4,889 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
Simulations of the tidal interaction and mass transfer of a star in an eccentric orbit around an intermediate-mass black hole: the case of HLX-1
The X-ray source HLX-1 near the spiral galaxy ESO 243-49 is currently the
best intermediate-mass black hole candidate. It has a peak bolometric
luminosity of erg s, which implies a mass inflow rate of
MSun yr, but the origin of this mass is unknown. It has
been proposed that there is a star on an eccentric orbit around the black hole
which transfers mass at pericentre. To investigate the orbital evolution of
this system, we perform stellar evolution simulations using mesa and SPH
simulations of a stellar orbit around an intermediate-mass black hole using fi.
We run and couple these simulations using the amuse framework. We find that
mass is lost through both the first and second Lagrange points and that there
is a delay of up to 10 days between the pericentre passage and the peak mass
loss event. The orbital evolution timescales we find in our simulations are
larger than what is predicted by analytical models, but these models fall
within the errors of our results. Despite the fast orbital evolution, we are
unable to reproduce the observed change in outburst period. We conclude that
the change in the stellar orbit with the system parameters investigated here is
unable to account for all observed features of HLX-1.Comment: accepted for publication in mnra
Evolution of binary stars and the effect of tides on binary populations
We present a rapid binary evolution algorithm that enables modelling of even
the most complex binary systems. In addition to all aspects of single star
evolution, features such as mass transfer, mass accretion, common-envelope
evolution, collisions, supernova kicks and angular momentum loss mechanisms are
included. In particular, circularization and synchronization of the orbit by
tidal interactions are calculated for convective, radiative and degenerate
damping mechanisms. We use this algorithm to study the formation and evolution
of various binary systems. We also investigate the effect that tidal friction
has on the outcome of binary evolution. Using the rapid binary code, we
generate a series of large binary populations and evaluate the formation rate
of interesting individual species and events. By comparing the results for
populations with and without tidal friction we quantify the hitherto ignored
systematic effect of tides and show that modelling of tidal evolution in binary
systems is necessary in order to draw accurate conclusions from population
synthesis work. Tidal synchronism is important but because orbits generally
circularize before Roche-lobe overflow the outcome of the interactions of
systems with the same semi-latus rectum is almost independent of eccentricity.
It is not necessary to include a distribution of eccentricities in population
synthesis of interacting binaries, however, the initial separations should be
distributed according to the observed distribution of semi-latera recta rather
than periods or semi-major axes.Comment: 36 pages, 12 figures, to be published in the Monthly Notices of the
Royal Astronomical Societ
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
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
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