728 research outputs found
Helium in Double-Detonation Models of Type Ia Supernovae
The double-detonation explosion model has been considered a candidate for
explaining astrophysical transients with a wide range of luminosities. In this
model, a carbon-oxygen white dwarf star explodes following detonation of a
surface layer of helium. One potential signature of this explosion mechanism is
the presence of unburned helium in the outer ejecta, left over from the surface
helium layer. In this paper we present simple approximations to estimate the
optical depths of important He I lines in the ejecta of double-detonation
models. We use these approximations to compute synthetic spectra, including the
He I lines, for double-detonation models obtained from hydrodynamical explosion
simulations. Specifically, we focus on photospheric-phase predictions for the
near-infrared 10830 \AA~and 2 m lines of He I. We first consider a double
detonation model with a luminosity corresponding roughly to normal SNe Ia. This
model has a post-explosion unburned He mass of 0.03 and our
calculations suggest that the 2 m feature is expected to be very weak but
that the 10830 \AA~feature may have modest opacity in the outer ejecta.
Consequently, we suggest that a moderate-to-weak He I 10830 \AA~feature may be
expected to form in double-detonation explosions at epochs around maximum
light. However, the high velocities of unburned helium predicted by the model
(~km~s) mean that the He I 10830 \AA~feature may be
confused or blended with the C I 10690~\AA~line forming at lower velocities. We
also present calculations for the He I 10830 \AA~and 2 m lines for a lower
mass (low luminosity) double detonation model, which has a post-explosion He
mass of 0.077 . In this case, both the He I features we consider are
strong and can provide a clear observational signature of the double-detonation
mechanism.Comment: 12 pages, 11 figures, accepted by A&
The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa
The UV spectra of nova-like variables are dominated by emission from the
accretion disk, modified by scattering in a wind emanating from the disk. Here
we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes
which have been observed with the Hubble Space Telescope in the
far-ultraviolet, in an attempt to constrain the geometry and the ionization
structure of their winds. Using our Monte Carlo radiative transfer code we
computed spectra for simply-parameterized axisymmetric biconical outflow models
and were able to find plausible models for both systems. These reproduce the
primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in
and out of eclipse. The distribution of these ions in the wind models is
similar in both cases as is the extent of the primary scattering regions in
which these lines are formed. The inferred mass loss rates are 6% to 8% of the
mass accretion rates for the systems. We discuss the implication of our point
models for our understanding of accretion disk winds in cataclysmic variables.Comment: 13 pages, 15 figures and 4 tables. Published in Ap
On the relativistic iron line and soft excess in the Seyfert 1 galaxy Markarian 335
We report on a 133 ks XMM-Newton observation of the Seyfert 1 galaxy
Markarian 335. The 0.4-12 keV spectrum contains an underlying power law
continuum, a soft excess below 2 keV, and a double-peaked iron emission feature
in the 6-7 keV range. We investigate the possibility that the double-peaked
emission might represent the characteristic signature of the accretion disc.
Detailed investigations show that a moderately broad, accretion disc line is
most likely present, but that the peaks may be owing to narrower components
from more distant material. The peaks at 6.4 and 7 keV can be identified,
respectively, with the molecular torus in active galactic nucleus unification
schemes, and very highly ionized, optically thin gas filling the torus. The
X-ray variability spectra on both long (~100 ks) and short (~1 ks) timescales
disfavour the recent suggestion that the soft excess is an artifact of
variable, moderately ionized absorption.Comment: 6 pages, 2 figures, accepted for publication in MNRA
The Impact of Accretion Disk Winds on the Optical Spectra of Cataclysmic Variables
Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted
absorption or P-Cygni profiles associated with ultraviolet (UV) resonance
lines. These features imply the existence of powerful accretion disk winds in
CVs. Here, we use our Monte Carlo ionization and radiative transfer code to
investigate whether disk wind models that produce realistic UV line profiles
are also likely to generate observationally significant recombination line and
continuum emission in the optical waveband. We also test whether outflows may
be responsible for the single-peaked emission line profiles often seen in
high-state CVs and for the weakness of the Balmer absorption edge (relative to
simple models of optically thick accretion disks). We find that a standard disk
wind model that is successful in reproducing the UV spectra of CVs also leaves
a noticeable imprint on the optical spectrum, particularly for systems viewed
at high inclination. The strongest optical wind-formed recombination lines are
H and He II . We demonstrate that a higher-density outflow
model produces all the expected H and He lines and produces a recombination
continuum that can fill in the Balmer jump at high inclinations. This model
displays reasonable verisimilitude with the optical spectrum of RW Trianguli.
No single-peaked emission is seen, although we observe a narrowing of the
double-peaked emission lines from the base of the wind. Finally, we show that
even denser models can produce a single-peaked H line. On the basis of
our results, we suggest that winds can modify, and perhaps even dominate, the
line and continuum emission from CVs.Comment: 15 pages, 13 figures. Accepted to MNRA
Line-driven Disk Winds in Active Galactic Nuclei: The Critical Importance of Ionization and Radiative Transfer
Accretion disk winds are thought to produce many of the characteristic
features seen in the spectra of active galactic nuclei (AGN) and quasi-stellar
objects (QSOs). These outflows also represent a natural form of feedback
between the central supermassive black hole and its host galaxy. The mechanism
for driving this mass loss remains unknown, although radiation pressure
mediated by spectral lines is a leading candidate. Here, we calculate the
ionization state of, and emergent spectra for, the hydrodynamic simulation of a
line-driven disk wind previously presented by Proga & Kallman (2004). To
achieve this, we carry out a comprehensive Monte Carlo simulation of the
radiative transfer through, and energy exchange within, the predicted outflow.
We find that the wind is much more ionized than originally estimated. This is
in part because it is much more difficult to shield any wind regions
effectively when the outflow itself is allowed to reprocess and redirect
ionizing photons. As a result, the calculated spectrum that would be observed
from this particular outflow solution would not contain the ultraviolet
spectral lines that are observed in many AGN/QSOs. Furthermore, the wind is so
highly ionized that line-driving would not actually be efficient. This does not
necessarily mean that line-driven winds are not viable. However, our work does
illustrate that in order to arrive at a self-consistent model of line-driven
disk winds in AGN/QSO, it will be critical to include a more detailed treatment
of radiative transfer and ionization in the next generation of hydrodynamic
simulations.Comment: 13 pages, 10 figures - Accepted for publication in Ap
Type Iax SNe as a few-parameter family
We present direct spectroscopic modeling of five Type Iax supernovae (SNe)
with the one dimensional Monte Carlo radiative transfer code TARDIS. The
abundance tomography technique is used to map the chemical structure and
physical properties of the SN atmosphere. Through via fitting of multiple
spectral epochs with self-consistent ejecta models, we can then constrain the
location of some elements within the ejecta. The synthetic spectra of the
best-fit models are able to reproduce the flux continuum and the main
absorption features in the whole sample. We find that the mass fractions of
IGEs and IMEs show a decreasing trend toward the outer regions of the
atmospheres using density profiles similar to those of deflagration models in
the literature. Oxygen is the only element, which could be dominant at higher
velocities. The stratified abundance structure contradicts the well-mixed
chemical profiles predicted by pure deflagration models. Based on the derived
densities and abundances, a template model atmosphere is created for the SN Iax
class and compared to the observed spectra. Free parameters are the scaling of
the density profile, the velocity shift of the abundance template, and the peak
luminosity. The results of this test support the idea that all SNe Iax can be
described by a similar internal structure, which argues for a common origin of
this class of explosions.Comment: 21 pages, 7 tables, 16 figures, accepted by MNRA
Type Ia supernovae from exploding oxygen-neon white dwarfs
The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most
of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs
(WDs), but for many of the explosion scenarios, particularly those involving
the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch
WD), there is also a possibility of having an oxygen-neon (ONe) WD as
progenitor. We simulate detonations of ONe WDs and calculate synthetic
observables from these models. The results are compared with detonations in CO
WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic
explosion simulations of detonations in initially hydrostatic ONe WDs for a
range of masses below the Chandrasekhar mass (M Ch), followed by detailed
nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The
results are used to calculate synthetic spectra and light curves, which are
then compared with observations of SNe Ia. We also perform binary evolution
calculations to determine the number of SNe Ia involving ONe WDs relative to
the number of other promising progenitor channels. The ejecta structures of our
simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs.
There are, however, small systematic deviations in the mass fractions and the
ejecta velocities. These lead to spectral features that are systematically less
blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions
are similar to those obtained from CO models. Our binary evolution calculations
show that a significant fraction (3-10%) of potential progenitor systems should
contain an ONe WD. The comparison of our ONe models with our CO models of
comparable mass (1.2 Msun) shows that the less blueshifted spectral features
fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure
2D simulations of the double-detonation model for thermonuclear transients from low-mass carbon-oxygen white dwarfs
Thermonuclear explosions may arise in binary star systems in which a carbon–oxygen (CO) white dwarf (WD) accretes helium-rich material from a companion star. If the accretion rate allows a sufficiently large mass of helium to accumulate prior to ignition of nuclear burning, the helium surface layer may detonate, giving rise to an astrophysical transient. Detonation of the accreted helium layer generates shock waves that propagate into the underlying CO WD. This might directly ignite a detonation of the CO WD at its surface (an edge-lit secondary detonation) or compress the core of the WD sufficiently to trigger a CO detonation near the centre. If either of these ignition mechanisms works, the two detonations (helium and CO) can then release sufficient energy to completely unbind the WD. These ‘double-detonation’ scenarios for thermonuclear explosion of WDs have previously been investigated as a potential channel for the production of Type Ia supernovae from WDs of ∼ 1  M⊙. Here we extend our 2D studies of the double-detonation model to significantly less massive CO WDs, the explosion of which could produce fainter, more rapidly evolving transients. We investigate the feasibility of triggering a secondary core detonation by shock convergence in low-mass CO WDs and the observable consequences of such a detonation. Our results suggest that core detonation is probable, even for the lowest CO core masses that are likely to be realized in nature. To quantify the observable signatures of core detonation, we compute spectra and light curves for models in which either an edge-lit or compression-triggered CO detonation is assumed to occur. We compare these to synthetic observables for models in which no CO detonation was allowed to occur. If significant shock compression of the CO WD occurs prior to detonation, explosion of the CO WD can produce a sufficiently large mass of radioactive iron-group nuclei to significantly affect the light curves. In particular, this can lead to relatively slow post-maximum decline. If the secondary detonation is edge-lit, however, the CO WD explosion primarily yields intermediate-mass elements that affect the observables more subtly. In this case, near-infrared observations and detailed spectroscopic analysis would be needed to determine whether a core detonation occurred. We comment on the implications of our results for understanding peculiar astrophysical transients including SN 2002bj, SN 2010X and SN 2005E
On duality for nonsmooth Lipschitz optimization problems
We present some duality theorems for a non-smooth Lipschitz vector optimization problem. Under generalized invexity assumptions on the functions the duality theorems do not require constraint qualifications
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