706 research outputs found
Numerical Tests of Rotational Mixing in Massive Stars with the new Population Synthesis Code BONNFIRES
We use our new population synthesis code BONNFIRES to test how surface
abundances predicted by rotating stellar models depend on the numerical
treatment of rotational mixing, such as spatial resolution, temporal resolution
and computation of mean molecular weight gradients. We find that even with
identical numerical prescriptions for calculating the rotational mixing
coefficients in the diffusion equation, different timesteps lead to a deviation
of the coefficients and hence surface abundances. We find the surface
abundances vary by 10-100% between the model sequences with short timestep of
0.001Myr to model sequences with longer timesteps. Model sequences with
stronger surface nitrogen enrichment also have longer main-sequence lifetimes
because more hydrogen is mixed to the burning cores. The deviations in
main-sequence lifetimes can be as large as 20%. Mathematically speaking, no
numerical scheme can give a perfect solution unless infinitesimally small
timesteps are used. However, we find that the surface abundances eventually
converge within 10% between modelling sequences with sufficiently small
timesteps below 0.1Myr. The efficiency of rotational mixing depends on the
implemented numerical scheme and critically on the computation of the mean
molecular weight gradient. A smoothing function for the mean molecular weight
gradient results in stronger rotational mixing. If the discretization scheme or
the computational recipe for calculating the mean molecular weight gradient is
altered, re-calibration of mixing parameters may be required to fit
observations. If we are to properly understand the fundamental physics of
rotation in stars, it is crucial that we minimize the uncertainty introduced
into stellar evolution models when numerically approximating rotational mixing
processes.Comment: 8 pages, 6 figures, accepted by A&
BONNSAI: a Bayesian tool for comparing stars with stellar evolution models
Powerful telescopes equipped with multi-fibre or integral field spectrographs
combined with detailed models of stellar atmospheres and automated fitting
techniques allow for the analysis of large number of stars. These datasets
contain a wealth of information that require new analysis techniques to bridge
the gap between observations and stellar evolution models. To that end, we
develop BONNSAI (BONN Stellar Astrophysics Interface), a Bayesian statistical
method, that is capable of comparing all available observables simultaneously
to stellar models while taking observed uncertainties and prior knowledge such
as initial mass functions and distributions of stellar rotational velocities
into account. BONNSAI can be used to (1) determine probability distributions of
fundamental stellar parameters such as initial masses and stellar ages from
complex datasets, (2) predict stellar parameters that were not yet
observationally determined and (3) test stellar models to further advance our
understanding of stellar evolution. An important aspect of BONNSAI is that it
singles out stars that cannot be reproduced by stellar models through
hypothesis tests and posterior predictive checks. BONNSAI can be
used with any set of stellar models and currently supports massive
main-sequence single star models of Milky Way and Large and Small Magellanic
Cloud composition. We apply our new method to mock stars to demonstrate its
functionality and capabilities. In a first application, we use BONNSAI to test
the stellar models of Brott et al. (2011a) by comparing the stellar ages
inferred for the primary and secondary stars of eclipsing Milky Way binaries.
Ages are determined from dynamical masses and radii that are known to better
than 3%. We find that the stellar models reproduce the Milky Way binaries well.
BONNSAI is available through a web-interface at
http://www.astro.uni-bonn.de/stars/bonnsai.Comment: Accepted for publication in A&A; 15 pages, 10 figures, 4 tables;
BONNSAI is available through a web-interface at
http://www.astro.uni-bonn.de/stars/bonnsa
Super and massive AGB stars - IV. Final fates - Initial to final mass relation
We explore the final fates of massive intermediate-mass stars by computing
detailed stellar models from the zero age main sequence until near the end of
the thermally pulsing phase. These super-AGB and massive AGB star models are in
the mass range between 5.0 and 10.0 Msun for metallicities spanning the range
Z=0.02-0.0001. We probe the mass limits M_up, M_n and M_mass, the minimum
masses for the onset of carbon burning, the formation of a neutron star, and
the iron core-collapse supernovae respectively, to constrain the white
dwarf/electron-capture supernova boundary. We provide a theoretical initial to
final mass relation for the massive and ultra-massive white dwarfs and specify
the mass range for the occurrence of hybrid CO(Ne) white dwarfs. We predict
electron-capture supernova (EC-SN) rates for lower metallicities which are
significantly lower than existing values from parametric studies in the
literature. We conclude the EC-SN channel (for single stars and with the
critical assumption being the choice of mass-loss rate) is very narrow in
initial mass, at most approximately 0.2 Msun. This implies that between ~ 2-5
per cent of all gravitational collapse supernova are EC-SNe in the metallicity
range Z=0.02 to 0.0001. With our choice for mass-loss prescription and computed
core growth rates we find, within our metallicity range, that CO cores cannot
grow sufficiently massive to undergo a Type 1.5 SN explosion.Comment: 15 pages, 7 figures, accepted for publication in MNRA
The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars
We investigate the behaviour of asymptotic giant branch (AGB) stars between
metallicities Z = 10-4 and Z = 10-8 . We determine which stars undergo an
episode of flash-driven mixing, where protons are ingested into the intershell
convection zone, as they enter the thermally pulsing AGB phase and which
undergo third dredge-up. We find that flash-driven mixing does not occur above
a metallicity of Z = 10-5 for any mass of star and that stars above 2 M do not
experience this phenomenon at any metallicity. We find carbon ingestion (CI),
the mixing of carbon into the tail of hydrogen burning region, occurs in the
mass range 2 M to around 4 M . We suggest that CI may be a weak version of the
flash-driven mechanism. We also investigate the effects of convective
overshooting on the behaviour of these objects. Our models struggle to explain
the frequency of CEMP stars that have both significant carbon and nitrogen
enhancement. Carbon can be enhanced through flash-driven mixing, CI or just
third dredge up. Nitrogen can be enhanced through hot bottom burning and the
occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio
may be a good indicator of the mass of the primary AGB stars.Comment: 15 pages, 13 figures, 1 table, accepted by MNRA
Spindown of massive rotating stars
Models of rapidly rotating massive stars at low metallicities show
significantly different evolution and higher metal yields compared to
non-rotating stars. We estimate the spin-down time-scale of rapid rotating
non-convective stars supporting an alpha-Omega dynamo. The magnetic dynamo
gives rise to mass loss in a magnetically controlled stellar wind and hence
stellar spin down owing to loss of angular momentum. The dynamo is maintained
by strong horizontal rotation-driven turbulence which dominates over the Parker
instability. We calculate the spin-down time-scale and find that it could be
relatively short, a small fraction of the main-sequence lifetime. The spin-down
time-scale decreases dramatically for higher surface rotations suggesting that
rapid rotators may only exhibit such high surface velocities for a short time,
only a small fraction of their main-sequence lifetime.Comment: Accepted by MNRA
V605 Aquilae: a born again star, a nova or both?
V605 Aquilae is today widely assumed to have been the result of a final
helium shell flash occurring on a single post-asymptotic giant branch star. The
fact that the outbursting star is in the middle of an old planetary nebula and
that the ejecta associated with the outburst is hydrogen deficient supports
this diagnosis. However, the material ejected during that outburst is also
extremely neon rich, suggesting that it derives from an oxygen-neon-magnesium
star, as is the case in the so-called neon novae. We have therefore attempted
to construct a scenario that explains all the observations of the nebula and
its central star, including the ejecta abundances. We find two scenarios that
have the potential to explain the observations, although neither is a perfect
match. The first scenario invokes the merger of a main sequence star and a
massive oxygen-neon-magnesium white dwarf. The second invokes an
oxygen-neon-magnesium classical nova that takes place shortly after a final
helium shell flash. The main drawback of the first scenario is the inability to
determine whether the ejecta would have the observed composition and whether a
merger could result in the observed hydrogen-deficient stellar abundances
observed in the star today. The second scenario is based on better understood
physics, but, through a population synthesis technique, we determine that its
frequency of occurrence should be very low and possibly lower than what is
implied by the number of observed systems. While we could not envisage a
scenario that naturally explains this object, this is the second final flash
star which, upon closer scrutiny, is found to have hydrogen-deficient ejecta
with abnormally high neon abundances. These findings are in stark contrast with
the predictions of the final helium shell flash and beg for an alternative
explanation.Comment: 8 pages, 1 figures, 2 tables, accepted for MNRAS. Better title and
minor corrections compared to previous versio
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