629 research outputs found
Towards an understanding of Type Ia supernovae from a synthesis of theory and observations
Motivated by the fact that calibrated light curves of Type Ia supernovae (SNe
Ia) have become a major tool to determine the expansion history of the
Universe, considerable attention has been given to, both, observations and
models of these events over the past 15 years. Here, we summarize new
observational constraints, address recent progress in modeling Type Ia
supernovae by means of three-dimensional hydrodynamic simulations, and discuss
several of the still open questions. It will be be shown that the new models
have considerable predictive power which allows us to study observable
properties such as light curves and spectra without adjustable non-physical
parameters. This is a necessary requisite to improve our understanding of the
explosion mechanism and to settle the question of the applicability of SNe Ia
as distance indicators for cosmology. We explore the capabilities of the models
by comparing them with observations and we show how such models can be applied
to study the origin of the diversity of SNe Ia.Comment: 26 pages, 13 figures, Frontiers of Physics, in prin
Three-dimensional simulations of the interaction between Type Ia supernova ejecta and their main sequence companions
The identity of the progenitor systems of SNe Ia is still uncertain. In the
single-degenerate (SD) scenario, the interaction between the SN blast wave and
the outer layers of a main sequence (MS) companion star strips off H-rich
material which is then mixed into the ejecta. Strong contamination of the SN
ejecta with stripped material could lead to a conflict with observations of SNe
Ia. This constrains the SD progenitor model. In this work, our previous
simulations based on simplified progenitor donor stars have been updated by
adopting more realistic progenitor-system models that result from fully
detailed, state-of-the-art binary evolution calculations. We use Eggleton's
stellar evolution code including the optically thick accretion wind model and
the possibility of the effects of accretion disk instabilities to obtain
realistic models of companions for different progenitor systems. The impact of
the SN blast wave on these companion stars is followed in three-dimensional
hydrodynamic simulations employing the SPH code GADGET3. We find that the
stripped masses range from 0.11 to 0.18 M_sun. The kick velocity is between 51
and 105 km/s. We find that the stripped mass and kick velocity depend on the
ratio of the orbital separation to the radius of a companion. They can be
fitted by a power law for a given companion model. However, the structure of
the companion star is also important for the amount of stripped material. With
more realistic companion star models than in previous studies, our simulations
show that the H masses stripped from companions are inconsistent with the best
observational limits (< 0.01 M_sun) derived from nebular spectra. However, a
rigorous forward modeling based on impact simulations with radiation transfer
is required to reliably predict observable signatures of the stripped H and to
conclusively assess the viability of the considered SN Ia progenitor scenario.Comment: 14 pages, 13 figures, accepted for publication by A&
Constraints on the origin of the first light from SN2014J
We study the very early lightcurve of supernova 2014J (SN 2014J) using the
high-cadence broad-band imaging data obtained by the Kilodegree Extremely
Little Telescope (KELT), which fortuitously observed M 82 around the time of
the explosion, starting more than two months prior to detection, with up to 20
observations per night. These observations are complemented by observations in
two narrow-band filters used in an H survey of nearby galaxies by the
intermediate Palomar Transient Factory (iPTF) that also captured the first days
of the brightening of the \sn. The evolution of the lightcurves is consistent
with the expected signal from the cooling of shock heated material of large
scale dimensions, \gsim 1 R_{\odot}. This could be due to heated material of
the progenitor, a companion star or pre-existing circumstellar environment,
e.g., in the form of an accretion disk. Structure seen in the lightcurves
during the first days after explosion could also originate from radioactive
material in the outer parts of an exploding white dwarf, as suggested from the
early detection of gamma-rays. The model degeneracy translates into a
systematic uncertainty of days on the estimate of the first light
from SN 2014J.Comment: Accepted by ApJ. Companion paper by Siverd et al, arXiv:1411.415
Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor
HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass
too low to ignite core helium burning and is therefore considered as a
pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M 0.3 Msun) are
He-core objects produced by the evolution of compact binary systems. We present
in this paper a detailed abundance analysis of HD 188112 based on
high-resolution Hubble Space Telescope (HST) near and far-ultraviolet
spectroscopy. We also constrain the mass of the star's companion. We use hybrid
non-LTE model atmospheres to fit the observed spectral lines and derive the
abundances of more than a dozen elements as well as the rotational broadening
of metallic lines. We confirm the previous binary system parameters by
combining radial velocities measured in our UV spectra with the already
published ones. The system has a period of 0.60658584 days and a WD companion
with M 0.70 Msun. By assuming a tidally locked rotation, combined with
the projected rotational velocity (v sin i = 7.9 0.3 km s) we
constrain the companion mass to be between 0.9 and 1.3 Msun. We further discuss
the future evolution of the system as a potential progenitor of a
(underluminous) type Ia supernova. We measure abundances for Mg, Al, Si, P, S,
Ca, Ti, Cr, Mn, Fe, Ni, and Zn, as well as for the trans-iron elements Ga, Sn,
and Pb. In addition, we derive upper limits for the C, N, O elements and find
HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE
effects on the line strength of some ionic species such as Si II and Ni II. The
metallic abundances indicate that the star is metal-poor, with an abundance
pattern most likely produced by diffusion effects.Comment: Accepted for publication in A&
No trace of a single-degenerate companion in late spectra of SNe 2011fe and 2014J
Left-over, ablated material from a possible non-degenerate companion can
reveal itself after about one year in spectra of Type Ia SNe (SNe Ia). We have
searched for such material in spectra of SN 2011fe (at 294 days after the
explosion) and for SN 2014J (315 days past explosion). The observations are
compared with numerical models simulating the expected line emission. The
spectral lines sought for are H-alpha, [O I] 6300 and [Ca II] 7291,7324, and
the expected width of these lines is about 1000 km/s. No signs of these lines
can be traced in any of the two supernovae. When systematic uncertainties are
included, the limits on hydrogen-rich ablated gas in SNe 2011fe and 2014J are
0.003 M_sun and 0.0085 M_sun, respectively, where the limit for SN 2014J is the
second lowest ever, and the limit for SN 2011fe is a revision of a previous
limit. Limits are also put on helium-rich ablated gas. These limits are used,
in conjunction with other data, to argue that these supernovae can stem from
double-degenerate systems, or from single-degenerate systems with a spun
up/spun down super-Chandrasekhar white dwarf. For SN 2011fe, other types of
hydrogen-rich donors can likely be ruled out, whereas for SN 2014J a
main-sequence donor system with large intrinsic separation is still possible.
Helium-rich donor systems cannot be ruled out for any of the two supernovae,
but the expected short delay time for such progenitors makes this possibility
less likely, especially for SN 2011fe. The broad [Ni II] 7378 emission in SN
2014J is redshifted by about +1300 km/s, as opposed to the known blueshift of
roughly -1100 km/s for SN 2011fe. [Fe II] 7155 is also redshifted in SN 2014J.
SN 2014J belongs to a minority of SNe Ia that both have a nebular redshift of
[Fe II] 7155 and [Ni II] 7378, and a slow decline of the Si II 6355 absorption
trough just after B-band maximum.Comment: 13 pages, submitted to A&
Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha
Stellar evolution models predict the existence of hybrid white dwarfs (WDs)
with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with
masses ~1.1 Msun, hybrid WDs in a binary system may easily approach the
Chandrasekhar mass (MCh) by accretion and give rise to a thermonuclear
explosion. Here, we investigate an off-centre deflagration in a near-MCh hybrid
WD under the assumption that nuclear burning only occurs in carbon-rich
material. Performing hydrodynamics simulations of the explosion and detailed
nucleosynthesis post-processing calculations, we find that only 0.014 Msun of
material is ejected while the remainder of the mass stays bound. The ejecta
consist predominantly of iron-group elements, O, C, Si and S. We also calculate
synthetic observables for our model and find reasonable agreement with the
faint Type Iax SN 2008ha. This shows for the first time that deflagrations in
near-MCh WDs can in principle explain the observed diversity of Type Iax
supernovae. Leaving behind a near-MCh bound remnant opens the possibility for
recurrent explosions or a subsequent accretion-induced collapse in faint Type
Iax SNe, if further accretion episodes occur. From binary population synthesis
calculations, we find the rate of hybrid WDs approaching MCh to be on the order
of 1 percent of the Galactic SN Ia rate.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in MNRA
The peculiar Type Ia supernova iPTF14atg: Chandrasekhar-mass explosion or violent merger?
iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN
2002es, is the first SN Ia for which a strong UV flash was observed in the
early-time light curves. This has been interpreted as evidence for a
single-degenerate (SD) progenitor system where such a signal is expected from
interactions between the SN ejecta and the non-degenerate companion star. Here,
we compare synthetic observables of multi-dimensional state-of-the-art
explosion models for different progenitor scenarios to the light curves and
spectra of iPTF14atg. From our models, we have difficulties explaining the
spectral evolution of iPTF14atg within the SD progenitor channel. In contrast,
we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and
0.76 solar masses, respectively, provides an excellent match to the spectral
evolution of iPTF14atg from 10d before to several weeks after maximum light.
Our merger model does not naturally explain the initial UV flash of iPTF14atg.
We discuss several possibilities like interactions of the SN ejecta with the
circum-stellar medium and surface radioactivity from a He ignited merger that
may be able to account for the early UV emission in violent merger models.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Gamma-ray diagnostics of Type Ia supernovae: Predictions of observables from three-dimensional modeling
Besides the fact that the gamma-ray emission due to radioactive decays is
responsible for powering the light curves of Type Ia supernovae (SNe Ia), gamma
rays themselves are of particular interest as a diagnostic tool because they
provide a direct way to obtain deeper insights into the nucleosynthesis and the
kinematics of these explosion events. Focusing on two of the most broadly
discussed SN Ia progenitor scenarios - a delayed detonation in a
Chandrasekhar-mass white dwarf (WD) and a violent merger of two WDs - we use
three-dimensional explosion models and perform radiative transfer simulations
to obtain synthetic gamma-ray spectra. Both chosen models produce the same mass
of 56Ni and have similar optical properties that are in reasonable agreement
with the recently observed supernova SN 2011fe. In contrast to the optical
regime, the gamma-ray emission of our two chosen models proves to be rather
different. The almost direct connection of the emission of gamma rays to
fundamental physical processes occuring in SNe Ia permits additional
constraints concerning several explosion model properties that are not easily
accessible within other wavelength ranges. Proposed future MeV missions such as
GRIPS will resolve all spectral details only for nearby SNe Ia, but hardness
ratio and light curve measurements still allow for a distinction of the two
different models at 10 and 16 Mpc for an exposure time of 10^6 s, respectively.
The possibility to detect the strongest line features up to the Virgo distance
will offer the opportunity to build up a first sample of SN Ia detections in
the gamma-ray energy range and underlines the importance of future space
observatories for MeV gamma rays.Comment: 10 pages, 8 figures, accepted for publication by A&
Prospect of Studying Hard X- and Gamma-Rays from Type Ia Supernovae
We perform multi-dimensional, time-dependent radiation transfer simulations
for hard X-ray and gamma-ray emissions, following radioactive decays of 56Ni
and 56Co, for two-dimensional delayed detonation models of Type Ia supernovae
(SNe Ia). The synthetic spectra and light curves are compared with the
sensitivities of current and future observatories for an exposure time of 10^6
seconds. The non-detection of the gamma-ray signal from SN 2011fe at 6.4 Mpc by
SPI on board INTEGRAL places an upper limit for the mass of 56Ni of \lesssim
1.0 Msun, independently from observations in any other wavelengths. Signals
from the newly formed radioactive species have not been convincingly measured
yet from any SN Ia, but the future X-ray and gamma-ray missions are expected to
deepen the observable horizon to provide the high energy emission data for a
significant SN Ia sample. We predict that the hard X-ray detectors on board
NuStar (launched in 2012) or ASTRO-H (scheduled for launch in 2014) will reach
to SNe Ia at \sim15 Mpc, i.e., one SN every few years. Furthermore, according
to the present results, the soft gamma-ray detector on board ASTRO-H will be
able to detect the 158 keV line emission up to \sim25 Mpc, i.e., a few SNe Ia
per year. Proposed next generation gamma-ray missions, e.g., GRIPS, could reach
to SNe Ia at \sim20 - 35 Mpc by MeV observations. Those would provide new
diagnostics and strong constraints on explosion models, detecting rather
directly the main energy source of supernova light.Comment: 14 pages, 7 figures, 1 table, accepted for publication in Ap
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