1,093 research outputs found
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
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
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
The Past and Future History of Regulus
We show how the recent discovery of a likely close white dwarf companion to
the well known star Regulus, one of the brightest stars in the sky, leads to
considerable insight into the prior evolutionary history of this star,
including the cause of its current rapid rotation. We infer a relatively narrow
range for the initial masses of the progenitor system: M_{10} = 2.3 +/- 0.2
M_sun and M_{20} = 1.7 +/- 0.2 M_sun, where M_{10} and M_{20} are the initial
masses of the progenitors of the white dwarf and Regulus, respectively. In this
scenario, the age of the Regulus system would exceed 1 Gyr. We also show that
Regulus, with a current orbital period of 40 days, has an interesting future
ahead of it. This includes (i) a common envelope phase, and, quite possibly,
(ii) an sdB phase, followed by (iii) an AM CVn phase with orbital periods < 1
hr. Binary evolution calculations are presented in support of this scenario. We
also discuss alternative possibilities, emphasizing the present uncertainties
in binary evolution theory. Thus, this one particular star system illustrates
many different aspects of binary stellar evolution.Comment: PDFLaTeX, 9 pages with 8 figure
Face-on accretion onto a protoplanetary disc
Globular clusters (GCs) are known to harbor multiple stellar populations. To
explain these observations Bastian et al. suggested a scenario in which a
second population is formed by the accretion of enriched material onto the
low-mass stars in the initial GC population. The idea is that the low-mass,
pre-main sequence stars sweep up gas expelled by the massive stars of the same
generation into their protoplanetary disc as they move through the GC core. We
perform simulations with 2 different smoothed particle hydrodynamics codes to
investigate if a low-mass star surrounded by a protoplanetary disc can accrete
the amount of enriched material required in this scenario. We focus on the gas
loading rate onto the disc and star as well as on the lifetime of the disc. We
find that the gas loading rate is a factor of 2 smaller than the geometric
rate, because the effective cross section of the disc is smaller than its
surface area. The loading rate is consistent for both codes, irrespective of
resolution. The disc gains mass in the high resolution runs, but loses angular
momentum on a time scale of 10^4 yrs. Two effects determine the loss of
(specific) angular momentum in our simulations: 1) continuous ram pressure
stripping and 2) accretion of material with no azimuthal angular momentum. Our
study and previous work suggest that the former, dominant process is mainly
caused by numerical rather than physical effects, while the latter is not. The
latter process causes the disc to become more compact, increasing the surface
density profile at smaller radii. The disc size is determined in the first
place by the ram pressure when the flow first hits the disc. Further evolution
is governed by the decrease in the specific angular momentum of the disc. We
conclude that the size and lifetime of the disc are probably not sufficient to
accrete the amount of mass required in Bastian et al.'s scenario.Comment: Accepted for publication in A&A, 15 pages, 5 figures, 4 table
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
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