127 research outputs found
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
The formation of the eccentric-orbit millisecond pulsar J1903+0327 and the origin of single millisecond pulsars
The millisecond pulsar J1903+0327 is accompanied by an ordinary G-dwarf star
in an unusually wide (\,days) and eccentric () orbit. The standard model for producing MSPs fails to explain the
orbital characteristics of this extraordinary binary, and alternative binary
models are unable to explain the observables. We present a triple-star model
for producing MSPs in relatively wide eccentric binaries with a normal
(main-sequence) stellar companion. We start from a stable triple system
consisting of a Low-Mass X-ray Binary (LMXB) with an orbital period of at least
1 day, accompanied by a G-dwarf in a wide and possibly eccentric orbit.
Variations in the initial conditions naturally provide a satisfactory
explanation for the unexplained triple component in the eclipsing soft X-ray
transient 4U~2129+47 or the cataclysmic variable EC 19314-5915. The best
explanation for J1903, however, results from the expansion of the orbit of the
LMXB, driven by the mass transfer from the evolving donor star to its neutron
star companion, which causes the triple eventually to becomes dynamically
unstable. Using numerical computations we show that, depending on the precise
system configuration at the moment the triple becomes dynamically unstable, the
ejection of each of the three components is possible. If the donor star of the
LMXB is ejected, a system resembling J1903, will result. If the neutron star is
ejected, a single MSP results. This model therefore also provides a
straightforward mechanism for forming single MSP in the Galactic disk. We
conclude that the Galaxy contains some 30--300 binaries with characteristics
similar to J1903, and about an order of magnitude fewer single millisecond
pulsars produced with the proposed triple scenario.Comment: ApJ accepted for publicatio
Transients Among Binaries with Evolved Low-Mass Companions
We show that stable disk accretion should be very rare among low-mass X-ray
binaries and cataclysmic variables whose evolution is driven by the nuclear
expansion of the secondary star on the first giant branch. Stable accretion is
confined to neutron-star systems where the secondary is still relatively
massive, and some supersoft white dwarf accretors. All other systems, including
all black-hole systems, appear as soft X-ray transients or dwarf novae. All
long-period neutron-star systems become transient well before most of the
envelope mass is transferred, and remain transient until envelope exhaustion.
This complicates attempts to compare the numbers of millisecond pulsars in the
Galactic disk with their LMXB progenitors, and also means that the pulsar spin
rates are fixed in systems which are transient rather than steady, contrary to
common assumption. The long-period persistent sources Sco X-2, LMC X-2, Cyg X-2
and V395 Car must have minimum companion masses > 0.75 Msun if they contain
neutron stars, and still larger masses if they contain black holes. The
companion in the neutron-star transient GRO J1744-2844 must have a mass <0.87
Msun. The existence of any steady sources at all at long periods supports the
ideas that (a) the accretion disks in many, if not all, LMXBs are strongly
irradiated by the central source, and (b) mass transfer is thermally unstable
in long-period supersoft X-ray sources.Comment: 10 pages, Latex, 1 ps figure, Ap.J., accepted Feb. 15, 199
Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries
The evolution of binaries consisting of evolved main sequence stars (1 <
M_d/Msun < 3.5) with white dwarf companions (0.7 < M_wd/Msun < 1.2) is
investigated through the thermal mass transfer phase. Taking into account the
stabilizing effect of a strong, optically thick wind from the accreting white
dwarf surface, we have explored the formation of several evolutionary groups of
systems for progenitors with initial orbital periods of 1 and 2 days. The
numerical results show that CO white dwarfs can accrete sufficient mass to
evolve to a Type Ia supernova and ONeMg white dwarfs can be built up to undergo
accretion induced collapse for donors more massive than about 2 Msun. For
donors less massive than ~2 Msun the system can evolve to form a He and CO or
ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1
Msun) in systems characterized by 1.6 < M_d/Msun < 1.9 and 0.8 < M_wd/Msun < 1
such that sub Chandrasekhar mass models for Type Ia supernovae, involving off
center helium ignition, are possible for progenitor systems evolving via the
Case A mass transfer phase. For systems characterized by mass ratios > 3 the
system likely merges as a result of the occurrence of a delayed dynamical mass
transfer instability. A semi-analytical model is developed to delineate these
phases which can be easily incorporated in population synthesis studies of
these systems.Comment: 9 pages, 6 figures, Latex, emulateapj style, ApJ accepte
Theoretical Considerations on the Properties of Accreting Millisecond Pulsars
We examine a number of evolutionary scenarios for the recently discovered
class of accretion-powered millisecond X-ray pulsars in ultracompact binaries,
including XTE J0929-314 and XTE J1751-305, with orbital periods of 43.6 and
42.4 minutes, respectively. We focus on a particular scenario that can
naturally explain the present-day properties of these systems. This model
invokes a donor star that was either very close to the TAMS (i.e.,
main-sequence turnoff) at the onset of mass transfer or had sufficient time to
evolve during the mass-transfer phase. We have run a systematic set of detailed
binary evolution calculations with a wide range of initial donor masses and
degrees of (nuclear) evolution at the onset of mass transfer. In general, the
models whose evolutionary tracks result in the best fits to these ultracompact
binaries start mass transfer with orbital periods of ~15 hr, then decrease to a
minimum orbital period of less than or about 40 minutes, and finally evolve
back up to about 43 minutes. We also carry out a probability analysis based on
the measured mass functions of XTE J0929-314 and XTE J1751-305, and combine
this with the results of our binary evolution models and find that the donor
stars currently have masses in the range of about 0.012 - 0.025 solar masses,
and radii of about 0.042 - 0.055 solar radii, and that these radii are likely
to be factors of about 1.1 - 1.3 times larger than the corresponding
zero-temperature ones. We also find that the interiors of the donors are
largely composed of helium and that the surface hydrogen abundances are almost
certainly less than 10% (by mass).Comment: 16 pages, 6 figures, 3 table
Detached white dwarf main-sequence star binaries
We considered the formation of detached white dwarf main-sequence star (WDMS)
binaries through seven evolutionary channels subdivided according to the
evolutionary process that gives rise to the formation of the white dwarf or its
helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type
evolution), dynamically unstable Roche-lobe overflow (common-envelope
evolution), or stellar winds (single star evolution). We examine the
sensitivity of the population to changes in the amount of mass lost from the
system during stable Roche-lobe overflow, the common-envelope ejection
efficiency, and the initial mass ratio or initial secondary mass distribution.
In the case of a flat initial mass ratio distribution, the local space density
of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to
10^{-4}/pc^3 when the initial mass ratio distribution is approximately
proportional to the inverse of the initial mass ratio. More than 75% of the
WDMS binary population stems from wide systems in which both components evolve
as if they were single stars. The remaining part of the population is dominated
by systems in which the white dwarf is formed in a common-envelope phase. The
birthrate of WDMS binaries forming through a common-envelope phase is about 10
times larger than the birthrate of WDMS binaries forming through a stable
Roche-lobe overflow phase. The ratio of the number of helium white dwarf
systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf
systems derived from large samples of observed WDMS binaries by, e.g., future
planet-search missions such as SuperWASP, COROT, and Kepler may furthermore
constrain the common-envelope ejection efficiency.Comment: 22 pages, accepted for publication in A&
The bifurcation periods in low-mass X-ray binaries: the effect of magnetic braking and mass loss
The bifurcation period in low-mass X-ray binaries is the initial orbital pe-
riod which separates the formation of converging systems (which evolve with
decreasing orbital periods until the donor becomes degenerate) from the
diverging systems (which evolve with increasing orbital periods until the donor
star loses its envelope and a wide detached binary is formed). We calculate
systematically the bifurcation periods of binary systems with a 1.4M_\sun
neutron star and a 0.5-2M_\sun donor star, taking into account different kinds
of magnetic braking and mass loss mechanisms. Our results show that the
saturated magnetic braking can considerably decrease the values of bifurcation
period compared to the traditional magnetic braking, while the influence of
mass loss mechanisms on bifurcation periods is quite weak. We also develop a
semi-analytical method to compute the bifurcation period, the result of which
agrees well with the numerical method in the leading order.Comment: 18 pages, 4 figures, 1 table. Accepted to be published in Ap
A new mass-ratio for the X-ray Binary X2127+119 in M15?
The luminous low-mass X-ray binary X2127+119 in the core of the globular
cluster M15 (NGC 7078), which has an orbital period of 17 hours, has long been
assumed to contain a donor star evolving off the main sequence, with a mass of
0.8 solar masses (the main-sequence turn-off mass for M15). We present
orbital-phase-resolved spectroscopy of X2127+119 in the H-alpha and He I 6678
spectral region, obtained with the Hubble Space Telescope. We show that these
data are incompatible with the assumed masses of X2127+119's component stars.
The continuum eclipse is too shallow, indicating that much of the accretion
disc remains visible during eclipse, and therefore that the size of the donor
star relative to the disc is much smaller in this high-inclination system than
the assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I 6678
emission, which has a velocity that implies an association with the stream-disc
impact region, remains unchanged through eclipse, implying that material from
the impact region is always visible. This should not be possible if the
previously-assumed mass ratio is correct. In addition, we do not detect any
spectral features from the donor star, which is unexpected for a 0.8 solar-mass
sub-giant in a system with a 17-hour period.Comment: 6 pages, 4 figures, accepted by A&
The Formation of Low-Mass Transient X-Ray Binaries
We consider constraints on the formation of low-mass X-ray binaries
containing neutron stars (NLMXBs) arising from the presence of soft X-ray
transients among these systems. We show that in short-period systems driven by
angular momentum loss these constraints require the secondary at the beginning
of mass transfer to have a mass > 1.2 M_sun, and to be significantly
nuclear-evolved. As a consequence a comparatively large fraction of such
systems appear as soft X-ray transients even at short periods, as observed.
Moreover the large initial secondary masses account for the rarity of NLMXBs at
periods less than 3 hr. In contrast, NLMXB populations forming with large kick
velocities would not have these properties, suggesting that the kick velocity
is generally small compared to the pre-SN orbital velocity in a large fraction
of systems. We derive constraints on progenitor system parameters and on the
strength of magnetic braking.Comment: Accepted for publication in ApJ, 19 pages, 4 figure
- …