246 research outputs found
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&
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
Going from 3D to 1D: A one-dimensional approach to common-envelope evolution
The common-envelope (CE) phase is a crucial stage in binary star evolution
because the orbital separation can shrink drastically while ejecting the
envelope of a giant star. Three-dimensional (3D) hydrodynamic simulations of CE
evolution are indispensable to learning about the mechanisms that play a role
during the CE phase. While these simulations offer great insight, they are
computationally expensive. We propose a one-dimensional (1D) model to simulate
the CE phase within the stellar evolution code by using a
parametric drag force prescription for dynamical drag and adding the released
orbital energy as heat into the envelope. We compute CE events of a
asymptotic giant-branch star and a point mass
companion with mass ratios of 0.25, 0.50, and 0.75, and compare them to 3D
simulations of the same setup. The 1D CE model contains two free parameters,
which we demonstrate are both needed to fit the spiral-in behavior and the
fraction of ejected envelope mass of the 1D method to the 3D simulations. For
mass ratios of 0.25 and 0.50, we find good-fitting 1D simulations, while for a
mass ratio of 0.75, we do not find a satisfactory fit to the 3D simulation as
some of the assumptions in the 1D method are no longer valid. In all our
simulations, we find that the released recombination energy is important to
accelerate the envelope and drive the ejection.Comment: Accepted for publication in A&A, 16 pages, 9 figure
Faint calcium-rich transient from the double-detonation of a carbon-oxygen white dwarf star
We have computed a three-dimensional hydrodynamic simulation of the merger
between a massive () helium white dwarf (He WD) and a low-mass
() carbon-oxygen white dwarf (CO WD). Despite the low mass of the
primary, the merger triggers a thermonuclear explosion as a result of a double
detonation, producing a faint transient and leaving no remnant behind. This
type of event could also take place during common-envelope mergers whenever the
companion is a CO WD and the core of the giant star has a sufficiently large He
mass. The spectra show strong Ca lines throughout the first few weeks after the
explosion. The explosion only yields of Ni, resulting
in a low-luminosity SN Ia-like lightcurve that resembles the Ca-rich transients
within this broad class of objects, with a peak magnitude of mag and a rather slow decline rate of mag. Both, its lightcurve-shape and spectral
appearance, resemble the appearance of Ca-rich transients, suggesting such
mergers as a possible progenitor scenario for this class of events.Comment: Submitted to A&A letters, posted on ArXiv after positive referee
report. 7 pages, 4 figure
Scalable stellar evolution forecasting: Deep learning emulation vs. hierarchical nearest neighbor interpolation
Many astrophysical applications require efficient yet reliable forecasts of
stellar evolution tracks. One example is population synthesis, which generates
forward predictions of models for comparison with observations. The majority of
state-of-the-art population synthesis methods are based on analytic fitting
formulae to stellar evolution tracks that are computationally cheap to sample
statistically over a continuous parameter range. Running detailed stellar
evolution codes, such as MESA, over wide and densely sampled parameter grids is
prohibitively expensive computationally, while stellar-age based linear
interpolation in-between sparsely sampled grid points leads to intolerably
large systematic prediction errors. In this work, we provide two solutions of
automated interpolation methods that find satisfactory trade-off points between
cost-efficiency and accuracy. We construct a timescale-adapted evolutionary
coordinate and use it in a two-step interpolation scheme that traces the
evolution of stars from zero age main sequence all the way to the end of core
helium burning while covering a mass range from to . The feedforward neural network regression model (first
solution) that we train to predict stellar surface variables can make millions
of predictions, sufficiently accurate over the entire parameter space, within
tens of seconds on a 4-core CPU. The hierarchical nearest neighbor
interpolation algorithm (second solution) that we hard-code to the same end
achieves even higher predictive accuracy, the same algorithm remains applicable
to all stellar variables evolved over time, but it is two orders of magnitude
slower. Our methodological framework is demonstrated to work on the MIST data
set. Finally, we discuss prospective applications and provide guidelines how to
generalize our methods to higher dimensional parameter spaces.Comment: Submitted to A&
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
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
A quantitative evaluation of the extent of fluralaner uptake by ticks (Ixodes ricinus, Ixodes scapularis) in fluralaner (Bravecto TM ) treated vs. untreated dogs using the parameters tick weight and coxal index
Background Fluralaner is a new antiparasitic drug that was recently introduced
as Bravecto TM chewable tablets for the treatment of tick and flea
infestations in dogs. Most marketed tick products exert their effect via
topical application and contact exposure to the parasite. In contrast,
Bravecto TM delivers its acaricidal activity through systemic exposure. Tick
exposure to fluralaner occurs after attachment to orally treated dogs, which
induces a tick-killing effect within 12 h. The fast onset of killing lasts
over the entire treatment interval (12 weeks) and suggests that only marginal
uptake by ticks is required to induce efficacy. Three laboratory studies were
conducted to quantify the extent of uptake by comparison of ticks’ weight and
coxal index obtained from Bravecto TM -treated and negative-control dogs.
Methods Three studies were conducted using experimental tick infestation with
either Ixodes ricinus or Ixodes scapularis after oral administration of
fluralaner to dogs. All studies included a treated (Bravecto TM chewable
tablets, MSD Animal Health) and a negative control group. Each study had a
similar design for assessing vitality and weighing of ticks collected from
dogs of both groups. Additionally, in one study the coxal index (I. ricinus)
was calculated as a ratio of tick’s ventral coxal gap and dorsal width of
scutum. Tick weight data and coxal indices from Bravecto TM -treated and
negative-control groups were compared via statistical analysis. Results Ticks
collected from Bravecto TM -treated dogs weighed significantly less (p ≤
0.0108) than ticks collected from negative-control dogs, and their coxal index
was also significantly lower (p < 0.0001). The difference in tick weights was
demonstrated irrespective of the tick species investigated (I. ricinus, I.
scapularis). At some assessments the mean tick weights of Bravecto TM -treated
dogs were significantly lower than those of unfed pre-infestation (baseline)
ticks. The demonstrated tick-killing efficacy was in the range of 94.6 – 100
%. Conclusions Tick weights and coxal indices confirm that a minimal uptake
results in a sufficient exposure of ticks to fluralaner (Bravecto TM ) and
consequently in a potent acaricidal effect
Thermonuclear explosions of rapidly differentially rotating white dwarfs: Candidates for superluminous Type Ia supernovae?
The observed sub-class of "superluminous" Type Ia supernovae lacks a
convincing theoretical explanation. If the emission of such objects were
powered exclusively by radioactive decay of 56Ni formed in the explosion, a
progenitor mass close to or even above the Chandrasekhar limit for a
non-rotating white dwarf star would be required. Masses significantly exceeding
this limit can be supported by differential rotation. We, therefore, explore
explosions and predict observables for various scenarios resulting from
differentially rotating carbon-oxygen white dwarfs close to their respective
limit of stability. Specifically, we have investigated a prompt detonation
model, detonations following an initial deflagration phase ("delayed
detonation" models), and a pure deflagration model. In postprocessing steps, we
performed nucleosynthesis and three-dimensional radiative transfer
calculations, that allow us, for the first time, to consistently derive
synthetic observables from our models. We find that all explosion scenarios
involving detonations produce very bright events. The observables predicted for
them, however, are inconsistent with any known subclass of Type Ia supernovae.
Pure deflagrations resemble 2002cx-like supernovae and may contribute to this
class. We discuss implications of our findings for the explosion mechanism and
for the existence of differentially rotating white dwarfs as supernova
progenitors.Comment: 12 pages, 9 figures, 2 tables, accepted for publication in A&A. Model
data are available from the Heidelberg Supernova Model Archive (HESMA) at
https://hesma.h-its.org
Long-term evolution of a magnetic massive merger product
About 10% of stars more massive than have
strong, large-scale surface magnetic fields and are being discussed as
progenitors of highly-magnetic white dwarfs and magnetars. The origin of these
fields remains uncertain. Recent 3D magnetohydrodynamical simulations have
shown that strong magnetic fields can be generated in the merger of two massive
stars. Here, we follow the long-term evolution of such a 3D merger product in a
1D stellar evolution code. During a thermal relaxation phase after the
coalescence, the merger product reaches critical surface rotation, sheds mass
and then spins down primarily because of internal mass readjustments. The spin
of the merger product after thermal relaxation is mainly set by the
co-evolution of the star-torus structure left after coalescence. This evolution
is still uncertain, so we also consider magnetic braking and other
angular-momentum-gain and -loss mechanisms that may influence the final spin of
the merged star. Because of core compression and mixing of carbon and nitrogen
in the merger, enhanced nuclear burning drives a transient convective core that
greatly contributes to the rejuvenation of the star. Once the merger product
relaxed back to the main sequence, it continues its evolution similar to that
of a genuine single star of comparable mass. It is a slow rotator that matches
the magnetic blue straggler Sco. Our results show that merging is a
promising mechanism to explain some magnetic massive stars and it may also be
key to understand the origin of the strong magnetic fields of highly-magnetic
white dwarfs and magnetars.Comment: 17 pages (incl. appendix), 14 figures, 2 tables; accepted for
publication in MNRA
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