD, a core-collapse SN, and a CSM interaction scenario. Overall,
Hachinger et al. (2012) found the interaction scenario to be the most
promising in that it does not require the progenitor to be super-massive.
Taubenberger et al. (2013) furthered this discussion in conjunction with their
late time comparisons and conclude that the models of Hachinger et al. (2012)
do not simultaneously match the peak brightness and decline of SN 2009dc (see
also Yamanaka et al. 2013). Following the interaction scenario of Hachinger et
al. (2012), Taubenberger et al. (2013) propose a non-violent merger model that
produces $\sim$1M⊙ of 56Ni and is enshrouded by $\sim$0.6$-$0.7M⊙ of
C$+$O-rich material. In order to reconcile the low 56Ni production,
Taubenberger et al. (2013) note that additional luminosity from interaction
with CSM is required during the first two months post-explosion. Further
support for CSM interaction comes from the observed suppression of the double
peak in the _I_ -band, which is thought to arise from a breaking of ejecta
stratification in the outermost layers (Kasen, 2006; Kamiya et al., 2012).

It is not yet clear if SN 2003fg, 2006gz (A.32), 2007if (A.34), and SN 2009dc
are the result of a single super-Chandrasekhar mass WD star, given that even
in our galaxy there is no observational evidence for the existence of such a
system. Likewise, there is no direct observational evidence for the presence
of very rapidly rotating massive WD stars, either single WDs or in binary
systems as well. In fact, no double-degenerate close binary systems with a
total mass amounting to super-Chandrasekhar mass configurations that can merge
in Hubble-time have been found (Parthasarathy et al., 2007). Therefore, our
current understanding of the origin of over-luminous SN Ia is limited, and
more observations are needed. For example, progress has been made with the
recent discovery of 24 merging WD systems via the extremely low mass Survey
(see Kilic et al. 2012 and references therein), however it is unclear if any
are systems that would produce a normal SN Ia.

###  4.4 The Peculiar SN 2002cx-like Class of SN

####  4.4.1 SN 2002cx in CGCG-044-035

Li et al. (2003) considered SN 2002cx as “the most peculiar known SN Ia”
(Wood-Vasey et al., 2002b). They obtained photometric and spectroscopic
observations which revealed it to be unique among all observed SN Ia. Li et
al. (2003) described SN 2002cx as having SN 1991T-like pre-maximum spectrum, a
SN 1991bg-like luminosity, and expansion velocities roughly half those of
normal SN Ia.

Photometrically, SN 2002cx has a broad peak in the _R_ -band, a plateau phase
in the _I_ -band, and a slow late time decline. The _B_ $-$ _V_ color
evolution are described as nearly normal, while the _V_ $-$ _R_ and _V_ $-$
_I_ colors are redder than normal. Spectra of SN 2002cx during early phases
evolve rapidly and are dominated by lines from IMEs and IPEs, but the features
are weak overall. In addition, emission lines are present around 7000 Å during
post-maximum light phases, while the late time nebular spectrum shows narrow
lines of iron and cobalt.

Jha et al. (2006a) presented late time spectroscopy of SN 2002cx, which
includes spectra at 227 and 277 days post-maximum light. They considered it as
a prototype of a new subclass of SN Ia. The spectra do _not_ appear to be
dominated by the forbidden emission lines of iron, which is not expected
during the “nebular phase,” where instead they find a number of P Cygni
profiles of Fe II at exceptionally low expansion velocities of $\sim$ 700 km
s-1 (Branch et al., 2004a). A tentative identification of O I $\lambda$7774 is
also reported for SN 2002cx, suggesting the presence of oxygen-rich material.
Currently, it is difficult to explain all the observed photometric and
spectroscopic properties of SN 2002cx using the standard SN Ia models (see
Foley et al. 2013). However, the spectral characteristics of SN 2002cx support
pure deflagration or failed-detonation models that leave behind a bound
remnant instead of delayed detonations (Jordan et al., 2012; Kromer et al.,
2013a; Hillebrandt et al., 2013).

####  4.4.2 SN 2005hk in UGC 00272

Phillips et al. (2007) presented extensive multi-color photometry and optical
spectroscopy of SN 2005hk (Quimby et al., 2005). Sahu et al. (2008) also
studied the spectrophotometric evolution SN 2005hk, covering pre-maximum phase
to around 400 days after the event. These datasets reveal that SN 2005hk is
_nearly_ identical in its observed properties to SN 2002cx. Both supernovae
exhibited high ionization SN 1991T-like pre-maximum light spectra but with low
peak luminosities like that of SN 1991bg. The spectra reveal that SN 2005hk,
like SN 2002cx, has expansion velocities that are roughly half those of
typical SN Ia.

The _R_ and _I_ -band light curves of both supernovae are also peculiar for
not displaying the secondary maximum observed for normal SN Ia. Phillips et
al. (2007) constructed a bolometric light curve from 15 days before to 60 days
after _B_ -band maximum. They conclude that the shape and exceptionally low
peak luminosity of the bolometric light curve, low expansion velocities, and
absence of a secondary maximum in the NIR light curves are in reasonable
agreement with model calculations of a three-dimensional deflagration that
produces 0.25 M⊙ of 56Ni. Note however that the low amount of continuum
polarization observed for SN 2005hk ($\sim$ 0.2%$-$0.4%) is far too similar to
that of more normal SN Ia to serve as an explanation for the spectroscopic
peculiarity of SN 2005hk, and possibly other SN 2002cx-like events (Chornock
et al., 2006; Maund et al., 2010a).

####  4.4.3 Sub-luminous SN 2007qd

McClelland et al. (2010) obtained multi-band photometry and multi-epoch
spectroscopy of SN 2007qd (Bassett et al., 2007). Its observed properties
place it broadly between those of the peculiar SN 2002cx and SN 2008ha (A.37).
Optical photometry indicate a fast rise-time and a peak absolute _B_ -band
magnitude of $-$15.4. McClelland et al. (2010) carried out spectroscopy of SN
2007qd near maximum brightness and detect signatures of IMEs. They find the
photospheric velocity to be 2800 km s-1 near maximum light, and note that this
is $\sim$ 4000 and 7000 km s-1 less than that inferred for SN 2002cx and
normal SN Ia, respectively. McClelland et al. (2010) find that the peak
luminosities of SN 2002cx-like objects are well correlated with their light
curve stretch and photospheric velocities.

####  4.4.4 SN 2009ku

SN 2009ku was discovered by Pan-STARS-1 as a SN Ia belonging to the peculiar
SN 2002cx class. Narayan et al. (2011) studied SN 2009ku and find that while
its multi-band light curves are similar to that of SN 2002cx, they are
slightly broader and have a later rise to _g_ -band maximum. Its peak
brightness was found to be M${}_{\emph{V}}$ = $-$18.4 and the ejecta velocity
at 18 days after maximum brightness was found to be $\sim$ 2000 km s-1.
Spectroscopically, SN 2009ku is similar to SN 2008ha (A.37). Narayan et al.
(2011) note that the high luminosity and low ejecta velocity for SN 2009ku is
not in agreement with the trend seen for SN 2002cx class of SN Ia. The
spectroscopic and photometric characteristics of SN 2009ku indicate that the
SN 2002cx class of SN Ia are not homogeneous, and that the SN 2002cx class of
events may have a significant dispersion in their progenitor population and/or
explosion physics (see also Kasliwal et al. 2012 for differences between this
class and sub-luminous “calcium-rich” transients).

###  4.5 PTF11kx and the “Ia-CSM” Class of SN Ia

####  4.5.1 PTF11kx: A case for single-degenerate scenarios?

Dilday et al. (2012) studied the photometric and spectroscopic properties of
another unique SN Ia event, PTF11kx. Using time series, high-resolution
optical spectra, they find direct evidence supporting a single-degenerate
progenitor system based on several narrow, temporal ($\sim$ 65 km s-1)
spectroscopic features of the hydrogen Balmer series, He I, Na I, Ti II, and
Fe II. In addition, and for the first time, PTF11kx observations reveal
strong, narrow, highly time-dependent Ca II absorption features that change
from saturated absorption signatures to emission lines within $\sim$ 40 days.

Dilday et al. (2012) considered the details of these observations and
concluded that the complex CSM environment that enshrouds PTF11kx is strongly
indicative of mass loss or “outflows,” prior to the onset of the explosion of
the progenitor system. Other SN Ia have exhibited narrow, temporal Na D lines
before (e.g., SN 2006X, 2007le; see Simon et al. 2009; Patat et al. 2009,
2010, 2011; Sternberg et al. 2011), but none have been reported as having
signatures of these particular ions, which are clearly present in the high-
resolution spectra of PTF11kx. On the whole, and during the earliest epochs,
Dilday et al. (2012) find that the underlying SN Ia spectroscopic component of
PTF11kx most closely resembles that of SN 1991T (Filippenko et al., 1992a;
Gómez & López, 1998) and 1999aa (Garavini et al., 2004).

As for the late time phases, Silverman et al. (2013b) studied spectroscopic
observations of PTF11kx from 124 to 680 days post-maximum light and find that
its nebular phase spectra are markedly different from those of normal SN Ia.
Specifically, the late time spectra of PTF11kx are void of the strong cobalt
and iron emission features typically seen in other SN 1991T/1999aa-like and
normal SN Ia events (e.g., Ruiz-Lapuente & Lucy 1992; Salvo et al. 2001;
Branch et al. 2003; Stehle et al. 2005; Kotak et al. 2005; McClelland et al.
2013; Silverman et al. 2013a). For the most part, the late time spectra of
PTF11kx are seen to be dominated by broad (FWHM $\sim$ 2000 km s-1) H$\alpha$
emission and strong Ca II emission features that are superimposed onto a
relatively blue, overly luminous continuum level that may be serving to wash
out the underlying SN Ia spectroscopic information. Silverman et al. (2013b)
note that the H$\alpha$ emission increases in strength for $\sim$1 yr before
decreasing. In addition, from the absence of strong H$\beta$, He I, and O I
emission, as well as a larger than normal late time luminosity, Silverman et
al. (2013b) conclude that PTF11kx indeed interacted with some form of CSM
material; possibly of multiply thin shells, shocked into radiative modes of
collisional excitation as the SN ejecta overtakes the slower-moving CSM.
However, it should be noted that it is not yet clear if the CSM originates
from a single-degenerate scenario or a H-rich layer of material that is
ejected prior to a double-degenerate merger event (Shen et al. 2013, see also
Soker et al. 2013).

####  4.5.2 SN 2002ic

Hamuy et al. (2003) detected a large H$\alpha$ emission in the spectra of SN
2002ic (Wood-Vasey et al., 2002a). Seven days before to 48 days after maximum
light, the optical spectra of SN 2002ic exhibit normal SN Ia spectral features
in addition to the strong H$\alpha$ emission. The H$\alpha$ emission line in
the spectrum of SN 2002ic consists of a narrow component atop a broad
component (FWHM of about 1800 km s-1). Hamuy et al. (2003) argue that the
broad component arose from ejecta$-$CSM interaction. By day $+$48, they find
that the spectrum is similar to that of SN 1990N. Hamuy et al. (2003) argue
that the progenitor system contained a massive AGB star, associated with a few
solar masses of hydrogen-rich CSM.

Kotak et al. (2004) obtained the first high resolution, high S/N spectrum of
SN 2002ic. The resolved H$\alpha$ line has a P Cygni-type profile, indicating
the presence of a dense, slow-moving outflow (about 100 km s-1). They also
find a relatively large and unusual NIR excess and argue that this is the
result of an infrared light-echo originating from the presence of CSM. They
estimate the mass of CSM to be more than 0.3 M⊙, produced by a progenitor mass
loss rate greater than 10-4 M⊙ yr-1. For the progenitor, Kotak et al. (2004)
favor a single-degenerate system with a post-AGB companion star.

Wood-Vasey et al. (2004) obtained pre-maximum and late time photometry of SN
2002ic and find that a non-SN Ia component of the light curve becomes
pronounced about 20 days post-explosion. They suggest the non-SN Ia component
to be due to heating from a shock interaction between SN ejecta and CSM. Wood-
Vasey et al. (2004) also suggest that the progenitor system consisted of a WD
and an AGB star in the protoplanetary nebula phase. Wood-Vasey et al. (2004)
and Sokoloski et al. (2006) proposed that a nova shell ejected from a
recurrent nova progenitor system, creating the evacuated region around the
explosion center of SN 2002ic. They suggest that the periodic shell ejections
due to nova explosions on a WD sweep up the slow wind from the binary
companion, creating density variations and instabilities that lead to
structure in the circumstellar medium. This type of phenomenon may occur in SN
Ia with recurrent nova progenitors, however Schaefer (2011) recently reported
on an ongoing observational campaign of recurrent novae (RN) orbital period
changes between eruptions. For at least two objects (CI Aquilae and U
Scorpii), he finds that the RN _lose_ mass, thus making RN unlikely
progenitors for SN Ia.

Nearly one year after the explosion, Wang et al. (2004) found that the
supernova had become fainter overall, but H$\alpha$ emission had brightened
and broadened compared to earlier observations. From their spectropolarimetry
observations, Wang et al. (2004) find that hydrogen-rich matter is
asymmetrically distributed. Likewise, Deng et al. (2004) also found evidence
of a hydrogen-rich asymmetric circumstellar medium. From their observations of
SN 2002ic, Wang et al. (2004) conclude that the event took place within a
“dense, clumpy, disk-like” circumstellar medium. They suggest that the star
responsible for SN 2002ic could either be a post-AGB star or WD companion (see
also Hachisu et al. 1999; Han & Podsiadlowski 2006).

####  4.5.3 SN 2005gj

Similar to SN 2002ic, Aldering et al. (2006) argue that SN 2005gj is a SN Ia
in a massive circumstellar envelope (see also Prieto et al. 2007), which is
located in a low metallicity host galaxy with a significant amount of star
formation. Their first spectrum of SN 2005gj shows a blue continuum level with
broad and narrow H$\alpha$ emission. Subsequent spectra reveal muted SN Ia
features combined with broad and narrow H$\gamma$, H$\beta$, H$\alpha$ and He
I $\lambda\lambda$5876, 7065 in emission, where high resolution spectra reveal
narrow P Cygni profiles. An inverted P Cygni profile for [O III] $\lambda$5007
was also detected, indicating top-lighting effects from CSM interaction
(Branch et al., 2000). From their early photometry of SN 2005gj, Aldering et
al. (2006) find that the interaction between the supernova ejecta and CSM was
much weaker for SN 2002ic. Notably, both Aldering et al. (2006) and Prieto et
al. (2007) agree that a SN 1991T-like spectrum can account for many of the
observed profiles with an assumed increase in continuum radiation from
interaction with the hydrogen-rich material.

Aldering et al. (2006) also find that the light curve and measured velocity of
the unshocked CSM imply mass loss as recent as 1998. This is in contrast to SN
2002ic, for which an inner cavity in the circumstellar matter was inferred
(Wood-Vasey et al., 2004). Furthermore, SN 1997cy, SN 2002ic, and SN 2005gj
all exhibit large CSM interactions and are from low-luminosity hosts.

Consistent with this interpretation for CSM interactions is the recent report
by Fox & Filippenko (2013) that a NIR re-brightening, possibly due to emission
from “warm” dust, took place at late times for both SN 2002ic and 2005gj.
Notably, and in contrast to SN 2002ic, Fox & Filippenko (2013) find that the
mid-IR luminosity of SN 2005gj increased to $\sim$ twice its early epoch
brightness.

##  5 Summary and Concluding Remarks

Observations of a significant number of SN Ia during the last two decades have
enabled us to document a larger expanse of their physical properties which is
manifested through spectrophotometric diversity. While in general SN Ia have
long been considered a homogeneous class, they do exhibit up to 3.5 mag
variations in the peak luminosity, whereas “normal” SN Ia dispersions are
$\sim$1 mag, and constitute several marginally distinct subtypes (Blondin et
al., 2012; Scalzo et al., 2012; Silverman et al., 2013d; Foley et al., 2013;
Dessart et al., 2013a). Consequently, the use of normal SN Ia for cosmological
purposes depends on empirical calibration methods (e.g., Bailey et al. 2009),
where one of the most physically relevant methods is the use of the width-
luminosity relation (Phillips, 1993; Phillips et al., 1999).

Understanding the physics and origin of the width-luminosity-relationship of
SN Ia light curves is an important aspect in the modeling of SN Ia (Khokhlov
et al., 1993; Lentz et al., 2000; Timmes et al., 2003; Nomoto et al., 2003;
Kasen & Woosley, 2007; Kasen et al., 2009; Meng et al., 2011; Blondin et al.,
2011). Brighter SN Ia often have broad light curves that decline slowly after
peak brightness. Slightly less bright or dimmer SN Ia have narrower and
relatively rapidly declining light curves. In addition, several SN Ia do not
follow the width-luminosity-relationship (e.g., SN 2001ay, 2004dt, 2010jn,
SCC, CL and SN 2002cx-like SN Ia), which reinforces the notion that a
significant number of physically relevant factors influence the diversity of
SN Ia overall (see Wang et al. 2012; Baron et al. 2012).

Despite the ever increasing number of caught-early supernovae, our perspective
on their general properties and individual peculiarities undergoes a continual
convergence toward a set of predictive standards with which models must be
seen to comply. The most recent observational example is that of SN 2012fr
(Maund et al., 2013; Childress et al., 2013c), a normal/low-velocity-gradient
SN Ia that has been added to the growing list of similar SN Ia that exhibit
stark evidence for a distinctly separate region of “high-velocity” material
($>$16,000 km s-1). While the origin of high velocity features in the spectra
of SN Ia is not well understood, it is concurrent with polarization signatures
in most cases which implies some amount of ejecta density asymmetries (e.g.,
Kasen et al. 2003; Wang & Wheeler 2008; Smith et al. 2011; Maund et al. 2013).
Furthermore, since understanding the temporal behavior of high velocity Si
II/Ca II depends on knowing the same for the photospheric component, studies
that focus on velocity gradients and potential velocity-plateaus of the
photospheric component could make clearer the significance of the physical
separation between these two regions of material (see Patat et al. 1996; Kasen
et al. 2003; Tanaka et al. 2008; Foley et al. 2012c; Parrent et al. 2012;
Scalzo et al. 2012; Childress et al. 2013c; Marion et al. 2013). However, it
is at least certain that all viable models that encompass “normal” SN Ia
conditions must account for the range of properties related to velocity
evolution (see Blondin et al. 2012 and references therein), the occasionally
observed however potentially under-detected signatures of C$+$O material at
both low and high velocities (Thomas et al., 2007; Parrent et al., 2011;
Thomas et al., 2011b; Folatelli et al., 2012; Silverman & Filippenko, 2012;
Piro & Nakar, 2013; Mazzali et al., 2013), a high-velocity region of either
clumps or an amorphous plumage of opaque Si-, Ca-based material (Gamezo et
al., 2004; Leonard et al., 2005; Wang et al., 2007; Maund et al., 2010b; Piro,
2011), and the supposed blue/red-shift of nebular lines emitted from the inner
IPE-rich material (Maeda et al., 2010b; McClelland et al., 2013; Silverman et
al., 2013a).

For at least normal SN Ia, there remain two viable explosion channels (with a
few sub- and super-MCh sub-channels) regardless of the hierarchical dominance
of each at various redshifts and/or ages of galactic constituents (c.f., Röpke
et al. 2012; Hachisu et al. 2012; Seitenzahl et al. 2013; Pakmor et al. 2013;
Moll et al. 2013; Claeys et al. 2014). Also, it may or may not be the case
that some SN Ia are 2$+$ subtypes viewed upon from various lines of sight
amidst variable CSM interaction (Maeda et al., 2010b; Foley et al., 2012a;
Scalzo et al., 2012; Leloudas et al., 2013; Dan et al., 2013; Moll et al.,
2013; Dessart et al., 2013a). However, with the current lack of complete
observational coverage in wavelength, time, and mode (i.e. spectrophotometric
and spectropolarimetric observations) for all SN Ia subtypes and “well-
observed” events, there is a limit for how much constraint can be placed on
many of the proposed explosion models and progenitor scenarios. That is,
despite observational indications for and theoretical consistencies with the
supposition of multiple progenitor channels, the observed diversity of SN Ia
does not yet necessitate that each spectrophotometric subtype be from a
distinctly separate explosive binary scenario than that of others within the
SN Ia family of observed events; particularly so for normal SN Ia.

For the purposes of testing the multifaceted predictions of theoretical
explosion models, time series spectroscopic observations of SN Ia serve to
visualize the post-explosion material of an unknown progenitor system. For
example during the summer of 2011 astronomers bore witness to SN 2011fe, the
best observed normal type Ia supernova of the modern era. The prompt discovery
and follow-up of this nearby event uniquely allowed for a more complete record
of observed properties than all previous well-observed events. More
specifically, the full range (in wavelength and time) of rapid spectroscopic
changes was documented with continual day-to-day follow-up into the object’s
post-maximum light phases and well beyond. However, the observational side of
visualizing other SN Ia remains inefficient without the logistical
coordination of many telescope networks (e.g., LCOGT; Brown et al. 2013),
telescopes large enough to make nearly all SN “nearby” in terms of improved
signal-to-noise ratios (e.g., The Thirty Meter Telescope, The Giant Magellan
Telescope), or a space-based facility dedicated to the study of such time
sensitive UV$-$optical$-$NIR transients.

Existing SN Ia surveys are currently acting toward optimizing a steady flow of
discoveries, while other programs have produced a significant number of
publicly available spectra (Richardson et al., 2001; Matheson et al., 2008;
Blondin et al., 2012; Yaron & Gal-Yam, 2012; Silverman et al., 2012c;
Folatelli et al., 2013). However, for the longterm future we believe it is
imperative to begin a discussion of a larger (digital) network of
international collaboration by way of (data-) cooperative competition like
that done for both The Large Hadron Collider Experiment and Fermi Lab’s
Tevatron, with multiple competing experiments centered about mutual goals and
mutual resources. Otherwise we feel the simultaneous collection of even very
high quality temporal datasets by multiple groups will continue to create an
inefficient pursuit of over-observing the most high profile event(s) of the
year with a less than complete dataset.

Such observational pursuits require an increasingly focused effort toward
observing bright and nearby events. For example, 206 supernovae were reported
in 1999 and 67 were brighter than 18th magnitude while only three reached
$\sim$ 13 magnitude212121Quoted from the archives page of
http://www.rochesterastronomy.org/snimages/.. By 2012 the number of found
supernovae increased to 1045 while 78 were brighter than 16th magnitude and
five brighter than 13th magnitude. This clearly indicates that supernovae
caught early are more prevalent than $\sim$ 15 years ago and it is worthwhile
for multiple groups to continually increase collaborative efforts for the
brightest events. Essentially this could be accomplished without interfering
with spectrum-limited high[er]-z surveys by considering a distance threshold
($\lesssim$ 10$-$30 Mpc) as part of the public domain. Additionally, surveys
that corroborate the immediate release of discoveries would further increase
the number of well-observed events and could be supplemented and sustained
with staggered observations given that there are two celestial hemispheres,
unpredictable weather patterns, and caught-early opportunities nearly every
week during active surveying.

In conclusion, to extract details of the spectroscopic behavior for all SN Ia
subtypes, during all phases, larger samples of well-observed events are
essential, beginning from as close to the onset of the explosion as possible
(e.g., SN 1999ac, 2009ig, 2011fe, 2012cg, 2012fr), where SN Ia homogeneity
diverges the most (see Zheng et al. 2013 for the most recent instance in SN
2013dy). Near-continuous temporal observations are most important for at least
the first 1$-$2 months post-explosion and biweekly to monthly follow-up
thereafter for $\sim$ 1 year. SN Ia spectra are far too complicated to do so
otherwise. Even normal SN Ia deserve UV$-$optical$-$IR spectroscopic follow-up
at a 1:1 to 2:1 ratio between days passed and spectrum taken, whenever
possible, given that fine differences between normal SN Ia detail the variance
in explosion mechanism parameters and initial conditions of their unobserved
progenitor systems. It is through such observing campaigns that the true
diversity to the underlying nature of SN Ia events will be better understood.

Acknowledgements

This work was supported in part by NSF grant AST-0707704, and US DOE Grant DE-
FG02- 07ER41517, and by SFB 676, GRK 1354 from the DFG. Support for Program
number HST-GO- 12298.05-A was provided by NASA through a grant from the Space
Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Incorporated, under NASA contract
NAS5-26555. We wish to acknowledge the use of the Kurucz & Bell (1995) line
list and colorbrewer2.org for the construction of Figure 3.

This review was made possible by collaborative discussions at the 2011 UC-UC-
HIPACC International AstroComputing Summer School on Computational Explosive
Astrophysics. We would like to thank Dan Kasen and Peter Nugent for organizing
the program and providing the environment for a productive summit, and we hope
that such programs for summer learning opportunities will continue in the
future. We worked on this review during the visits of MP to the Homer L. Dodge
Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA.,
McDonnell Center for the Space Science, Department of Physics, Washington
University in St. Louis, USA., National Astronomical Observatory of Japan
(NAOJ), Mitaka, Tokyo, Japan., and Inter-University Centre for Astronomy and
Astrophysics (IUCAA), Pune, India.

MP is thankful to Prof. David Branch, Prof. Eddie Baron, Prof. Ramanath
Cowsik, Prof. Shoken Miyama, Prof. Masahiko Hayashi, Prof. Yoichi Takeda,
Prof. Wako Aoki, Prof. Ajit Kembhavi, Prof. Kandaswamy Subramanian, and Prof.
T. Padamanabhan for their kind support, encouragement, and hospitality. JTP
would like to thank the University of Oklahoma Supernova Group, Rollin Thomas,
and Alicia Soderberg for several years of support and many enlightening
discussions on reading supernova spectra. JTP wishes to acknowledge helpful
discussions with B. Dilday, R. A. Fesen, R. Foley, M. L. Graham, D. A. Howell,
G. H. Marion, D. Milisavljevic, P. Milne, D. Sand, and S. Valenti, as well as
S. Perlmutter for an intriguing conversation on “characteristic information of
SN Ia” at the 221st American Astronomical Society Meeting in Long Beach, CA.
JTP is also indebted to Natalie Buckley-Medrano for influential comments on
the text and figures presented here.

Finally, we would like to pay special tribute to our referee, Michael
Childress, whose critical comments and suggestions were substantially helpful
for the presentation of this review.

Table 1 : References for Spectra in Figures 5$-$13 SN Name | References
---|---
SN 1981B | Branch et al. 1983
SN 1986G | Cristiani et al. 1992
SN 1989B |  Barbon et al. 1990;
| Wells et al. 1994
SN 1990N |  Mazzali et al. 1993;
| Gómez & López 1998
SN 1991T |  Filippenko et al. 1992a;
| Gómez & López 1998
SN 1991bg |  Filippenko et al. 1992b;
| Turatto et al. 1996
SN 1994D |  Patat et al. 1996;
|  Ruiz-Lapuente 1997;
|  Gómez & López 1998;
| Blondin et al. 2012
SN 1994ae | Blondin et al. 2012
SN 1995D |  Sadakane et al. 1996;
| Blondin et al. 2012
SN 1996X |  Salvo et al. 2001;
| Blondin et al. 2012
SN 1997br |  Li et al. 1999;
| Blondin et al. 2012
SN 1997cn | Turatto et al. 1998
SN 1998aq | Branch et al. 2003
SN 1998bu |  Jha et al. 1999;
| Matheson et al. 2008
SN 1998de |  Modjaz et al. 2001;
| Matheson et al. 2008
SN 1998es | Matheson et al. 2008
SN 1999aa | Garavini et al. 2004
SN 1999ac |  Garavini et al. 2005;
|  Phillips et al. 2006;
| Matheson et al. 2008
SN 1999by |  Garnavich et al. 2004;
| Matheson et al. 2008
SN 1999ee | Hamuy et al. 2002
SN 2000E | Valentini et al. 2003
SN 2000cx | Li et al. 2001
SN 2001V | Matheson et al. 2008
SN 2001ay | Krisciunas et al. 2011
SN 2001el | Wang et al. 2003
SN 2002bo |  Benetti et al. 2004;
| Blondin et al. 2012
SN 2002cx | Li et al. 2003
Table 2 : References for Spectra in Figures 5$-$13 SN Name | References
---|---
SN 2002dj |  Pignata et al. 2008;
| Blondin et al. 2012
SN 2002er | Kotak et al. 2005
SN 2003cg |  Elias-Rosa et al. 2006;
| Blondin et al. 2012
SN 2003du |  Gerardy et al. 2004;
|  Anupama et al. 2005b;
|  Leonard et al. 2005;
|  Stanishev et al. 2007;
| Blondin et al. 2012
SN 2003hv |  Leloudas et al. 2009;
| Blondin et al. 2012
SN 2004S | Krisciunas et al. 2007
SN 2004dt |  Leonard et al. 2005;
|  Altavilla et al. 2007;
| Blondin et al. 2012
SN 2004eo | Pastorello et al. 2007a
SN 2005am | Blondin et al. 2012
SN 2005cf |  Garavini et al. 2007;
|  Wang et al. 2009b;
| Bufano et al. 2009
SN 2005cg | Quimby et al. 2006b
SN 2005hk |  Chornock et al. 2006;
|  Phillips et al. 2007;
| Blondin et al. 2012
SN 2005hj | Quimby et al. 2007
SN 2006D | Blondin et al. 2012
SN 2006X |  Wang et al. 2008a;
|  Yamanaka et al. 2009b;
| Blondin et al. 2012
SN 2006bt | Foley et al. 2010b
SN 2006gz | Hicken et al. 2007
SN 2007ax | Blondin et al. 2012
SN 2007if |  Silverman et al. 2011;
| Blondin et al. 2012
SN 2008J | Taddia et al. 2012
SN 2008ha | Foley et al. 2009
SN 2009dc |  Silverman et al. 2011;
| Taubenberger et al. 2011
PTF09dav | Sullivan et al. 2011b
SN 2011fe | Parrent et al. 2012
SN 2011iv | Foley et al. 2012b

Table 3 : References for M${}_{\emph{B}}$(Peak) and $\Delta$m15(_B_) plotted in Figure 14: 1981$-$1992 SN Name | References
---|---
SN 1981B |  Leibundgut et al. 1993;
|  Saha et al. 1996;
|  Hamuy et al. 1996;
| Saha et al. 2001b
SN 1984A | Barbon et al. 1989
SN 1986G |  Filippenko et al. 1992b;
|  Ruiz-Lapuente & Lucy 1992;
| Leibundgut et al. 1993
SN 1989B |  Barbon et al. 1990;
|  Wells et al. 1994;
|  Richmond et al. 1995;
|  Saha et al. 1999;
|  Contardo et al. 2000;
| Saha et al. 2001a
SN 1990N |  Saha et al. 1997;
|  Lira et al. 1998;
| Saha et al. 2001a
SN 1991T |  Leibundgut et al. 1993;
|  Lira et al. 1998;
|  Phillips et al. 1999;
|  Krisciunas et al. 2004;
|  Contardo et al. 2000;
|  Saha et al. 2001b;
| Tsvetkov et al. 2011
SN 1991bg |  Leibundgut et al. 1993;
|  Turatto et al. 1996;
|  Mazzali et al. 1997;
| Contardo et al. 2000
SN 1992A |  Leibundgut et al. 1993;
|  Hamuy et al. 1996;
|  Drenkhahn & Richtler 1999;
| Contardo et al. 2000
SN 1992K | Hamuy et al. 1994
SN 1992al | Misra et al. 2005
SN 1992bc |  Maza et al. 1994;
| Contardo et al. 2000
SN 1992bo |  Maza et al. 1994;
| Contardo et al. 2000
Table 4 : References for M${}_{\emph{B}}$(Peak) and $\Delta$m15(_B_) plotted in Figure 14: 1994$-$1999 SN Name | References
---|---
SN 1994D |  Hoflich et al. 1995;
|  Richmond et al. 1995;
|  Patat et al. 1996;
|  Vacca & Leibundgut 1996;
|  Drenkhahn & Richtler 1999;
| Contardo et al. 2000
SN 1994ae | Contardo et al. 2000
SN 1995D |  Sadakane et al. 1996;
| Contardo et al. 2000
SN 1996X |  Phillips et al. 1999;
| Salvo et al. 2001
SN 1997br | Li et al. 1999
SN 1997cn | Turatto et al. 1998
SN 1998aq |  Riess et al. 1999;
| Saha et al. 2001a
SN 1998bu |  Jha et al. 1999;
|  Hernandez et al. 2000;
| Saha et al. 2001a
SN 1998de | Modjaz et al. 2001
SN 1998es |  Jha et al. 2006b;
| Tsvetkov et al. 2011
SN 1999aa |  Krisciunas et al. 2000;
|  Li et al. 2003;
| Tsvetkov et al. 2011
SN 1999ac |  Jha et al. 2006b;
| Phillips et al. 2006
SN 1999aw | Strolger et al. 2002
SN 1999by |  Vinkó et al. 2001;
|  Howell et al. 2001;
|  Garnavich et al. 2004;
| Sullivan et al. 2011b
SN 1999ee |  Stritzinger et al. 2002;
| Krisciunas et al. 2004

Table 5 : References for M${}_{\emph{B}}$(Peak) and $\Delta$m15(_B_) plotted in Figure 14: 2000$-$2005 SN Name | References
---|---
SN 2000E | Valentini et al. 2003
SN 2000cx |  Li et al. 2001;
|  Candia et al. 2003;
| Sollerman et al. 2004
SN 2001V | Vinkó et al. 2003
SN 2001ay | Krisciunas et al. 2011
SN 2001el | Krisciunas et al. 2003
SN 2002bo |  Benetti et al. 2004;
| Stehle et al. 2005
SN 2002cv | Elias-Rosa et al. 2008
SN 2002cx | Li et al. 2003
SN 2002dj | Pignata et al. 2008
SN 2002er | Pignata et al. 2004
SN 2003cg | Elias-Rosa et al. 2006
SN 2003du |  Anupama et al. 2005b;
|  Stanishev et al. 2007;
| Tsvetkov et al. 2011
SN 2003fg |  Howell et al. 2006;
| Yamanaka et al. 2009b
| Scalzo et al. 2010
SN 2003hv | Leloudas et al. 2009
SN 2004S |  Misra et al. 2005;
| Krisciunas et al. 2007
SN 2004dt | Altavilla et al. 2007
SN 2004eo | Pastorello et al. 2007a
SN 2005am | Brown et al. 2005
SN 2005bl |  Taubenberger et al. 2008;
| Hachinger et al. 2009
SN 2005cf |  Pastorello et al. 2007b;
| Wang et al. 2009b
SN 2005hk | Phillips et al. 2007
Table 6 : References for M${}_{\emph{B}}$(Peak) and $\Delta$m15(_B_) plotted in Figure 14: 2006$-$2012 SN Name | References
---|---
SN 2006X | Wang et al. 2008b
SN 2006bt |  Hicken et al. 2009b;
| Foley et al. 2010b
SN 2006gz |  Hicken et al. 2007;
| Scalzo et al. 2010
SN 2007ax | Kasliwal et al. 2008
SN 2007if | Scalzo et al. 2010
SN 2007qd | McClelland et al. 2010
SN 2008J | Taddia et al. 2012
SN 2008ha | Foley et al. 2009
SN 2009dc |  Yamanaka et al. 2009b;
|  Scalzo et al. 2010;
|  Silverman et al. 2011;
| Taubenberger et al. 2011
SN 2009ig | Foley et al. 2012c
SN 2009ku | Narayan et al. 2011
SN 2009nr |  Khan et al. 2011b;
| Tsvetkov et al. 2011
PTF09dav | Sullivan et al. 2011b
SN 2010jn | Hachinger et al. 2013
SN 2011fe |  Richmond & Smith 2012;
|  Munari et al. 2013;
| Pereira et al. 2013
SN 20011iv | Foley et al. 2012b
SN 2012cg |  Silverman & Filippenko 2012;
| Munari et al. 2013
SN 2012fr | Childress et al. 2013c

##  Appendix A Some recent SN Ia, Continued

###  A.1 Peculiar SN 1997br in ESO 576-G40

Li et al. (1999) presented observations of the peculiar SN 1991T-like, SN
1997br (Bao Supernova Survey et al., 1997). Hatano et al. (2002) analyzed the
spectra of SN 1997br and raised the question of whether or not Fe III and Ni
III features in the early spectra are produced by 54Fe and 58Ni rather than by
56Fe and 56Ni. In addition, Hatano et al. (2002) discussed the issue of SN
1991T-like events as more powerful versions of normal SN Ia, rather than a
physically distinct subgroup of events.

###  A.2 SN 1997cn in NGC 5490

Turatto et al. (1998) studied the faint SN 1997cn, which is located in an
elliptical host galaxy (Li et al., 1997; Turatto et al., 1997). Like SN
1991bg, spectra of SN 1997cn show a deep Ti II trough between 4000 and 5000 Å,
strong Ca II IR3 absorption features, a large $\mathcal{R}$(“Si II”), and slow
mean expansion velocities.

###  A.3 SN 1997ff and other “farthest known” SN Ia

With a redshift of z = 1.7, SN 1997ff was the most distant SN Ia discovered at
that time (Riess et al., 2001; Benítez et al., 2002). There have been
$\sim$110 high-z (1 $<$ z $<$ 2) SN Ia discoveries since SN 1997ff (Riess et
al., 2004, 2007; Suzuki et al., 2012), with the most recent and and one of the
most distant SN Ia known being “SN UDS10Wil” at z = 1.914 (Jones et al.,
2013). With these and future observations of “highest-z” SN Ia, constraints on
DTD timescales (Strolger et al., 2010; Graur et al., 2013) and dark energy
(Rubin et al., 2013) will certainly improve.

###  A.4 SN 1998aq in NGC 3982

Branch et al. (2003) used SYNOW to study 29 optical spectra of the normal SN
1998aq (Hurst et al., 1998), covering 9 days before to 241 days after maximum
light (days $-$9 and $+$241, respectively). Notably, they find evidence for C
II down to 11,000 km s-1, $\sim$ 3000 km s-1 below the cutoff of carbon in the
pure deflagration model, W7 (Nomoto et al., 1984).

###  A.5 SN 1999aa in NGC 2595

From day $-$11 to day $+$58, Garavini et al. (2004) obtained 25 optical
spectra of SN 1999aa (Armstrong & Schwartz, 1999). While SN 1999aa appears SN
1991T-like, Garavini et al. (2004) note that the Ca II absorption feature
strengths are between those of SN 1991T (SS) and the SN 1990N (CN), along with
a phase transition to normal SN Ia characteristics that sets in earlier than
SN 1991T. Subsequently, they suggest SN 1999aa to be a link between SN
1991T-likes and spectroscopically normal SN Ia. Evidence of carbon-rich
material is also found in SN 1999aa; decisively as C II $\lambda$6580,
tentatively as C III $\lambda$4649 (see Parrent et al. 2011). A schematic
representation of their SYNOW fitting results is also presented, showing the
inference of Co II, Ni II, and Ni III during the pre-maximum phases. These
results deserve further study from more detailed models.

###  A.6 SN 1999ac in NGC 2841

Between day $-$15 and day $+$42, Garavini et al. (2005) obtained spectroscopic
observations of the unusual SN 1999ac (Modjaz et al., 1999). The pre-maximum
light spectra are similar to that of SN 1999aa-like, while appearing
spectroscopically normal during later epochs. Garavini et al. (2005) find
evidence of a fairly conspicuous, heavily blended C II $\lambda$6580 feature
in the day $-$15 spectrum with approximate ejection velocities $>$16,000 km
s-1. By day $-$9, the C II absorption feature is weak or absent amidst
blending with the neighboring 6100 Å feature. This alone indicates that
studies cannot fully constrain SN Ia models without spectra prior to day
$-$10.

###  A.7 SN 1999aw in a low luminosity host galaxy

Strolger et al. (2002) find SN 1999aw to be a luminous, slow-declining SN Ia,
similar to 1999aa-like events. Strolger et al. (2002) derive a peak luminosity
of 1.51 x 1043 and a 56Ni mass of 0.76 M⊙.

###  A.8 SN 1999by in NGC 2841

Vinkó et al. (2001) presented and discussed the first three pre-maximum light
spectra of SN 1999by (Arbour et al., 1999), where they find it to be a sub-
luminous SN Ia similar to SN 1991bg (Filippenko et al., 1992b; Leibundgut et
al., 1993; Turatto et al., 1996), SN 1992K (Hamuy et al., 1994), SN 1997cn
(Turatto et al., 1998), and SN 1998de (Modjaz et al., 2001); in addition, the
list of sub-luminous SN Ia include SN 1957A, 1960H, 1971I, 1980I, 1986G (see
Branch et al. 1993; Doull & Baron 2011 and references therein) and several
other recently discovered under-luminous SN Ia (Howell, 2001; McClelland et
al., 2010; Hachinger et al., 2009). Pre-maximum spectra of SN 1999by show
relatively strong features due to O, Mg, and Si, which are due to explosive
carbon burning. In addition, blue wavelength regions reveal spectra dominated
by Ti II and some other IPEs.

Meanwhile, Höflich et al. (2002) studied the infrared spectra of SN 1999by,
covering from day $-$4 to day $+$14\. Post-maximum spectra show features which
can be attributed to incomplete Si burning, while further support for
incomplete burning comes from the detection of a pre-maximum C II absorption
feature (Garnavich et al., 2004). Höflich et al. (2002) analyzed the spectra
through the construction of an extended set of delayed detonation models
covering the entire range of normal to sub-luminous SN Ia. They estimate the
56Ni mass for SN 1999by to be on the order of 0.1 M⊙. Garnavich et al. (2004)
obtained _UBVRIJHK_ light curves of SN 1999by. From the photometry of SN
1999by, the recent Cepheid distance to NGC 2841 (Macri et al., 2001), and
minimal dust extinction along the line-of-sight, Garnavich et al. (2004)
derive a peak absolute magnitude of M${}_{\emph{B}}$ = $-$17.15.

In order to assess the role of material asymmetries as being responsible for
the observed peculiarity of sub-luminous SN Ia, Howell et al. (2001) obtained
polarization spectra of SN 1999by near maximum light. They find relatively low
levels of polarization (0.3%$-$0.8%), however significant enough to be
consistent with a 20% departure from spherical symmetry (Maund et al., 2010b).

###  A.9 SN 1999ee in IC 5179

From day $-$10 to day $+$53, Stritzinger et al. (2002) obtained well-sampled
_UBVRIz_ light curves of SN 1999ee (Maza et al., 1999). They find the _B_
-band light curve is broader than normal SN Ia, however sitting toward the
over-luminous end of SN Ia peak brightnesses, with M${}_{\emph{B}}$ = $-$19.85
$\pm$ 0.28 and $\Delta$m15 = 0.94.

Hamuy et al. (2002) obtained optical and infrared spectroscopy of SN 1999ee
between day $-$9 and day $+$42\. Before maximum light, the spectra of SN
1999ee are normal, with relatively strong Si II 6100 Å absorption, however
within the SS subtype (Branch et al., 2009). Hamuy et al. (2002) compared the
infrared spectra of SN 1999ee to that of other SN Ia out to 60 days post-
explosion, and find similar characteristics for SN 1999ee and 1994D (Meikle et
al., 1996).

###  A.10 SN 2000E in NGC 6951

Valentini et al. (2003) obtained _UBVRIJHK_ photometry and optical spectra of
SN 2000E, which is located in a spiral galaxy. Optical spectra were obtained
from 6 days before _B_ -band maximum to 122 days after _B_ -band maximum. The
photometric observations span 230$+$ days, starting at day $-$16\. The
photometric light curves of SN 2000E are similar to other SS SN Ia, however SN
2000E is classified as a slowly declining, spectroscopically “normal” SN Ia
similar to SN 1990N. Valentini et al. (2003) estimate the 56Ni mass to be 0.9
M⊙ from the bolometric light curve.

###  A.11 SN 2000cx in NGC 524

One of the brightest supernovae observed in the year 2000 was the peculiar SN
2000cx, located in an S0 galaxy (Yu et al., 2000; Li et al., 2001; Candia et
al., 2003). It was classified as a SN Ia with a spectrum resembling that of
the peculiar SN 1991T (Chornock et al., 2000). Sollerman et al. (2004)
obtained late time _BVRIJH_ light curves of SN 2000cx covering 360 to 480 days
after maximum. During these epochs, they find relatively constant NIR
magnitudes, indicating the increasing importance (with time) of the NIR
contribution to the bolometric light curve.

Branch et al. (2004b) decomposed the photospheric-phase spectra of SN 2000cx
with SYNOW. Apart from confirming HVFs of Ca II IR3 (which are consistent with
primordial abundances; Thomas et al. 2004), Branch et al. (2004b) also find
HVFs of Ti II. They attribute the odd behavior of SN 2000cx’s _B_ -band light
curve to the time-dependent behavior of these highly line blanketing Ti II
absorption signatures. Branch et al. (2004b) find an absorption feature near
4530 Å in the spectra of SN 2000cx that can be tentatively associated with
H$\beta$ at high velocities, however this feature is more likely due to C III
$\lambda$4649 or S II/Fe II instead (see Parrent et al. 2011 and references
therein).

Rudy et al. (2002) obtained 0.8$-$2.5 $\mu$m spectra of SN 2000cx at day $-$7
and day $-$8 before maximum light. From the $\lambda$10926 line of Mg II, they
find that carbon-burning has taken place up to $\sim$25,000 km s-1. Given the
SS subtype nature of SN 2000cx, the early epoch IR spectra of Rudy et al.
(2002) are valuable for comparison with other SN Ia IR datasets.

###  A.12 SN 2001V in NGC 3987

Vinkó et al. (2003) presented photometry of SN 2001V (Jha et al., 2001). They
find that SN 2001V is over-luminous, relative to the majority of SN Ia.
Spectroscopic observations, spanning from day $-$14 to day $+$106, can be
found in Matheson et al. (2008) and reveal it to be a SS SN Ia, consistent
with its observed brightness.

###  A.13 Slowly declining SN 2001ay in IC 4423

Krisciunas et al. (2011) obtained optical and near infrared photometry, and
optical and UV spectra of SN 2001ay (Swift et al., 2001). They find maximum
light Si II and Mg II line velocities of $\sim$ 14,000 km s-1, with Si III and
S II near 9,000 km s-1. SN 2001ay is one of the most slowly declining SN Ia.
However, a $\Delta$m15(_B_) = 0.68 is odd given it is not over-luminous like
SCC SN Ia slow decliners. In fact, the 56Ni yield of 0.58 is comparable to
that of many normal SN Ia. Baron et al. (2012) note this apparent WLR
violation is related to a decrease in $\gamma$-ray trapping deeper within the
ejecta due to an overall outward shift of 56Ni, thus creating a fast rise in
brightness followed by a slow decline caused by enhanced heating of the outer
regions of material, which is a consequence of the larger expansion opacities.

###  A.14 SN 2001el in NGC 1448

Krisciunas et al. (2003) obtained well-sampled _UBVRIJHK_ light curves of the
nearby (about 18 Mpc) and normal SN 2001el (Monard et al., 2001), from day
$-$11 to day $+$142\. Because Krisciunas et al. (2003) obtained _UBVRI_ and
_JHK_ light curves, they were able to measure a true optical$-$NIR reddening
value (A${}_{\emph{V}}$ = 0.57 mag along the line-of-sight) for the first
time.

Mattila et al. (2005) obtained early time high resolution and low resolution
optical spectra of SN 2001el. They estimate the mass loss rate (assuming
10$-$50 km s-1 wind velocities) from the progenitor system of SN 2001el to be
no greater than 9 x 10-6 M⊙ yr-1 and 5 x 10-5 M⊙ yr-1, respectively. The low
resolution spectrum was obtained 400 days after maximum light with no apparent
signatures of hydrogen Balmer lines. High velocity Ca II was detected out to
34,000 km s-1, while the 6100 Å absorption feature is suspected of harboring
high velocity Si II (see also Kasen et al. 2003).

###  A.15 SN 2002bo in NGC 3190

Between day $-$13 and day $+$102, Benetti et al. (2004) collected optical and
NIR spectra and photometry of the BL SN 2002bo (Cacella et al., 2002;
Krisciunas et al., 2004). Estimates on host galaxy extinction from Na D
equivalent width measurements are consistent with the inferred color excess
determined by comparison to the Lira relation (Lira, 1995; Riess et al., 1996;
Phillips et al., 1999). From the time-evolution of the 6100 Å absorption
feature, Benetti et al. (2004) find that SN 2002bo is an intermediary between
the BL SN 1984A and the CN SN 1994D. Benetti et al. (2004) also discuss SN
2002bo argue that some of the IME high velocity material may be primordial,
while most is produced during the explosion and possibly by prolonged burning
in a delayed detonation. This interpretation is also consistent with a lack of
any clear signatures of unburned carbon. Stehle et al. (2005) studied the
abundance stratification by fitting a series of spectra with a Monte Carlo
code and found that the elements synthesized in different stages of burning
are not completely mixed within the ejecta. In the case of SN 2002bo, they
derived the total mass of 56Ni to be 0.52 M⊙. Similar to SN 2001ay’s fast rise
to maximum light (Baron et al., 2012), Stehle et al. (2005) attribute SN
2002bo’s fast rise to outward mixing of 56Ni.

###  A.16 SN 2002cv in NGC 3190

The NIR photometry of SN 2002cv reveal an obscured SN Ia (Di Paola et al.,
2002); more than 8 magnitudes of visual extinction. Both optical and NIR
spectroscopy indicate SN 2002cv is most similar to SN 1991T (Meikle et al.,
2002; Filippenko et al., 2002). It should also be noted that the SS SN 2002cv
and the BL SN 2002bo share the same host galaxy. Elias-Rosa et al. (2008)
obtained and analyzed VRIJHK photometry, in addition to a sampling of optical
and NIR spectroscopy near and after maximum light, and find a best fit value
for the ratio between inferred extinction and reddening, RV = 1.59 $\pm$ 0.07
whereas 3.1 is often assumed for normal SN Ia (however see Tripp 1998; Astier
et al. 2006; Krisciunas et al. 2006; Folatelli et al. 2010). They suggest this
to indicate varying mean grain sizes for the dust along the line of sight
toward SN 2002bo and 2002cv.

###  A.17 SN 2002dj in NGC 5018

For two years, and starting from day $-$11, Pignata et al. (2008) monitored
the optical and IR behaviors of the SN 2002bo-like, high-velocity gradient SN
2002dj (Hutchings & Li, 2002). The dataset presented make it one of the most
well-observed SN 1984A-like SN Ia and is a valuable tool for the discussion of
SN Ia diversity.

###  A.18 SN 2002er in UGC 10743

From day $-$11 to day $+$215, Kotak et al. (2005) carried out spectroscopic
follow-up for the reddened, CN SN 2002er (Wood-Vasey et al., 2002c). By
contrast with the photometric behavior seen for SN 1992A, 1994D, and 1996X, SN
2002er stands out for its slightly delayed second peak in the _I_ -band and
similarly for _V_ and _R_ -bands. Pignata et al. (2004); Kotak et al. (2005)
estimated the mass of 56Ni to be on the order of 0.6 to 0.7 M⊙, where the
uncertainty in the exact distance to SN 2002er was the primary limitation.

###  A.19 SN 2003du in UGC 09391

For 480 days, and starting from day $-$13, Stanishev et al. (2007) monitored
the CN SN 2003du. From modeling of the bolometric light curve, Stanishev et
al. (2007) estimate the mass of 56Ni to between 0.6 and 0.8 M⊙. Like other
normal SN Ia, the early spectra of SN 2003du contain HVFs of Ca II and a 6100
Å feature that departs from being only due to photospheric Si II, suggesting
either a distinctly separate region of HV Si II or the radial extension of
opacities from below.

Tanaka et al. (2011) studied the chemical composition distribution in the
ejecta of SN 2003du by modeling a one year extended time series of optical
spectra. Tanaka et al. (2011) do not find SN 2003du to be as fully mixed as a
some 3D deflagration models. Specifically, from their modeling Tanaka et al.
(2011) that the a core of stable IPEs supersedes 56Ni out to $\sim$ 3000 km
s-1 ($\lesssim$ 0.2 in mass coordinate). Atop this 0.65 M⊙ of 56Ni are layers
of IMEs, while the outermost layers consist of oxygen, some silicon, and no
more than 0.016 M⊙ of carbon above 10,500 km s-1.

###  A.20 SN 2003gs in NGC 936

Krisciunas et al. (2009) obtained near-maximum to late time optical and NIR
observations of SN 2003gs, offering a chance to study the post-maximum light
bolometric behavior of a fast declining SN Ia that was sub-luminous at optical
wavelengths, but of standard luminosity in NIR bands at maximum light.
Krisciunas et al. (2009) find $\Delta$m15(_B_) = 1.83 $\pm$ 0.02 and discuss
comparisons to other fast decliners; namely, SN 2003hv (Leloudas et al.,
2009), SN 2004gs (Folatelli et al., 2010; Contreras et al., 2010), SN 2005bl
(Taubenberger et al., 2008; Folatelli et al., 2010; Wood-Vasey et al., 2008),
and SN 2005ke, 2006gt, and 2006mr (Folatelli et al., 2010; Contreras et al.,
2010). In particular, Krisciunas et al. (2009) note that, in contrast to
normal and over-luminous SN Ia, the delay in the time of _J_ -band maximum
from that of the _B_ -band, for fast decliners, is inversely proportional to
the peak NIR magnitude. Furthermore, they discussed the possibility for two
subsets of FAINT$-$CL fast decliners; those that do and do not show a _J_
-band peak before the _B_ -band (see also Kattner et al. 2012); respectively,
SN 1986G, 2003gs, 2003hv, and 2006gt, and SN 1991bg, 1999by, 2005bl, 2005ke,
and 2006mr.

Krisciunas et al. (2009) conclude that differences in NIR opacity within the
outer layers are responsible for dissimilar $\gamma$-ray trapping, and
therefore longer _J_ -band than _B_ -band diffusion times for FAINT$-$CL SN Ia
that are fainter in the NIR. However, the origin (differences of explosion
mechanism and/or progenitor systems) for this apparent ‘bimodal’ difference in
NIR opacity is not clear.

For SN 2003gs, Krisciunas et al. (2009) used _UBVRIJHK_ photometry and
Arnett’s rule (Arnett 1982, but also see Stritzinger & Leibundgut 2005) to
estimate 0.25 M⊙ of 56Ni was produced during the explosion. As for the optical
spectra, SN 2003gs is similar to SN 2004eo (A.23) in that it is found to have
absorption signatures that are consistent with its photometric
characteristics; a FAINT$-$CL SN Ia with a larger than normal $\mathcal{R}$(Si
II) and the presence of Ti II features near 4000$-$4500 Å.

###  A.21 SN 2003hv in NGC 1201

Leloudas et al. (2009) studied SN 2003hv out to very late phases (day $+$786).
Notably, this seemingly spectroscopically normal SN Ia has $\Delta$m15(_B_) =
1.61, while the late time light curves show a deficit in flux that follow the
decay of radioactive 56Co, assuming full and instantaneous positron trapping.
Leloudas et al. (2009) consider this as possibly due to a redistribution of
flux for SN 2003hv (a.k.a. an infrared catastrophe, see Axelrod 1980) from a
dense clumping of inner material, and would also explain the flat-topped
nebular emission lines (Motohara et al., 2006; Gerardy et al., 2007).

Mazzali et al. (2011) also studied the nebular spectrum of 2003hv and consider
it as a non-standard event. They note that its late time flux deficit,
compared to normal SN Ia, could be due to SN 2003hv having a lower mean
density structure, possibly consistent with a sub-Chandrasekhar mass origin.

Motohara et al. (2006) presented NIR Subaru Telescope spectra of SN 2003du,
2003hv, and 2005W during their late phase evolution ($+$200 days post-maximum
light). For both SN 2003du and 2003hv, they find a flat-topped [Fe II]
$\lambda$16440 emission feature that is blue-shifted by $\sim$ 2000 km s-1
from the SN rest frame (FWHM $\sim$ 4000 km s-1). Motohara et al. (2006)
further argue that the [Fe II] emission would be rounded on top if the
neutron-rich Fe-peak isotopes produced in the explosion were thoroughly mixed
with the surrounding distribution of 56Ni; for SN 2003du and 2003hv they
suggest that this is not the case. In fact, they find that SN 1991T and 2005W
(see their Fig. 1), at least, may represent instances where the inner most
regions have been thoroughly mixed.

Similarly, Gerardy et al. (2007) looked to address the nature of the
thermonuclear burning front by utilizing late time ($+$135 days) mid-IR
(5.2$-$15.2 $\mu$m) _Spitzer Space Telescope_ spectra of SN 2003hv and 2005df.
In particular, Gerardy et al. (2007) find direct evidence in SN 2005df for a
small inner zone of nickel that is surrounded by 56Co and an asymmetric shell-
like structure of Ar. While it is not clear _why_ a supposed initial
deflagration phase of a DDT explosion mechanism produces little to no mixing
for these two SN Ia, the observations of Gerardy et al. (2007) give strong
support for a stratified abundance tomography like those seen in DDT-like
models; the various species of material are restricted to radially confined
zones, which is inconsistent with the large-scale mixing that is expected to
occur in 3D deflagration models. This is also in agreement with X-ray
observations of the Tycho supernova remnant (Badenes et al., 2006) in addition
to optical and UV line resonance absorption imaging of SNR 1885 in M31 (Fesen
et al., 2007).

###  A.22 SN 2004dt in NGC 0799

Wang et al. (2006) and Altavilla et al. (2007) studied the early spectral
evolution of SN 2004dt from more than a week before optical maximum, when line
profiles show matter moving at velocities as high as 25,000 km s-1. The
variation of the polarization across some Si II lines approaches 2%, making SN
2004dt one of the most highly polarized SN Ia observed and an outlier in the
polarization-nebular velocity plane (Maund et al., 2010b). In contrast with
the polarization associated with Si II, Wang et al. (2006) find that the
strong 7400 Å O I$-$Mg II absorption complex shows little or no polarization
signature. Wang et al. (2006) conclude this is due to a spherical geometry of
oxygen-rich material encompassing a lumpy distribution of IMEs.

###  A.23 SN 2004eo in NGC 6928

Pastorello et al. (2007a) presented optical and infrared observations of the
transitional normal, CL SN 2004eo (Nakano et al., 2004). The light curves and
spectra appear normal (M${}_{\emph{B}}$ = $-$19.08) while exhibiting low mean
expansion velocities and a fast declining _B_ -band light curve
($\Delta$m15(_B_) = 1.46). The observed properties of SN 2004eo signify it is
intermediate between FAINT, LVG, and HVG SN Ia. Mazzali et al. (2008) also
consider SN 2004eo as a spectroscopically normal SN Ia that produced 0.43
$\pm$ 0.05 M⊙ of 56Ni.

###  A.24 SN 2005am in NGC 2811

Between day $-$4 and day $+$69, Brown et al. (2005) obtained UV, optical, and
X-ray observations with the _Swift_ satellite of the SN 1992A-like SN 2005am
(Kirshner et al., 1993; Modjaz et al., 2005). They place an upper limit on SN
2005am’s X-ray luminosity (0.3$-$10 keV) of 6 x 1039 erg s-1.

###  A.25 Under-luminous SN 2005bl in NGC 4070

Both Taubenberger et al. (2008) and Hachinger et al. (2009) studied the sub-
luminous SN 2005bl with observations made between day $-6$ and day $+$66, and
carried out spectral analysis (“abundance tomography”) of SN 2005bl (Morrell
et al., 2005). They find it to be one of incomplete burning similar to SN
1991bg and 1999by. Compared to SN 1991bg, a noteworthy difference of SN 1999by
is the likely presence of carbon C II in pre-maximum spectra (Taubenberger et
al., 2008), whereas C I $\lambda$10691 is also clearly detected in NIR spectra
(Höflich et al., 2002). However, this is likely a biased comparison to SN
1991bg given that the earliest spectrum obtained was on day $-$1 (potentially
too late to detect unburned material via C II $\lambda$6580). To our knowledge
no conspicuous C I $\lambda$10691 absorption features have been documented for
other SN Ia. For example, C I $\lambda$10691 is present in the pre-maximum
spectra of SN 2011fe but it is not a conspicuous signature. Similarly, pre-
maximum spectra of SN 2005bl show less conspicuous detections of C I and C II
but still indicate low burning efficiency with a significant amount of
leftover unburned material (Taubenberger et al., 2008). Hachinger et al.
(2009) suggest that a detonation at low pre-expanded densities is responsible
for the abundance stratification of IMEs seen in the spectra of SN 2005bl.
This would also explain the remaining carbon-rich material seen for some CL SN
Ia when caught early enough.

###  A.26 SN 2005cf in MCG-01-39-003

Wang et al. (2009b) studied UV$-$optical$-$NIR observations of the normal SN
2005cf (see also Pastorello et al. 2007b). During the early evolution of the
spectrum, HVFs of Ca II and Si II are found to be present above 18,000 km s-1
(confirming observations of Garavini et al. 2007). Gall et al. (2012) studied
the NIR spectra of SN 2005cf at epochs from day $-$10 to day $+$42, which show
clear signatures of Co II during post-maximum phases. In addition, they
attribute fluorescence emission in making the underlying shape of the SED.

###  A.27 SN 2005cg in a low-luminosity, star forming host

Quimby et al. (2006b) presented and discussed the spectroscopic evolution and
light curve of the SS SN 2005cg, which was discovered by ROTSE-IIIc. Pre-
maximum spectra reveal HVFs of Ca II and Si II out to $\sim$ 24,000 km s-1 and
Quimby et al. (2006b) find good consistency between observed and modeled Si II
profiles. They interpret the steep rise in the blue wing of the Si II to be an
indication of circumstellar interaction given that abundance estimates for
HVFs suggest modest amounts of swept up material ($\sim$ 10-4 $-$ 10-3 M⊙; see
Quimby et al. 2006b; Branch et al. 2006).

###  A.28 SN 2005hj

Quimby et al. (2007) obtained optical spectra of the SS SN 2005hj during pre-
maximum and post-maximum light phases. From a ROTSE-IIIb unfiltered light
curve, SN 2005hj reached an over-luminous peak absolute magnitude of $-$19.6
(assuming z = 0.0574). Interestingly, the sharp and shallow 6100 Å feature
remains fairly stagnant at $\sim$ 10,600 km s-1 near and after maximum light,
with a sudden decrease at later epochs. Similar to Quimby et al. (2006b),
Quimby et al. (2007) find this also consistent with the interpretation that
CSM is influencing spectral profiles of SN 1999aa-like SN Ia (see also Scalzo
et al. 2010, 2012).

###  A.29 SN 2006D in MCG-01-33-034

Thomas et al. (2007) obtained the spectra of the spectroscopically normal SN
2006D from day $-$7 to day $+$13\. The spectra show one of the clearest
signatures of carbon-rich material at photospheric velocities observed in a
_normal_ SN Ia (below 10,000 km s-1). The 6300 Å carbon feature becomes weaker
with time and disappears as the photosphere recedes and the SN reaches maximum
brightness. These observations$-$like all SN Ia diversity studies$-$underscore
the importance of obtaining spectra of SN Ia during all phases. If [O I] and
[C I] lines are present in the spectra during post-maximum light phases at
velocities below 10,000 km s-1, this would indicate the presence of unburned
matter. These particular lines have not been detected, however the absence of
said signatures does not imply a complete lack of C+O material at low
velocities (Baron et al., 2003; Kozma et al., 2005).

###  A.30 SN 2006X in M100

Wang et al. (2008b) presented _UBVRI_ and _JK_ light curves and optical
spectroscopy of the reddened BL SN 2006X (Stockdale et al., 2006; Immler,
2006; Quimby et al., 2006a) and find high mean expansion velocities during
pre-maximum light phases ($\gtrsim$ 20,000 km s-1). Wang et al. (2008b)
suggest the observed properties of SN 2006X may be due to interaction with
CSM. Yamanaka et al. (2009b) also presented and discussed the early spectral
evolution. They note that the $\mathcal{R}$(Si II) ratio is unusually low for
such a high-velocity gradient SN Ia. However, rather than this being an
indication of low effective temperature, they suggest that the low
$\mathcal{R}$(Si II) value is due to line-blending, likely from a higher
velocity component of Si II. Both Wang et al. (2008b) and Yamanaka et al.
(2009b) find the observed properties of SN 2006X to be consistent with
characteristics of delayed detonation models.

Equipped with high resolution spectra of narrow Na D signatures spanning
$\sim$100 days post-maximum light, Patat et al. (2007) infer the presence of
intervening CSM and argue a mass loss history associated with SN 2006X in the
decades prior to explosion. In fact, at least half of all SN Ia with narrow
rest frame, blue-shifted Na D absorption profiles are associated with high
ejecta velocities (Sternberg et al., 2011; Foley et al., 2012a). This
indicates that CSM outflows are either present in some explosion scenarios
_or_ associated with all progenitors scenarios at some point during the lead
up to the explosion.

Patat et al. (2009) later discussed the VLT spectropolarimetry of SN 2006X. In
particular, they find that the presence of the high-velocity Ca II is
coincident with a relatively high polarization signature ($\sim$1.4%) at day
$-$10, that diminishes by only $\sim$15% near maximum light, and is still
present 41 days later. Patat et al. (2009) note that this day $+$40 detection
is not seen for SN 2001el (Wang et al., 2003) or SN 2004du (Leonard et al.,
2005). As for the high-velocity Si II, its polarization signature is seen to
peak ($\sim$1.1%) at day $-$6, drop by $\sim$30% near maximum, and is
undetected well into the post-maximum phase. While the findings of
spectropolarimetry studies of SN Ia are thought to be associated with, for
example, “deflagration phase plumes” with time-dependent photospheric covering
fractions, Patat et al. (2009) are unable to conclude why SN 2006X exhibits a
sizable post-maximum light re-polarization signature by day $+$39.

###  A.31 SN 2006bt in CGCG 108-013

Foley et al. (2010b) obtained optical light curves and spectra of transitional
CN/CL SN 2006bt (Lee & Li, 2006). The _B_ -band decline rate, $\Delta$m15(B) =
1.09, is within the range that is observed for normal SN Ia, however SN 2006bt
shows a larger than normal $\mathcal{R}$(Si II), slightly lower mean expansion
velocities, and a lack of a double peak in the _I_ -band; CL SN 1991bg-like
properties. A tentative C II 6300 feature is identified, however with a
minimum at $\sim$ 6450 Å. Foley et al. (2010b) suggest this inferred lower
projected Doppler velocity could be accounted for by a clump of carbon offset
from the line of sight _at_ photospheric velocities. Because of an association
within a halo population of its passive host galaxy, Foley et al. (2010b)
conclude that the progenitor was also likely to be from an old population of
stars.

###  A.32 Over-luminous SN 2006gz in IC 1277

Hicken et al. (2007) studied SN 2006gz (Prieto et al., 2006a) and estimated a
peak intrinsic _V_ -band brightness of $-$19.74 and $\Delta$m15(_B_) = 0.69,
implying M(56Ni) $\sim$ 1.0$-$1.2 M⊙ (assuming $R_{V}$ = 2.1$-$3.1; see also
Maeda et al. 2009). The spectroscopic signatures during early phases are
relatively narrow on account of slightly lower mean expansion velocities. At
two weeks before maximum light, Hicken et al. (2007) attributed a relatively
strong 6300 Å feature to C II $\lambda$6580 that diminishes in strength by day
$-$10 (Prieto et al., 2006b). Compared to a 5 Å equivalent width 6300 Å
absorption feature observed in the CN SN 1990N (Leibundgut et al., 1991;
Jeffery et al., 1992), the absorption feature has an observed equivalent width
of 25 Å in the early spectra of SN 2006gz (Hicken et al., 2007). So far,
spectroscopic modeling that incorporate signatures of C II $\lambda$6580
predict carbon mass fractions, X(C), that span an order of magnitude and are
broadly consistent with both single- and double-degenerate scenarios.

Maeda et al. (2009) obtained Subaru and Keck observations of 2006gz at late-
phases. Interestingly, SN 2006gz shows relatively weak pillars of iron
emission that are usually seen in most SN Ia subtypes.

###  A.33 Extremely faint, SN 2007ax in NGC 2577

SN 2007ax was a very faint, red, and peculiar SN Ia. Kasliwal et al. (2008)
find that it shares similarities with a sub-luminous SN 2005ke (Immler et al.,
2006; Hughes et al., 2007; Patat et al., 2012) and also shows clear excess UV
emission $\sim$ 20 days post-maximum light. Based on the small amount of
synthesized 56Ni that is inferred (0.05 $-$ 0.09 M⊙), along with SN Ia-like
expansion velocities near maximum light ($\sim$9000 km s-1), Kasliwal et al.
(2008) conclude that SN 2007ax is not compatible with a number of theoretical
models that have been proposed to explain FAINT$-$CL SN Ia.

###  A.34 Over-luminous SN 2007if

Scalzo et al. (2010) find that SN 2007if qualifies as a SCC SN Ia, i.e. it is
over-luminous (M${}_{\emph{V}}$ = $-$20.4), has a slow-rise to peak brightness
(trise = 24 days), the early spectra contain signatures of stronger than
normal C II, and SN 2007if resides in a low-luminosity host (M${}_{\emph{g}}$
= $-$14.10). Despite having a red _B_ $-$  _V_ color ($+$0.16) at _B_ -band
maximum, signs of host reddening via Na D lines appear negligible. Utilizing
Keck observations of the young metal-poor host galaxy, Childress et al. (2011)
concluded that SN 2007if is likely to have originated from a young, metal-poor
progenitor. From the H$\alpha$ line of the host galaxy, Yuan et al. (2010)
derived a redshift of 0.0736.

Based on the bolometric light curve and the sluggish Si II velocity evolution,
Scalzo et al. (2010) conclude that SN 2007if was the death of a super-
Chandrasekhar mass progenitor. They estimate the total mass of the system to
be 2.4 M⊙, with 1.6 M⊙ of 56Ni, and 0.3 to 0.5 M⊙ in the form of a C$+$O
envelope. Given the possibility that other over-luminous events could
potentially stem from similar super-Chandrasekhar mass origins, Scalzo et al.
(2012) searched the SNFactory sample (based on a criterion of SN 1991T/2007if-
like selections) and found four additional super-Chandrasekhar mass
candidates.

###  A.35 SN 2007on in NGC 1404

SN 2007on was found associated with the elliptical galaxy, NGC 1404 (Pollas &
Klotz, 2007). Voss & Nelemans (2008) reported the discovery of the progenitor
of SN 2007on based on a detected X-ray source in pre-supernova archival X-ray
images, located 0.9” $\pm$ 1.3” (later 1.15” $\pm$ 0.27”; Roelofs et al. 2008)
from the position of SN 2007on within its host galaxy. However, Roelofs et al.
(2008) later reevaluated the detection of the progenitor of SN 2007on and
concluded that given the offset discrepancy between the X-ray source and the
SN location, the probability for a connection is of order 1 percent. However,
should SN Ia progenitors reveal themselves to be producers of pre-explosive
X-ray sources, Voss & Nelemans (2008) suggest this would be consistent with a
merger model with an accretion disc, formed from the disrupted companion star
rather than an explosion immediately upon or soon after the merger of the two
stars.

###  A.36 SN 2008J $-$ heavily reddened SN 2002ic-like in MGC-02-07-033

Taddia et al. (2012) studied SN 2008J (Thrasher et al., 2008), which provides
additional observational evidence for hydrogen-rich CSM around an otherwise SN
1991T-like SS SN Ia. They obtained a NIR spectrum extending up to 2.2 $\mu$m,
and find that SN 2008J is affected by a visual extinction of 1.9 mag.

###  A.37 Sub-luminous SN 2008ha in UGC 12682

Foley et al. (2010a) studied the optical spectrum of SN 2008ha near maximum
brightness (Puckett et al., 2008; Soderberg, 2009). It is found to be a dim
thermonuclear SN Ia with uncommonly slow projected expansion velocities.
Carbon features at maximum light indicate that carbon-rich material is present
to significant depths in the SN ejecta. Consequently, Foley et al. (2010a)
conclude that SN 2008ha was a failed deflagration since late time imaging and
spectroscopy also give support to this idea (Kromer et al., 2013a).

###  A.38 SN 2009nr in UGC 8255

Khan et al. (2011b) discuss the photometric and spectroscopic observations of
the over-luminous (M${}_{\emph{V}}$ = $-$19.6, $\Delta$m15(_B_) = 0.95) SS SN
2009nr (Balanutsa & Lipunov, 2010). Similarly, Tsvetkov et al. (2011) made
_UBVRI_ photometric observations of SN 2009nr. They estimate that 0.78 $-$
1.07 M⊙ of 56Ni was synthesized during the explosion. Khan et al. (2011b) also
find SN 2009nr is at a projected distance of 13.0 kpc from the nucleus of its
star-forming host galaxy. In turn, this indicates that the progenitor of SN
2009nr was _not_ associated with a young stellar population, i.e. SN 2009nr
may not have originated from a “prompt” progenitor channel as is often assumed
for SN Ia of its subtype.

###  A.39 Peculiar, sub-luminous PTF09dav

Sullivan et al. (2011b) studied the peculiar PTF09dav discovered by the
Palomar Transient Factory. Sullivan et al. (2011b) find it to be faint
(M${}_{\emph{B}}$ = $-$15.5) compared to SN 1991bg, and does not satisfy the
faint end of the WLR. Sullivan et al. (2011b) find estimates for both the 56Ni
mass (0.019 M⊙) and ejecta mass (0.36 M⊙) significantly low for thermonuclear
supernovae. The spectra are also consistent with signatures of Sc II, Mn I, Ti
II, Sr II and low velocities of $\sim$6000 km s-1. The host galaxy of PTF09dav
is not clear, however it appears this transient is not associated with
massive, old stellar populations. Sullivan et al. (2011b) conclude that the
observed properties of PTF09dav cannot be explained by the known models of
sub-luminous SN Ia.

Notably, Kasliwal et al. (2012) recently presented late time spectra of
PTF09dav (and other similar low luminosity transients). They confirm that this
class of objects look nothing like SN Ia at all on account of little to no
late-time iron emission, but instead with prominent emission from calcium in
the NIR (Perets et al., 2010), confirming previous suspicions of Sullivan et
al. (2011b).

###  A.40 PTF10ops, another peculiar cross-type SN Ia

Maguire et al. (2011) presented optical photometric and spectroscopic
observations of a somewhat peculiar and sub-luminous SN Ia, PTF10ops ($-$17.77
mag). Spectroscopically, this object has been noted as belonging to the CL
class of SN Ia on account of the presence of conspicuous Ti II absorption
features blue ward of 5000 Å, in addition to a larger than normal
$\mathcal{R}$(Si II) ratio (partially indicative of cooler effective
temperatures). Photometrically, PTF10ops overlaps “normal” SN Ia properties in
$\Delta$m15(_B_) (1.12 $\pm$ 0.06 mag) and its rise-time to maximum light
(19.6 days). Maguire et al. (2011) estimate $\sim$0.17 M⊙ of 56Ni was produced
during the explosion, which is well below what is expected for LVG$-$CN SN Ia.

Maguire et al. (2011) also note that either PTF10ops remains without a visible
host galaxy, or it resides within the outskirts of a massive spiral galaxy
located at least 148 kpc away, which would be consistent with a possible
influence of low metallicities or an old progenitor population. Maguire et al.
(2011) suggest the progenitor could have been the merger of two compact
objects (Pakmor et al., 2010), however time series spectrum synthesis is
needed to confirm.

###  A.41 SN 2010jn in NGC 2929

The BL SN 2010jn was discovered by the Palomar Transient Factory (PTF10ygu) 15
days before it reached maximum light. Hachinger et al. (2013) performed
spectroscopic analysis of the photospheric phase observations and find that
the outer layers of SN 2010jn are rich in iron-group elements. At such high
velocities ($>$16,000 km s-1), iron-group elements have been tentatively
identified in the spectra of SN Ia before (Hatano et al., 1999a) and may also
be a ubiquitous property of SN Ia. However, more early epoch, time series
observations are needed in order to test and confirm such claims. For SN
2010jn at least, Hachinger et al. (2013) favor a Chandrasekhar-mass delayed
detonation, where the presence of iron-group elements within the outermost
layers may be a consequence of outward mixing via hydrodynamical instabilities
prior to or during the explosion (see Piro 2011, 2012).

###  A.42 SN 2011iv in NGC 1404

Foley et al. (2012b) presented the first maximum-light UV through NIR spectrum
of a SN Ia (SN 2011iv; Drescher et al. 2011). Despite having a normal looking
spectrum, SN 2011iv declined in brightness fairly quickly ($\Delta$m15(_B_) =
1.69). Since the UV region of a SN Ia spectrum is extremely sensitive to the
composition of the outer layers, they offer the potential for strong
constraints as soon as observational UV spectroscopic diversity is better
understood.

###  A.43 SN 2012cg in NGC 4424

Silverman et al. (2012d) presented early epoch observations of the nearby
spectroscopically normal SN 2012cg (Kandrashoff et al., 2012; Cenko et al.,
2012; Marion et al., 2012), discovered immediately after the event ($\sim$1.5
days after). Compared to the width of other normal SN Ia _B_ -band light
curves, Silverman et al. (2012d) find that SN 2012cg’s light curve relatively
narrow for its peak absolute brightness, with $t_{rise}$ = 17.3 days
(coincident photometry was also presented by Munari et al. 2013). Mean
expansion velocities within 2.5 days of the event were found to be more than
14,000 km s-1, while the earliest observations show high-velocity components
of both Si II and Ca II. The C II $\lambda\lambda$6580, 7234 absorption
features were also detected very early.

Johansson et al. (2013) obtained upper limits on dust emission via far
infrared _Herschel Space Observatory_ flux measurements in the vicinity of the
recent and nearby SN 2011by, 2011fe, and 2012cg. From non-detections during
post-maximum epochs at 70 $\mu$m and 160 $\mu$m band-passes and archival image
measurements, Johansson et al. (2013) exclude dust masses $\gtrsim$ 7 x 10-3
M⊙ for SN 2011fe, and $\gtrsim$ 10-1 M⊙ for SN 2011by and 2012cg for $\sim$
500 K dust temperatures, $\sim$ 1017 cm dust shell radii, and peak SN
bolometric luminosities of $\sim$ 109 L⊙.

###  A.44 SN 2000cx-like, SN 2013bh

Silverman et al. (2013c) discussed recent observations of SN 2013bh and found
it similar to SN 2000cx on all accounts, with slightly higher mean expansion
velocities. Silverman et al. (2013c) note that both of these SN Ia reside on
the fringes of their spiral host galaxies. In addition, both SN 2000cx and
2013bh lack narrow Na D lines that would otherwise indicate an environment of
CSM. Given the extreme similarities between SN 2000cx and 2013bh, Silverman et
al. (2013c) suggest identical explosion scenarios for both events.

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