24 research outputs found
Magnetic field and unstable accretion during AM Herculis low states
A study of AM Her low states in September 1990 and 1991 and June-July 1997 is
reported from a coordinated campaign with observations obtained at the
Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical
Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory.
Spectra obtained at different dates when the source was in low states at a
comparable V magnitude, show the presence of strong Zeeman absorption features
and marked changes in emission lines with a day-to-day reappearance of the HeII
(4686\AA) emission lines in 1991. Despite this variability, the magnetic field
inferred from the fitting of the absorption spectrum with Zeeman hydrogen
splitting, is remarkably constant with a best value of (12.50.5)MG.
Detailed analysis of the UBVRI light curves shows the presence of repetitive
moderate amplitude ( 0.3-0.5 mag) flares predominantly red in colour.
These flares are attributed to small accretion events and are compared to the
large ( 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We
suggest that the general flaring activity observed during the low states is
generated by accretion events. The different characteristics of the flares
(colour and polarization) are the results of different shock geometries
depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6
Figures, Late
Implications of the PSR 1257+12 Planetary System for Isolated Millisecond Pulsars
The first extrasolar planets were discovered in 1992 around the millisecond
pulsar PSR 1257+12. We show that recent developments in the study of accretion
onto magnetized stars, plus the existence of the innermost, moon-sized planet
in the PSR 1257+12 system, suggest that the pulsar was born with approximately
its current rotation frequency and magnetic moment. If so, this has important
implications for the formation and evolution of neutron star magnetic fields as
well as for the formation of planets around pulsars. In particular, it suggests
that some and perhaps all isolated millisecond pulsars may have been born with
high spin rates and low magnetic fields instead of having been recycled by
accretion.Comment: 17 pages including one figure, uses aaspp4, accepted by Ap
White Dwarfs Near Black Holes: A New Paradigm for Type I Supernovae
We present calculations indicating the possibility of a new class of Type I
supernovae. In this new paradigm relativistic terms enhance the self gravity of
a carbon-oxygen white dwarf as it passes or orbits near a black hole. This
relativistic compression can cause the central density to exceed the threshold
for pycnonuclear reactions so that athermonuclear runaway ensues. We consider
three possible environments: 1) white dwarfs orbiting a low-mass black hole; 2)
white dwarfs encountering a massive black hole in a dense globular cluster; and
3) white dwarfs passing a supermassive black hole in a dense galactic core. We
estimate the rate at which such events could occur out to a redshift of z = 1.
Event rates are estimated to be significantly less than the rate of normal Type
Ia supernovae for all three classes. Nevertheless, such events may be frequent
enough to warrant a search for this new class of supernova. We propose several
observable signatures which might be used to identify this type of event and
speculate that such an event might have produced the observed
"mixed-morphology" Sgr A East supernova remnant in the Galactic core.Comment: Accepted for Publication in The Astrophysical Journa
Expected coalescence rates of NS-NS binaries for laser beam interferometers
The coalescence rate of two neutron stars (NS) is revisited. For estimation
of the number of bound NS-NS and the probability of their coalescence in a
timescale , the galactic star formation history, directly derived from
observations, and the evolution of massive stars are considered. The newly
established galactic merging rate is ,
while the local merging rate, including the contribution of elliptical
galaxies, is about a factor of two higher, . Using
the present data basis on galaxy distribution in the local universe and the
expected sensitivity of the first generation of laser beam interferometers, we
estimate that one event should occur every 125 years for LIGO and one event
each 148 years for VIRGO. The situation is considerably improved for
advanced-LIGO since we predict that 6 events per year should be detected
whereas for a recently proposed VIRGO new configuration, the event rate might
increase up to 3 events every two years.Comment: 15 pages, 3 figures accepted for publication in IJMP
Aperiodic optical variability of intermediate polars - cataclysmic variables with truncated accretion disks
We study the power spectra of the variability of seven intermediate polars
containing magnetized asynchronous accreting white dwarfs, XSS J00564+4548,IGR
J00234+6141, DO Dra, V1223 Sgr, IGR J15094-6649, IGR J16500-3307 and IGR
J17195-4100, in the optical band and demonstrate that their variability can be
well described by a model based on fluctuations propagating in a truncated
accretion disk. The power spectra have breaks at Fourier frequencies, which we
associate with the Keplerian frequency of the disk at the boundary of the white
dwarfs' magnetospheres. We propose that the properties of the optical power
spectra can be used to deduce the geometry of the inner parts of the accretion
disk, in particular: 1) truncation radii of the magnetically disrupted
accretion disks in intermediate polars, 2) the truncation radii of the
accretion disk in quiescent states of dwarf novaeComment: Accepted for publication in A&
Superhumps in Cataclysmic Binaries. XXIII. V442 Ophiuchi and RX J1643.7+3402
We report the results of long observing campaigns on two novalike variables:
V442 Ophiuchi and RX J1643.7+3402. These stars have high-excitation spectra,
complex line profiles signifying mass loss at particular orbital phases, and
similar orbital periods (respectively 0.12433 and 0.12056 d). They are
well-credentialed members of the SW Sex class of cataclysmic variables. Their
light curves are also quite complex. V442 Oph shows periodic signals with
periods of 0.12090(8) and 4.37(15) days, and RX J1643.7+3402 shows similar
signals at 0.11696(8) d and 4.05(12) d. We interpret these short and long
periods respectively as a "negative superhump" and the wobble period of the
accretion disk. The superhump could then possibly arise from the heating of the
secondary (and structures fixed in the orbital frame) by inner-disk radiation,
which reaches the secondary relatively unimpeded since the disk is not
coplanar.
At higher frequencies, both stars show another type of variability:
quasi-periodic oscillations (QPOs) with a period near 1000 seconds. Underlying
these strong signals of low stability may be weak signals of higher stability.
Similar QPOs, and negative superhumps, are quite common features in SW Sex
stars. Both can in principle be explained by ascribing strong magnetism to the
white dwarf member of the binary; and we suggest that SW Sex stars are
borderline AM Herculis binaries, usually drowned by a high accretion rate. This
would provide an ancestor channel for AM Hers, whose origin is still
mysterious.Comment: PDF, 41 pages, 4 tables, 16 figures; accepted, in press, to appear
December 2002, PASP; more info at http://cba.phys.columbia.edu
On the Change of the Inner Boundary of an Optically Thick Accretion Disk around White Dwarfs Using the Dwarf Nova SS Cyg as an Example
We present the results of our studies of the aperiodic optical flux
variability for SS Cyg, an accreting binary systemwith a white dwarf. The main
set of observational data presented here was obtained with the ANDOR/iXon
DU-888 photometer mounted on the RTT-150 telescope, which allowed a record(for
CCD photometers) time resolution up to 8 ms to be achieved. The power spectra
of the source's flux variability have revealed that the aperiodic variability
contains information about the inner boundary of the optically thick flow in
the binary system. We show that the inner boundary of the optically thick
accretion disk comes close to the white dwarf surface at the maximum of the
source's bolometric light curve, i.e., at the peak of the instantaneous
accretion rate onto the white dwarf, while the optically thick accretion disk
is truncated at distances 8.5e9 cm ~10 R_{WD} in the low state. We suggest that
the location of the inner boundary of the accretion disk in the binary can be
traced by studying the parameters of the power spectra for accreting white
dwarfs. In particular, this allows the mass of the accreting object to be
estimated.Comment: 9 pages, 7 figures, Published in Astronomy Letter
A model for the population of helium stars in the Galaxy I. Low-mass stars
By means of population synthesis we model the Galactic ensemble of helium
stars. It is assumed that all helium stars are formed in binaries. Under this
assumption, single helium stars are produced by the mergers of helium remnants
of components of close binaries (mainly, by merging helium white dwarfs) and by
disruption of binaries with helium components in supernovae explosions. The
estimate of the total birthrate of helium stars in the Galaxy is 0.043
yr, their total number is estimated as . The rate of
binarity in the total sample is 76%. We construct a subsample of low-mass
(M_{\rm He} \lesssim 2 \ms) helium stars limited by observational selection
effects: stellar magnitude (), ratio of stellar magnitudes
of components in binaries (), lower limit of the
semiamplitude of radial velocity that is necessary for discovery of binarity
( km/s). The parameters of this ``observable'' sample are in
satisfactory agreement with the parameters of the observed ensemble of sdB
stars. In particular, in the selection-limited sample binarity rate is 58%. We
analyze the relations between orbital periods and masses of helium stars and
their companions in systems with different combinations of components. We
expect that overwhelming majority (%) of unobserved components in
binary sdB stars are white dwarfs, predominantly, carbon-oxygen ones.Comment: 16 pages, 6 figures, submitted to Astronomy Reports, fig. 6
corrected, conclusions unchange
Evolution of the number of accreting white dwarfs with shell nuclear burning and of occurrence rate of SN Ia
We analyze temporal evolution of the number of accreting white dwarfs with
shell hydrogen burning in semidetached and detached binaries. We consider a
stellar system in which star formation lasts for 10 Gyr with a constant rate,
as well as a system in which the same amount of stars is formed in a single
burst lasting for 1 Gyr. Evolution of the number of white dwarfs is confronted
to the evolution of occurrence rate of events that usually are identified with
SN Ia or accretion-induced collapses, i.e. with accumulation of Chandrasekhar
mass by a white dwarf or a merger of a pair of CO white dwarfs with total mass
not lower than the Chandrasekhar one. In the systems with a burst of star
formation, at 10 Gyr observed supersoft X-ray sources, most probably, are
not precursors of SN Ia. The same is true for an overwhelming majority of the
sources in the systems with constant star formation rate. In the systems of
both kinds mergers of white dwarfs is the dominant SN Ia scenario. In symbiotic
binaries, accreting CO-dwarfs do not accumulate enough mass for SN Ia
explosion, while ONeMg-dwarfs finish their evolution by an accretion-induced
collapse with formation of a neutron star.Comment: 11 pages, 2 figures, accepted by Astronomy Letter
Population Synthesis for Neutron Star Systems with Intrinsic Kicks
We use a Monte Carlo binary synthesis code to model the formation and
evolution of neutron star systems including high-mass X-ray binaries, low-mass
X-ray binaries, double neutron star systems and radio pulsars. Our focus is on
the signature imprinted on such systems due to natal kicks to neutron stars
over and above that imparted by orbital motions. The code incorporates the
effect of the galactic potential (including rotation) on the velocities of
these systems. A comparison between our models and the observations leads us to
infer mean natal kicks between 400-500 km/s. Moreover, to be consistent with
all the data, we require a bimodal kick distribution with one peak in the
distribution near 0 km/s and the other above 600 km/s.Comment: 41 pages total, 24 text+tables pages, 17 figures, AASTeX, Accepted
for publication in Ap