195 research outputs found
On the presence of Silicon and Carbon in the pre-maximum spectrum of the Type Ia SN 1990N
The spectrum of the normal Type Ia SN 1990N observed very early on (14 days
before B maximum) was analysed by Fisher et al (1997), who showed that the
large width and the unusual profile of the strong line near 6000\AA can be
reproduced if the line is assumed to be due to \CII 6578, 6583\AA and if Carbon
is located in a high velocity shell. This line is one of the characterising
features of SNe Ia, and is usually thought to be due to \SiII. A Monte Carlo
spectrum synthesis code was used to investigate this suggestion further. The
result is that if a standard explosion model is used the mass enclosed in the
shell at the required high velocity (25,000--35,000 \kms) is too small to give
rise to a strong \CII line. At the same time, removing Silicon has a negative
effect on the synthetic spectrum at other wavelengths, and removing Carbon from
the lower velocity regions near the photosphere makes it difficult to reproduce
two weak lines which are naturally explained as \CII, one of them being the
line which Fisher et al (1997) suggested is responsible for the strong 6000\AA
feature. However, synthetic spectra confirm that although \SiII can reproduce
most of the observed 6000\AA line, the red wing of the line extends too far to
be compatible with a \SiII origin, and that the flat bottom of the line is also
not easy to reproduce. The best fit is obtained for a normal SN Ia abundance
mix at velocities near the photosphere (15,500-19,000 \kms) and an outer
Carbon-Silicon shell beyond 20,000 \kms. This suggests that mixing is not
complete in the outer ejecta of a SN Ia. Observations at even earlier epochs
might reveal to what extent a Carbon shell is unmixed.Comment: 12 pages, (4 figures). MNRAS, in pres
Oxygen Recombination in the nebular phase of supernovae 1998bw and 2002ap
Late-time spectra of stripped-envelope CC-SNe are dominated by strong [O {\sc
i}] 6300,6363 emission, caused by thermal electron excitation
of forbidden [O {\sc i}] transitions. The permitted O
{\sc i} 7774 \AA\ line is also often observed. This line cannot result from
thermal electron excitation of the oxygen ground state. In this work tests are
performed to verify whether the line can be powered by oxygen recombination
alone, using the examples of two of the best studied type Ic SNe, 1998bw and
2002ap.
Temperature-dependent effective recombination coefficients for neutral oxygen
are calculated using available atomic data. Missing atomic data are computed in
a temperature range typical for SN nebulae. Core ejecta models for SNe 1998bw
and 2002ap are obtained from modelling their nebular emission spectra so that
oxygen recombination line formation is computed consistently with oxygen
forbidden line emission.
While SN 2002ap can be explained well by a one dimensional shell model, this
seems not to be possible for SN 1998bw, for which a two dimensional model is
found. At very late epochs the formation of the O {\sc i} 7774 \AA\ line can be
explained by recombination radiation for both SNe, but at earlier epochs strong
absorption is present which may determine the strength of this line even at
200 days
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
Multi-Dimensional Simulations for Early Phase Spectra of Aspherical Hypernovae: SN 1998bw and Off-Axis Hypernovae
Early phase optical spectra of aspherical jet-like supernovae (SNe) are
presented. We focus on energetic core-collapse SNe, or hypernovae. Based on
hydrodynamic and nucleosynthetic models, radiative transfer in SN atmosphere is
solved with a multi-dimensional Monte-Carlo radiative transfer code, SAMURAI.
Since the luminosity is boosted in the jet direction, the temperature there is
higher than in the equatorial plane by ~ 2,000 K. This causes anisotropic
ionization in the ejecta. Emergent spectra are different depending on viewing
angle, reflecting both aspherical abundance distribution and anisotropic
ionization. Spectra computed with an aspherical explosion model with kinetic
energy 20 x 10^{51} ergs are compatible with those of the Type Ic SN 1998bw if
~ 10-20% of the synthesized metals are mixed out to higher velocities. The
simulations enable us to predict the properties of off-axis hypernovae. Even if
an aspherical hypernova explosion is observed from the side, it should show
hypernova-like spectra but with some differences in the line velocity, the
width of the Fe absorptions and the strength of the Na I line.Comment: 4 pages, 4 figures. Accepted for publication in The Astrophysical
Journal Letter
Helium Shell Detonations on Low Mass White Dwarfs as a Possible Explanation for SN 2005E
Recently several type Ib supernovae (SNe; with the prototypical SN 2005E)
have been shown to have atypical properties. These SNe are faint (absolute peak
magnitude of ~ -15) and fast SNe that show unique composition. They are
inferred to have low ejecta mass (a few tenths of a solar mass) and to be
highly enriched in calcium, but poor in silicon elements and nickel. These SNe
were therefore suggested to belong to a new class of calcium-rich faint SNe
explosions. Their properties were proposed to be the result of helium
detonations that may occur on helium accreting white dwarfs. In this paper we
theoretically study the scenario of helium detonations, and focus on the
results of detonations in accreted helium layers on low mass carbon-oxygen (CO)
cores. We present new results from one dimensional simulations of such
explosions, including their light curves and spectra. We find that when the
density of the helium layer is low enough the helium detonation produces large
amounts of intermediate elements, such as calcium and titanium, together with a
large amount of unburnt helium. Our results suggest that the properties of
calcium-rich faint SNe could indeed be consistent with the helium-detonation
scenario on small CO cores. Above a certain density (larger CO cores) the
detonation leaves mainly 56Ni and unburnt helium, and the predicted spectrum
will unlikely fit the unique features of this class of SNe. Finally, none of
our studied models reproduces the bright, fast evolving light curves of another
type of peculiar SNe suggested to originate in helium detonations (SNe 1885A,
1939B and 2002bj).Comment: 15 pages, 8 figure
The Type Ic SN 2007gr: a census of the ejecta from late-time optical-infrared spectra
Nebular spectra of Supernovae (SNe) offer an unimpeded view of the inner
region of the ejecta, where most nucleosynthesis takes place. Optical spectra
cover most, but not all of the emitting elements, and therefore offer only a
partial view of the products of the explosion. Simultaneous optical-infrared
spectra, on the other hand, contain emission lines of all important elements,
from C and O through to the Intermediate Mass Elements (IME) Mg, Si, S, Ca, and
to Fe and Ni. In particular, Si and S are best seen in the IR. The availability
of IR data makes it possible to explore in greater detail the results of the
explosion. SN\,2007gr is the first Type Ic SN for which such data are
available. Modelling the spectra with a NLTE code reveals that the inner ejecta
contain \sim 1 \Msun of material within a velocity of \,\kms.
%The spectrum is powered by \Nifs, in an amount (0.076 \Msun) consistent with
that %derived from the early-time data. The same mass of \Nifs\ derived from
the light curve peak (0.076 \Msun) was used to power the spectrum, yielding
consistent results. Oxygen is the dominant element, contributing \sim 0.8
\Msun. The C/O ratio is . IME account for \sim 0.1 \Msun. This
confirms that SN\,2007gr was the explosion of a low-mass CO core, probably the
result of a star of main-sequence mass \approx 15 \Msun. The ratios of the
\CaII\ lines, and of those of \FeII, are sensitive to the assumed degree of
clumping. In particular, the optical lines of [\FeII] become stronger, relative
to the IR lines, for higher degrees of clumping
Spectral luminosity indicators in SNe Ia - Understanding the R(SiII) line strength ratio and beyond
SNe Ia are good distance indicators because the shape of their light curves,
which can be measured independently of distance, varies smoothly with
luminosity. This suggests that SNe Ia are a single family of events. Similar
correlations are observed between luminosity and spectral properties. In
particular, the ratio of the strengths of the SiII \lambda 5972 and \lambda
6355 lines, known as R(SiII), was suggested as a potential luminosity
indicator. Here, the physical reasons for the observed correlation are
investigated. A Monte-Carlo code is used to construct a sequence of synthetic
spectra resembling those of SNe with different luminosities near B maximum. The
influence of abundances and of ionisation and excitation conditions on the
synthetic spectral features is investigated. The ratio R(SiII) depends
ssentially on the strength of SiII \lambda 5972, because SiII \lambda 6355 is
saturated. In less luminous objects, SiII \lambda 5972 is stronger because of a
rapidly increasing SiII/SiIII ratio. Thus, the correlation between R(SiII) and
luminosity is the effect of ionisation balance. The SiII \lambda 5972 line
itself may be the best spectroscopic luminosity indicator for SNe Ia, but all
indicators discussed show scatter which may be related to abundance
distributions.Comment: 10 pages, 16 figures. Accepted for publication in MNRA
The (54Fe+58Ni)/56Ni ratio as a second parameter for Type Ia supernova properties
A variation of the relative content of (54Fe+58Ni) versus 56Ni may be
responsible for the observed scatter of Type Ia Supernovae (SNe Ia) about a
mean relation between their intrinsic brightness and the shape of their light
curve. Synthetic light curves are computed of parametrised Chandrasekhar-mass
explosion models of constant kinetic energy, where the ejecta are divided into
an inner NSE zone, composed of (54Fe+58Ni) inside and 56Ni outside, an outer
zone with Intermediate Mass Elements and a CO zone. Both the size of the NSE
zone and the fraction of (54Fe+58Ni) v. 56Ni are varied systematically. Models
with the same original NSE content but different (54Fe+58Ni)/56Ni ratios reach
different peak brightness but have similar light curve shapes. Synthetic
spectra indicate that the V-band decline rate is not affected by the
(54Fe+58Ni)/56Ni ratio. While the 56Ni mass and the total NSE mass are the
dominant parameters determining the peak luminosity and the shape of the light
curve, respectively, a variation in the (54Fe+58Ni)/56Ni ratio, which may
depend on the metallicity of the progenitor (Timmes, Brown & Truran 2003) is
likely to account for a significant part of the observed scatter of local SNe
Ia about the mean brightness--decline rate relation.Comment: 7 pages, 2 figures; accepted by MNRA
Cosmological Implications of the Second Parameter of Type Ia Supernovae
Theoretical models predict that the initial metallicity of the progenitor of
a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This
can cause a deviation from the standard light curve calibration employed when
using SNe Ia as standardizable distance candles and, if there is a systematic
evolution of the metallicity of SN Ia progenitors, could affect the
determination of cosmological parameters. Here we show that this metallicity
effect can be substantially larger than has been estimated previously, when the
neutronisation in the immediate pre-explosion phase in the CO white dwarf is
taken into account, and quantitatively assess the importance of metallicity
evolution for determining cosmological parameters. We show that, in principle,
a moderate and plausible amount of metallicity evolution could mimic a
lambda-dominated, flat Universe in an open, lambda-free Universe. However, the
effect of metallicity evolution appears not large enough to explain the high-z
SN Ia data in a flat Universe, for which there is strong independent evidence,
without a cosmological constant. We also estimate the systematic uncertainties
introduced by metallicity evolution in a lambda-dominated, flat Universe. We
find that metallicity evolution may limit the precision with which Omega_m and
w can be measured and that it will be difficult to distinguish evolution of the
equation of state of dark energy from metallicity evolution, at least from SN
Ia data alone.Comment: 10 pages, 6 figures, constructive comments welcom
- …