1,208 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
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
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
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
On the Evolution of Thermonuclear Flames on Large Scales
The thermonuclear explosion of a massive white dwarf in a Type Ia supernova
explosion is characterized by vastly disparate spatial and temporal scales. The
extreme dynamic range inherent to the problem prevents the use of direct
numerical simulation and forces modelers to resort to subgrid models to
describe physical processes taking place on unresolved scales.
We consider the evolution of a model thermonuclear flame in a constant
gravitational field on a periodic domain. The gravitational acceleration is
aligned with the overall direction of the flame propagation, making the flame
surface subject to the Rayleigh-Taylor instability. The flame evolution is
followed through an extended initial transient phase well into the steady-state
regime. The properties of the evolution of flame surface are examined. We
confirm the form of the governing equation of the evolution suggested by
Khokhlov (1995). The mechanism of vorticity production and the interaction
between vortices and the flame surface are discussed. The results of our
investigation provide the bases for revising and extending previous
subgrid-scale model.Comment: 15 pages, 22 postscript figures. Accepted for publication by the
Astrophysical Journal. High resolution figures can be found at
http://flash.uchicago.edu/~zhang/research_paper.htm
A Physical Model for SN 2001ay, a normal, bright, extremely slowly declining Type Ia supernova
We present a study of the peculiar Type Ia supernova 2001ay (SN 2001ay). The
defining features of its peculiarity are: high velocity, broad lines, and a
fast rising light curve, combined with the slowest known rate of decline. It is
one magnitude dimmer than would be predicted from its observed value of
Delta-m15, and shows broad spectral features. We base our analysis on detailed
calculations for the explosion, light curves, and spectra. We demonstrate that
consistency is key for both validating the models and probing the underlying
physics. We show that this SN can be understood within the physics underlying
the Delta-m15 relation, and in the framework of pulsating delayed detonation
models originating from a Chandrasekhar mass, white dwarf, but with a
progenitor core composed of 80% carbon. We suggest a possible scenario for
stellar evolution which leads to such a progenitor. We show that the unusual
light curve decline can be understood with the same physics as has been used to
understand the Delta-m15 relation for normal SNe Ia. The decline relation can
be explained by a combination of the temperature dependence of the opacity and
excess or deficit of the peak luminosity, alpha, measured relative to the
instantaneous rate of radiative decay energy generation. What differentiates SN
2001ay from normal SNe Ia is a higher explosion energy which leads to a shift
of the Ni56 distribution towards higher velocity and alpha < 1. This result is
responsible for the fast rise and slow decline. We define a class of SN
2001ay-like SNe Ia, which will show an anti-Phillips relation.Comment: 35 pages, 14 figures, ApJ, in pres
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.
199
Constraints On The Delayed Transition to Detonation in Type Ia Supernovae
We investigate the possibility of a delayed detonation in a type Ia supernova
under the assumption that the transition to detonation is triggered by
turbulence only. Our discussion is based on the Zeldovich mechanism and
suggests that typical turbulent velocities present during the explosion are not
strong enough to allow this transition to occur. Although we are able to show
that in carbon-rich matter (e.g., C) the possibility of a
deflagration to detonation transition (DDT) is enhanced, even in this case the
turbulent velocities needed are larger than the expected value of on a length-scale of cm. Thus we
conclude that a DDT may not be a common event during a thermonuclear explosion
of a Chandrasekhar-mass white dwarf.Comment: 18 pages, 5 figures, accepted for publication in the Ap
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