227 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&
The occurrence of classical Cepheids in binary systems
Classical Cepheids, like binary stars, are laboratories for stellar evolution
and Cepheids in binary systems are especially powerful ones. About one-third of
Galactic Cepheids are known to have companions and Cepheids in eclipsing binary
systems have recently been discovered in the Large Magellanic Cloud. However,
there are no known Galactic binary Cepheids with orbital periods less than one
year. We compute population synthesis models of binary Cepheids to compare to
the observed period and eccentricity distributions of Galactic Cepheids as well
as to the number of observed eclipsing binary Cepheids in the LMC. We find that
our population synthesis models are consistent with observed binary properties
of Cepheids. Furthermore, we show that binary interaction on the red giant
branch prevents some red giant stars from becoming classical Cepheids. Such
interactions suggest that the binary fraction of Cepheids should be
significantly less than that of their main-sequence progenitors, and that
almost all binary Cepheids have orbital periods longer than one year. If the
Galactic Cepheid spectroscopic binary fraction is about 35%, then the
spectroscopic binary fraction of their intermediate mass main sequence
progenitors is about 40-45%.Comment: 7 pages, 3 figures, resubmitted to 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
The HD5980 multiple system: Masses and evolutionary status
New spectroscopic observations of the LBV/WR multiple system HD5980 in the
Small Magellanic Cloud are used to address the question of the masses and
evolutionary status of the two very luminous stars in the 19.3d eclipsing
binary system. Two distinct components of the N V 4944 A line are detected in
emission and their radial velocity variations are used to derive masses of 61
and 66 Mo, under the assumption that binary interaction effects on this atomic
transition are negligible. We propose that this binary system is the product of
quasi-chemically homogeneous evolution with little or no mass transfer. Thus,
both of these binary stars may be candidates for gamma-ray burst progenitors or
even pair instability supernovae. Analysis of the photospheric absorption lines
belonging to the third-light object in the system confirm that it consists of
an O-type star in a 96.56d eccentric orbit (e=0.82) around an unseen companion.
The 5:1 period ratio and high eccentricities of the two binaries suggest that
they may constitute a hierarchical quadruple system.Comment: 27 pages, 8 tables, 15 figures; accepted A
The Art of Modeling Stellar Mergers and the Case of the B[e] Supergiant R4 in the Small Magellanic Cloud
Most massive stars exchange mass with a companion, leading to evolution which
is altered drastically from that expected of stars in isolation. Such systems
are the result of unusual binary evolution pathways and, as such, may be used
to place stringent constraints on the physics of these interactions. We use the
R4 system's B[e] supergiant, which has been postulated to be the product of a
binary stellar merger, to guide our understanding of such outcomes by comparing
observations of R4 to the results of simulations of mergers performed with the
3d hydrodynamics code FLASH. Our approach tailors the simulation initial
conditions to the observed properties of R4 and implements realistic stellar
profiles generated by the 1d stellar evolution code MESA onto the 3d grid,
resolving the merger inspiral to within . We then map the
merger remnant into MESA to track its evolution on the HR diagram over a period
of years. This generates models for a B[e] supergiant with stellar
properties, age, and nebula structure in qualitative agreement with that of the
R4 system. Our calculations provide concrete evidence to support the idea that
R4 was originally a member of a triple system in which the inner binary merged
after its most massive member evolved off the main sequence, producing a new
object that is of similar mass yet significantly more luminous than the A
supergiant companion. The potential applications of the code framework
presented in this paper are wide ranging and can be used to generate models of
a variety of merger stellar remnants
Art of Modeling Stellar Mergers and the Case of the B[e] Supergiant R4 in the Small Magellanic Cloud
Most massive stars exchange mass with a companion, leading to evolution which is altered drastically from that expected of stars in isolation. Such systems result from unusual binary evolution pathways and can place stringent constraints on the physics of these interactions. We use the R4 binary system's B[e] supergiant, which has been postulated to be the product of a stellar merger, to guide our understanding of such outcomes by comparing observations of R4 to the results of simulating a merger with the 3D hydrodynamics code FLASH. Our approach tailors the simulation initial conditions to observed properties of R4 and implements realistic stellar profiles from the 1D stellar evolution code MESA onto the 3D grid, resolving the merger inspiral to within 0.02 R⊙. We map the merger remnant into MESA to track its evolution on the H-R diagram over a period of 10⁴ yr. This generates a model for a B[e] supergiant with stellar properties, age, and nebula structure in qualitative agreement with those of the R4 system. Our calculations provide evidence to support the idea that R4's B[e] supergiant was originally a member of a triple system in which the inner binary merged after its most massive member evolved off the main sequence, producing a new object of similar mass but significantly more luminosity than the A supergiant companion. The code framework presented in this paper, which was constructed to model tidal encounters, can be used to generate accurate models of a wide variety of merger stellar remnants
Evolution and final fate of massive post-common-envelope binaries
Mergers of neutron stars (NSs) and black holes (BHs) are nowadays observed
routinely thanks to gravitational-wave (GW) astronomy. In the isolated
binary-evolution channel, a common-envelope (CE) phase of a red supergiant
(RSG) and a compact object is crucial to sufficiently shrink the orbit and
thereby enable a merger via GW emission. Here, we use the outcomes of two
three-dimensional (3D) magneto-hydrodynamic CE simulations of an initially 10.0
solar-mass RSG with a 5.0 solar-mass BH and a 1.4 solar-mass NS, respectively,
to explore the further evolution and final fate of the post-CE binaries.
Notably, the 3D simulations reveal that the post-CE binaries are likely
surrounded by circumbinary disks (CBDs), which contain substantial mass and
angular momentum to influence the subsequent evolution. The binary systems in
MESA modelling undergo another phase of mass transfer (MT) and we find that
most donor stars do not explode in ultra-stripped supernovae (SNe), but rather
in Type Ib/c SNe. The final orbits of our models with the BH companion are too
wide, and NS kicks are actually required to sufficiently perturb the orbit and
thus facilitate a merger via GW emission. Moreover, by exploring the influence
of CBDs, we find that mass accretion from the disk widens the binary orbit,
while CBD-binary resonant interactions can shrink the separation and increase
the eccentricity depending on the disk mass and lifetime. Efficient resonant
contractions may even enable NS/BH to merge with the remnant He stars before a
second SN explosion, which may be observed as gamma-ray burst-like transients,
luminous fast blue optical transients and Thorne-\.Zytkow objects. For the
surviving post-CE binaries, the CBD-binary interactions may significantly
increase the GW-induced double compact merger fraction. We conclude that
accounting for CBD may be crucial to better understand observed GW mergers.Comment: 19 pages, 19 figures, 1 table. Submitted to A&A, comments welcom
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