1,331 research outputs found
Delayed Detonation at a Single Point in Exploding White Dwarfs
Delayed detonation in an exploding white dwarf, which propagates from an
off-center transition point, rather than from a spherical transition shell, is
described and simulated. The differences between the results of 2D simulations
and the 1D case are presented and discussed. The two dimensional effects become
significant in transition density below 3.e7 g/cm^3, where the energetics, the
production of Fe group elements and the symmetry of the explosion are all
affected. In the 2D case the explosion is less energetic and less Ni is
produced in the detonation phase of the explosion. For low transition density
the reduction in Ni mass can reach 20-30 percent. The asymmetry in abundances
between regions close to the transition point and regions far from that point
is large, and could be a source to polarization patterns in the emitted light.
We conclude that the spatial and temporal distribution of transition locations,
is an important parameter which must be included in delayed detonation models
for Type Ia supernovae. \Comment: 11 pages, 1 figur
Synthetic Spectra of Hydrodynamic Models of Type Ia Supernovae
We present detailed NLTE synthetic spectra of hydrodynamic SNe Ia models. We
make no assumptions about the form of the spectrum at the inner boundary. We
calculate both Chandrasekhar-mass deflagration models and sub-Chandrasekhar
``helium detonators.'' Gamma-ray deposition is handled in a simple, accurate
manner. We have parameterized the storage of energy that arises from the time
dependent deposition of radioactive decay energy in a reasonable manner, that
spans the expected range. We find that the Chandrasekhar-mass deflagration
model W7 of Nomoto etal shows good agreement with the observed spectra of SN
1992A and SN 1994D, particularly in the UV, where our models are expected to be
most accurate. The sub-Chandrasekhar models do not reproduce the UV deficit
observed in normal SNe Ia. They do bear some resemblance to sub-luminous SNe
Ia, but the shape of the spectra (i.e. the colors) are opposite to that of the
observed ones and the intermediate mass element lines such as Si II, and Ca II
are extremely weak, which seems to be a generic difficulty of the models.
Although the sub-Chandrasekhar models have a significant helium abundance
(unlike Chandrasekhar-mass models), helium lines are not prominent in the
spectra near maximum light and thus do not act as a spectral signature for the
progenitor.Comment: submitted to ApJ, 26 pages, 10 figures, uses LaTeX styles aasms4.sty
and natbib.sty Also available at: http://www.nhn.ou.edu/~baron
Recommended from our members
Gamma-Ray Light Curves and Spectra of Models for Type-Ia Supernovae
Based on detailed Monte Carlo calculations, we present gamma-ray energy deposition functions, gamma-ray light curves, and gamma-ray spectra for a large set of theoretical models of Type Ia supernovae including ''classical'' detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of Ni-56 synthesized in the event can be determined on the basis of the gamma-ray light curves and spectra if gamma-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times gamma-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the gamma-ray flux by CGRO experiment for our understanding of SN 1991 T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc), i.e., several models can be ruled out by the gamma-ray observations.Astronom
Detailed Spectral Modeling of a 3-D Pulsating Reverse Detonation Model: Too Much Nickel
We calculate detailed NLTE synthetic spectra of a Pulsating Reverse
Detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae.
While the hydro models are calculated in 3-D, the spectra use an angle averaged
hydro model and thus some of the 3-D details are lost, but the overall average
should be a good representation of the average observed spectra. We study the
model at 3 epochs: maximum light, seven days prior to maximum light, and 5 days
after maximum light. At maximum the defining Si II feature is prominent, but
there is also a prominent C II feature, not usually observed in normal SNe Ia
near maximum. We compare to the early spectrum of SN 2006D which did show a
prominent C II feature, but the fit to the observations is not compelling.
Finally we compare to the post-maximum UV+optical spectrum of SN 1992A. With
the broad spectral coverage it is clear that the iron-peak elements on the
outside of the model push too much flux to the red and thus the particular PRD
realizations studied would be intrinsically far redder than observed SNe Ia. We
briefly discuss variations that could improve future PRD models.Comment: 15 pages, 4 figures, submitted to Ap
Properties of Deflagration Fronts and Models for Type Ia Supernovae
Detailed models of the explosion of a white dwarf, which include
self-consistent calculations of the light curve and spectra, provide a link
between observational quantities and the underlying explosion.These
calculations assume spherical geometry and are based on parameterized
descriptions of the burning front during the deflagration phase. Recently,
first multi-dimensional calculations for nuclear burning fronts have been
performed. Although a fully consistent treatment of the burning fronts is
beyond the current state of the art, these calculations provided a new and
better understanding of the physics, and new descriptions for the flame
propagation have been proposed. Here, we have studied the influence on the
results of previous analyses of Type Ia Supernovae, namely, the nucleosynthesis
and structure of the expanding envelope. Our calculations are based on a set of
delayed detonation models with parameters that give a good account of the
optical and infrared light curves, and of the spectral evolution. In this
scenario, the burning front propagates first in a deflagration mode and,
subsequently, turns into a detonation. The explosions and light curves are
calculated using a one-dimensional Lagrangian radiation-hydro code, including a
detailed nuclear network.Comment: 9 pages, 4 figures, macros 'crckapb.sty'. The Astrophysical Journal
(accepted
On the Origin of the Type Ia Supernova Width-Luminosity Relation
Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining
B-band light curves than dimmer SNe Ia. We study the physical origin of this
width-luminosity relation (WLR) using detailed radiative transfer calculations
of Chandrasekhar mass SN Ia models. We find that the luminosity dependence of
the diffusion time (emphasized in previous studies) is in fact of secondary
relevance in understanding the model WLR. Instead, the essential physics
involves the luminosity dependence of the spectroscopic/color evolution of SNe
Ia. Following maximum-light, the SN colors are increasingly affected by the
development of numerous Fe II/Co II lines which blanket the B-band and, at the
same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia
are generally cooler, they experience an earlier onset of Fe III to Fe II
recombination in the iron-group rich layers of ejecta, resulting in a more
rapid evolution of the SN colors to the red. The faster B-band decline rate of
dimmer SNe Ia thus reflects their faster ionization evolution.Comment: 6 pages, submitted to Ap
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Detonating Failed Deflagration Model of Thermonuclear Supernovae I. Explosion Dynamics
We present a detonating failed deflagration model of Type Ia supernovae. In
this model, the thermonuclear explosion of a massive white dwarf follows an
off-center deflagration. We conduct a survey of asymmetric ignition
configurations initiated at various distances from the stellar center. In all
cases studied, we find that only a small amount of stellar fuel is consumed
during deflagration phase, no explosion is obtained, and the released energy is
mostly wasted on expanding the progenitor. Products of the failed deflagration
quickly reach the stellar surface, polluting and strongly disturbing it. These
disturbances eventually evolve into small and isolated shock-dominated regions
which are rich in fuel. We consider these regions as seeds capable of forming
self-sustained detonations that, ultimately, result in the thermonuclear
supernova explosion. Preliminary nucleosynthesis results indicate the model
supernova ejecta are typically composed of about 0.1-0.25 Msun of silicon group
elements, 0.9-1.2 Msun of iron group elements, and are essentially carbon-free.
The ejecta have a composite morphology, are chemically stratified, and display
a modest amount of intrinsic asymmetry. The innermost layers are slightly
egg-shaped with the axis ratio ~1.2-1.3 and dominated by the products of
silicon burning. This central region is surrounded by a shell of silicon-group
elements. The outermost layers of ejecta are highly inhomogeneous and contain
products of incomplete oxygen burning with only small admixture of unburned
stellar material. The explosion energies are ~1.3-1.5 10^51 erg.Comment: ApJ in press; 21 pages, 36 figures at reduced resolution; high
resolution version available at
http://flash.uchicago.edu/~tomek/Papers/DFD_I_r2.pd
SN 2005hj: Evidence for Two Classes of Normal-Bright SNe Ia and Implications for Cosmology
HET Optical spectra covering the evolution from about 6 days before to about
5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the
"Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxy
shows HII region lines at redshift of z=0.0574, which puts the peak unfiltered
absolute magnitude at a somewhat over-luminous -19.6. The spectra show weak and
narrow SiII lines, and for a period of at least 10 days beginning around
maximum light these profiles do not change in width or depth and they indicate
a constant expansion velocity of ~10,600 km/s. We analyzed the observations
based on detailed radiation dynamical models in the literature. Whereas delayed
detonation and deflagration models have been used to explain the majority of
SNe Ia, they do not predict a long velocity plateau in the SiII minimum with an
unvarying line profile. Pulsating delayed detonations and merger scenarios form
shell-like density structures with properties mostly related to the mass of the
shell, M_shell, and we discuss how these models may explain the observed SiII
line evolution; however, these models are based on spherical calculations and
other possibilities may exist. SN 2005hj is consistent with respect to the
onset, duration, and velocity of the plateau, the peak luminosity and, within
the uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun.
Our analysis suggests a distinct class of events hidden within the
Branch-normal SNe Ia. If the predicted relations between observables are
confirmed, they may provide a way to separate these two groups. We discuss the
implications of two distinct progenitor classes on cosmological studies
employing SNe Ia, including possible differences in the peak luminosity to
light curve width relation.Comment: ApJ accepted, 31 page
Maximum Brightness and Post-Maximum Decline of Light Curves of SN~Ia: A Comparison of Theory and Observations
We compare the observed correlations between the maximum brightness,
postmaximum decline rate and color at maximum light of Type Ia supernovae (SN
Ia) with model predictions.
The observations are based on a total of 40 SN Ia with 29 SN of the Calan
Tololo Supernova Search and 11 local SN which cover a range of 2 mag in the
absolute visual brightness.
The observed correlations are not tight, one dimensional relations.
Supernovae with the same postmaximum decline or the same color have a spread in
visual magnitude of about 0.7 mag. The dispersion in the color-magnitude
relation may result from uncertainties in the distance determinations or the
interstellar reddening within the host galaxy. The dispersion in the decline
rate-magnitude relation suggests that an intrinsic spread in the supernova
properties exists that cannot be accounted for by any single relation between
visual brightness and postmaximum decline.
Theoretical correlations are derived from a grid of models which encompasses
delayed detonations, pulsating delayed detonations, the merging scenario and
helium detonations.
We find that the observed correlations can be understood in terms of
explosions of Chandrasekhar mass white dwarfs.
Our models show an intrinsic spread in the relations of about 0.5 mag in the
maximum brightness and about 0.1 mag in the B-V color.
Our study provides strong evidence against the mechanism of helium detonation
for subluminous, red SN Ia.Comment: 7 pages, 3 figures, macros ''aaspp.sty'. LaTeX Style. Astrophysical
Journal Letters, submitted Jul. 1995, revised Aug. 1995, resubmitted Sep.
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