504 research outputs found
Spectroscopically Peculiar Type Ia Supernovae and Implications for Progenitors
In a recent paper Li et al. (2000) reported that 36 percent of 45 Type Ia
supernovae (SNe Ia) discovered since 1997 in two volume-limited supernova
searches were spectroscopically peculiar, and they suggested that because this
peculiarity rate is higher than that reported for an earlier observational
sample by Branch et al. (1993), it is now more likely that SNe Ia are produced
by more than one kind of progenitor. In this paper I discuss and clarify the
differences between the results of Li et al. and Branch et al. and I suggest
that multiple progenitor systems are now less likely than they were before.Comment: 11 pages; accepted by PASP; several minor changes, 2 references
added, main conclusions unchange
Spectral Consequences of Deviation from Spherical Composition Symmetry in Type Ia Supernovae
We investigate the prospects for constraining the maximum scale of clumping
in composition that is consistent with observed Type Ia supernova flux spectra.
Synthetic spectra generated without purely spherical composition symmetry
indicate that gross asymmetries make prominent changes to absorption features.
Motivated by this, we consider the case of a single unblended line forming in
an atmosphere with perturbations of different scales and spatial distributions.
Perturbations of about 1% of the area of the photodisk simply weaken the
absorption feature by the same amount independent of the line of sight.
Conversely, perturbations of about 10% of the area of the photodisk introduce
variation in the absorption depth which does depend on the line of sight. Thus,
1% photodisk area perturbations may be consistent with observed profile
homogeneity but 10% photodisk area perturbations can not. Based on this, we
suggest that the absence of significant variation in the depths of Si II 6355
absorption features in normal Type Ia spectra near maximum light indicates that
any composition perturbations in these events are quite small. This also
constrains future three-dimensional explosion models to produce ejecta profiles
with only small scale inhomogeneities.Comment: 11 pages, 6 figure
3D Models for High Velocity Features in Type Ia Supernovae
Spectral synthesis in 3-dimensional (3D) space for the earliest spectra of
Type Ia supernovae (SNe Ia) is presented. In particular, the high velocity
absorption features that are commonly seen at the earliest epochs (
days before maximum light) are investigated by means of a 3D Monte Carlo
spectral synthesis code. The increasing number of early spectra available
allows statistical study of the geometry of the ejecta. The observed diversity
in strength of the high velocity features (HVFs) can be explained in terms of a
``covering factor'', which represents the fraction of the projected photosphere
that is concealed by high velocity material. Various geometrical models
involving high velocity material with a clumpy structure or a thick torus can
naturally account for the observed statistics of HVFs. HVFs may be formed by a
combination of density and abundance enhancements. Such enhancements may be
produced in the explosion itself or may be the result of interaction with
circumstellar material or an accretion disk. Models with 1 or 2 blobs, as well
as a thin torus or disk-like enhancement are unlikely as a standard situation.Comment: 17 pages, 12 figures. Accepted for publication in the Astrophysical
Journa
On the High--Velocity Ejecta of the Type Ia Supernova 1994D
Synthetic spectra generated with the parameterized supernova
synthetic-spectrum code SYNOW are compared to spectra of the Type Ia SN 1994D
that were obtained before the time of maximum brightness. Evidence is found for
the presence of two-component Fe II and Ca II features, forming in high
velocity ( \kms) and lower velocity ( \kms) matter.
Possible interpretations of these spectral splits, and implications for using
early--time spectra of SNe Ia to probe the metallicity of the progenitor white
dwarf and the nature of the nuclear burning front in the outer layers of the
explosion, are discussed.Comment: 15 pages, 3 figures, 3 tables, Astrophysical Journal, in pres
A spectropolarimetric view on the nature of the peculiar Type I SN 2005hk
We report two spectropolarimetric observations of SN 2005hk, which is a close
copy of the "very peculiar" SN 2002cx, showing low peak luminosity, slow
decline, high ionization near peak and an unusually low expansion velocity of
only about 7,000 km s^-1. Further to the data presented by Chornock et al.,
(2006), at -4 days before maximum, we present data of this object taken on 9
November 2005 (near maximum) and 23 November (+ two weeks) that show the
continuum and most of the spectral lines to be polarized at levels of about
0.2-0.3%. At both epochs the data corresponds to the Spectropolarimetric Type
D1. The general low level of line polarization suggests that the line forming
regions for most species observed in the spectrum have a similar shape to that
of the photosphere, which deviates from a spherical symmetry by <10%. In
comparison with spectropolarimetry of Type Ia and Core-collapse SNe at similar
epochs, we find that the properties of SN 2005hk are most similar to those of
Type Ia SNe. In particular, we find the low levels of continuum and line
polarization to indicate that the explosion mechanism is approximately
spherical, with homogeneous ejecta (unlike the chemically segregated ejecta of
CCSNe). We discuss the possibility that SN 2005hk was the result of the pure
deflagration of a white dwarf and note the issues concerning this
interpretation.Comment: ApJ accepted, uses emulateapj, 16 pages, 10 figures, figures 3 and 4
update
Spectropolarimetry of the Type Ia SN 2007sr Two Months After Maximum Light
We present late time spectropolarimetric observations of SN 2007sr, obtained
with the VLT telescope at ESO Paranal Observatory when the object was 63 days
after maximum light. The late time spectrum displays strong line polarization
in the CaII absorption features. SN 2007sr adds to the case of some normal Type
Ia SNe that show high line polarization or repolarization at late times, a fact
that might be connected with the presence of high velocity features at early
times
Supernova 1996L: evidence of a strong wind episode before the explosion
Observations of the type II SN 1996L reveal the presence of a slowly
expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow
emission features are visible in the early spectra superposed on the normal SN
spectrum. Within about two months these features develop narrow symmetric
P-Cygni profiles. About 100 days after the explosion the light curve suddenly
flattens, the spectral lines broaden and the Halpha flux becomes larger than
what is expected from a purely radioactive model. These events are interpreted
as signatures of the onset of the interaction between the fast moving ejecta
and a slowly moving outer shell of matter ejected before the SN explosion. At
about 300 days the narrow lines disappear and the flux drops until the SN fades
away, suggesting that the interaction phase is over and that the shell has been
swept away. Simple calculations show that the superwind episode started 9 yr
before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3)
M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden
lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day
270 show relatively strong emission lines of HeI. These lines are narrower than
other emission lines coming from the SN ejecta, but broader than those from the
CSM. These high excitation lines are probably the result of non-thermal
excitation and ionization caused by the deposition of the gamma-rays emitted in
the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.
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