34 research outputs found
Cataclysmic Variables with Evolved Secondaries and the Progenitors of AM CVn Stars
We present the results of a systematic study of cataclysmic variables (CVs)
and related systems, combining detailed binary-population synthesis (BPS)
models with a grid of 120 binary evolution sequences calculated with a
Henyey-type stellar evolution code. In these sequences, we used 3 masses for
the white dwarf (0.6, 0.8, 1.0 Msun) and seven masses for the donor star in the
range of 0.6-1.4 Msun. The shortest orbital periods were chosen to have
initially unevolved secondaries, and the longest orbital period for each
secondary mass was taken to be just longer than the bifurcation period (16 - 22
hr), beyond which systems evolve towards long orbital periods. These
calculations show that systems which start with evolved secondaries near the
end or just after their main-sequence phase become ultra-compact systems with
periods as short as 7 min. These systems are excellent candidates for AM CVn
stars. Using a standard BPS code, we show how the properties of CVs at the
beginning of mass transfer depend on the efficiency for common-envelope (CE)
ejection and the efficiency of magnetic braking. In our standard model, where
CE ejection is efficient, some 10 per cent of all CVs have initially evolved
secondaries (with a central hydrogen abundance X_c < 0.4) and ultimately become
ultra-compact systems (implying a Galactic birthrate for AM CVn-like stars of
10^{-3} yr^{-1}). Almost all CVs with orbital periods longer than 5 hr are
found to have initially evolved or relatively massive secondaries. We show that
their distribution of effective temperatures is in good agreement with the
distribution of spectral types obtained by Beuermann et al. (1998).Comment: 16 pages, 6 figures (Fig. 4 in reduced format). Submitted to MNRA
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
Models of Ultraluminous X-Ray Sources with Intermediate-Mass Black Holes
We have computed models for ultraluminous X-ray sources ("ULXs") consisting
of a black-hole accretor of intermediate mass ("IMBH"; e.g., ~1000 Msun) and a
captured donor star. For each of four different sets of initial donor masses
and orbital separations, we computed 30,000 binary evolution models using a
full Henyey stellar evolution code. To our knowledge this is the first time
that a population of X-ray binaries this large has been carried out with other
than approximation methods, and it serves to demonstrate the feasibility of
this approach to large-scale population studies of mass-transfer binaries. In
the present study, we find that in order to have a plausible efficiency for
producing active ULX systems with IMBHs having luminosities > 10^{40} ergs/sec,
there are two basic requirements for the capture of companion/donor stars.
First, the donor stars should be massive, i.e., > 8 Msun. Second, the initial
orbital separations, after circularization, should be close, i.e., < 6-30 times
the radius of the donor star when on the main sequence. Even under these
optimistic conditions, we show that the production rate of IMBH-ULX systems may
fall short of the observed values by factors of 10-100.Comment: 5 pages, 2 figures, submitted to Ap
Hot subdwarf binaries - Masses and nature of their heavy compact companions
Neutron stars and stellar-mass black holes are the remnants of massive stars,
which ended their lives in supernova explosions. These exotic objects can only
be studied in relatively rare cases. If they are interacting with close
companions they become bright X-ray sources. If they are neutron stars, they
may be detected as pulsars. Only a few hundred such systems are presently known
in the Galaxy. However, there should be many more binaries with basically
invisible compact objects in non-interacting binaries. Here we report the
discovery of unseen compact companions to hot subdwarfs in close binary
systems. Hot subdwarfs are evolved helium-core-burning stars that have lost
most of their hydrogen envelopes, often due to binary interactions. Using
high-resolution spectra and assuming tidal synchronisation of the subdwarfs, we
were able to constrain the companion masses of 32 binaries. While most hot
subdwarf binaries have white-dwarf or late-type main sequence companions, as
predicted by binary evolution models, at least 5% of the observed subdwarfs
must have very massive companions: unusually heavy white dwarfs, neutron stars
and, in some cases, even black holes. We present evolutionary models which show
that such binaries can indeed form if the system has evolved through two
common-envelope phases. This new connection between hot subdwarfs, which are
numerous in the Galaxy, and massive compact objects may lead to a tremendous
increase in the number of known neutron stars and black holes and shed some
light on this dark population and its evolutionary link to the X-ray binary
population.Comment: 8 pages, 5 figures, to appear in the Journal of Physics Conference
Proceedings (JPCS) for the 16th European White Dwarf Workshop, Barcelona,
Spain, June 30 - July 11, 200
Cygnus X-2: the Descendant of an Intermediate-Mass X-Ray Binary
The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a
secondary that is much more luminous and hotter than is appropriate for a
low-mass subgiant. We present detailed binary-evolution calculations which
demonstrate that the present evolutionary state of Cyg X-2 can be understood if
the secondary had an initial mass of around 3.5 M_sun and started to transfer
mass near the end of its main-sequence phase (or, somewhat less likely, just
after leaving the main sequence). Most of the mass of the secondary must have
been ejected from the system during an earlier rapid mass-transfer phase. In
the present phase, the secondary has a mass of around 0.5 M_sun with a
non-degenerate helium core. It is burning hydrogen in a shell, and mass
transfer is driven by the advancement of the burning shell. Cyg X-2 therefore
is related to a previously little studied class of intermediate-mass X-ray
binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray
binaries presently classified as low-mass X-ray binaries may be descendants of
IMXBs and discuss some of the implications
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
Exploring the Nature of Weak Chandra Sources near the Galactic Centre
We present results from the first near-IR imaging of the weak X-ray sources
discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic
Centre (GC). These ~800 discrete sources, which contribute significantly to the
GC X-ray emission, represent an important and previously unknown population
within the Galaxy. From our VLT observations we will identify likely IR
counterparts to a sample of the hardest sources, which are most likely X-ray
binaries. With these data we can place constraints on the nature of the
discrete weak X-ray source population of the GC.Comment: In Proc. of ``Interacting Binaries: Accretion, Evolution, and
Outcomes'', eds. L. A. Antonelli et al., AIP, Cefalu, Sicily, 200
LMXB and IMXB Evolution: I. The Binary Radio Pulsar PSR J1614-2230
We have computed an extensive grid of binary evolution tracks to represent
low- and intermediate mass X-ray binaries (LMXBs and IMXBs). The grid includes
42,000 models which covers 60 initial donor masses over the range of 1-4 solar
masses and, for each of these, 700 initial orbital periods over the range of
10-250 hours. These results can be applied to understanding LMXBs and IMXBs:
those that evolve analogously to CVs; that form ultracompact binaries with
orbital periods in the range of 6-50 minutes; and that lead to wide orbits with
giant donors. We also investigate the relic binary recycled radio pulsars into
which these systems evolve. To evolve the donor stars in this study, we
utilized a newly developed stellar evolution code called "MESA" that was
designed, among other things, to be able to handle very low-mass and degenerate
donors. This first application of the results is aimed at an understanding of
the newly discovered pulsar PSR J1614-2230 which has a 1.97 solar masses
neutron star, orbital period = 8.7 days, and a companion star of 0.5 solar
mass. We show that (i) this system is a cousin to the LMXB Cyg X-2; (ii) for
neutron stars of canonical birth mass 1.4 solar masses, the initial donor stars
which produce the closest relatives to PSR J1614-2230 have a mass between
3.4-3.8 solar masses; (iii) neutron stars as massive as 1.97 solar masses are
not easy to produce in spite of the initially high mass of the donor star,
unless they were already born as relatively massive neutron stars; (iv) to
successfully produce a system like PSR J1614-2230 requires a minimum initial
neutron star mass of at least 1.6+-0.1 solar masses, as well as initial donor
masses and orbital period of ~ 4.25+-0.10 solar masses and ~49+-2 hrs,
respectively; and (v) the current companion star is largely composed of CO, but
should have a surface H abundance of ~10-15%.Comment: 9 pages, 6 figures, simulateapj style, accepted by Ap
Exploring the Nature of Weak Chandra Sources near the Galactic Centre
We present early results from the first IR imaging of the weak X-ray sources
discovered in a recent Chandra survey towards the Galactic Centre. From our VLT
observations we will identify likely counterparts to a sample of the hardest
sources in order to place constraints on the nature of this previously unknown
population.Comment: To appear in RevMexAA(SC) Conference Series, Proc. of IAU Colloquium
194, Compact Binaries in the Galaxy and Beyond, La Paz, Mexico, 17-21 Nov.,
200
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio