374 research outputs found
When Stars Collide
When two stars collide and merge they form a new star that can stand out
against the background population in a starcluster as a blue straggler. In so
called collision runaways many stars can merge and may form a very massive star
that eventually forms an intermediate mass blackhole. We have performed
detailed evolution calculations of merger remnants from collisions between main
sequence stars, both for lower mass stars and higher mass stars. These stars
can be significantly brighter than ordinary stars of the same mass due to their
increased helium abundance. Simplified treatments ignoring this effect give
incorrect predictions for the collision product lifetime and evolution in the
Hertzsprung-Russell diagram.Comment: 8 pages, 5 figures to appear in the proceedings for "Unsolved
Problems in Stellar Physics", Cambridge, 2-6 July 200
Building Blue Stragglers with Stellar Collisions
The evolution of stellar collision products in cluster simulations has
usually been modelled using simplified prescriptions. Such prescriptions either
replace the collision product with an (evolved) main sequence star, or assume
that the collision product was completely mixed during the collision.
It is known from hydrodynamical simulations of stellar collisions that
collision products are not completely mixed, however. We have calculated the
evolution of stellar collision products and find that they are brighter than
normal main sequence stars of the same mass, but not as blue as models that
assume that the collision product was fully mixed during the collision.Comment: 2 pages, 1 figure. To appear in the proceedings of Dynamical
Evolution of Dense Stellar Systems, IAU Symposium 24
Can low metallicity binaries avoid merging?
Rapid mass transfer in a binary system can drive the accreting star out of
thermal equilibrium, causing it to expand. This can lead to a contact system,
strong mass loss from the system and possibly merging of the two stars. In low
metallicity stars the timescale for heat transport is shorter due to the lower
opacity. The accreting star can therefore restore thermal equilibrium more
quickly and possibly avoid contact.
We investigate the effect of accretion onto main sequence stars with
radiative envelopes with different metallicities. We find that a low
metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher
accretion rate before it reaches a certain radius than a star at solar
metallicity. This could imply that up to two times fewer systems come into
contact during rapid mass transfer when we compare low metallicity. This factor
is uncertain due to the unknown distribution of binary parameters and the
dependence of the mass transfer timescale on metallicity. In a forthcoming
paper we will present analytic fits to models of accreting stars at various
metallicities intended for the use in population synthesis models.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 2 figure
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
AGB stars are responsible for producing a variety of elements, including
carbon, nitrogen, and the heavy elements produced in the slow neutron-capture
process (-elements). There are many uncertainties involved in modelling the
evolution and nucleosynthesis of AGB stars, and this is especially the case at
low metallicity, where most of the stars with high enough masses to enter the
AGB have evolved to become white dwarfs and can no longer be observed. The
stellar population in the Galactic halo is of low mass () and only a few observed stars have evolved beyond the first
giant branch. However, we have evidence that low-metallicity AGB stars in
binary systems have interacted with their low-mass secondary companions in the
past. The aim of this work is to investigate AGB nucleosynthesis at low
metallicity by studying the surface abundances of chemically peculiar very
metal-poor stars of the halo observed in binary systems. To this end we select
a sample of 15 carbon- and -element-enhanced metal-poor (CEMP-) halo
stars that are found in binary systems with measured orbital periods. With our
model of binary evolution and AGB nucleosynthesis, we determine the binary
configuration that best reproduces, at the same time, the observed orbital
period and surface abundances of each star of the sample. The observed periods
provide tight constraints on our model of wind mass transfer in binary stars,
while the comparison with the observed abundances tests our model of AGB
nucleosynthesis.Comment: 18 pages, 20 figures, accepted for publication on A&
Models for the Observable System Parameters of Ultraluminous X-ray Sources
We investigate the evolution of the properties of model populations of
ultraluminous X-ray sources (ULXs) consisting of a black-hole accretor in a
binary with a donor star. We have computed models corresponding to three
different populations of black-hole binaries; two invoke stellar-mass (~10
Msun) black hole accretors, and the third utilizes intermediate-mass (~1000
Msun) black holes (IMBHs). For each of the three populations, we computed
30,000 binary evolution sequences using a full Henyey stellar evolution code.
The optical flux from the model ULXs includes contributions from the accretion
disk, due to x-ray irradiation as well as intrinsic viscous heating, and that
due to the donor star. We present "probability images" for the ULX systems in
planes of color-magnitude, orbital period vs. X-ray luminosity, and luminosity
vs. evolution time. Estimates of the numbers of ULXs in a typical galaxy as
functions of time and of X-ray luminosity are also presented. Our model CMDs
are compared with six ULX counterparts that have been discussed in the
literature. Overall, the observed systems seem more closely related to model
systems with very high-mass donors (> ~25 Msun) in binaries with IMBH
accretors. However, significant difficulties remain with both the IMBH and
stellar-mass black hole models.Comment: 15 pages, 8 figures, submitted to ApJ on Oct 05, 200
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP- stars
Many observed CEMP stars are found in binary systems and show enhanced
abundances of -elements. The origin of the chemical abundances of these
CEMP- stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP- stars were formed.
For this purpose we compare a sample of CEMP- stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP- stars in three groups based on the observed
abundance of europium. In CEMP stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP stars. No measurement of europium is currently available for
CEMP- stars. On average our models reproduce well the abundances observed
in CEMP- stars, whereas in CEMP- stars and CEMP- stars the
abundances of the light- elements are systematically overpredicted by our
models and in CEMP- stars the abundances of the heavy- elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the -elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP- stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP- stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Binaries at Low Metallicity: ranges for case A, B and C mass transfer
The evolution of single stars at low metallicity has attracted a large
interest, while the effect of metallicity on binary evolution remains still
relatively unexplored. We study the effect of metallicity on the number of
binary systems that undergo different cases of mass transfer. We find that
binaries at low metallicity are more likely to start transferring mass after
the onset of central helium burning, often referred to as case C mass transfer.
In other words, the donor star in a metal poor binary is more likely to have
formed a massive CO core before the onset of mass transfer.
At solar metallicity the range of initial binary separations that result in
case C evolution is very small for massive stars, because they do not expand
much after the ignition of helium and because mass loss from the system by
stellar winds causes the orbit to widen, preventing the primary star to fill
its Roche lobe. This effect is likely to have important consequences for the
metallicity dependence of the formation rate of various objects through binary
evolution channels, such as long GRBs, double neutron stars and double white
dwarfs.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
The s-process in stellar population synthesis: a new approach to understanding AGB stars
Thermally pulsating asymptotic giant branch (AGB) stars are the main
producers of slow neutron capture (s-) process elements, but there are still
large uncertainties associated with the formation of the main neutron source,
13C, and with the physics of these stars in general. Observations of s-process
element enhancements in stars can be used as constraints on theoretical models.
For the first time we apply stellar population synthesis to the problem of
s-process nucleosynthesis in AGB stars, in order to derive constraints on free
parameters describing the physics behind the third dredge-up and the properties
of the neutron source. We utilize a rapid evolution and nucleosynthesis code to
synthesize different populations of s-enhanced stars, and compare them to their
observational counterparts to find out for which values of the free parameters
in the code the synthetic populations fit best to the observed populations.
These free parameters are the amount of third dredge-up, the minimum core mass
for third dredge-up, the effectiveness of 13C as a source of neutrons and the
size in mass of the 13C pocket. We find that galactic disk objects are
reproduced by a spread of a factor of two in the effectiveness of the 13C
neutron source. Lower metallicity objects can be reproduced only by lowering by
at least a factor of 3 the average value of the effectiveness of the 13C
neutron source needed for the galactic disk objects. Using observations of
s-process elements in post-AGB stars as constraints we find that dredge-up has
to start at a lower core mass than predicted by current theoretical models,
that it has to be substantial ( >~ 0.2) in stars with mass M <~ 1.5
M_sun and that the mass of the 13C pocket must be about 1/40 that of the
intershell region.Comment: 16 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
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