110 research outputs found
Stability Criteria for Mass Transfer in Binary Stellar Evolution
The evolution of a binary star system by various analytic approximations of
mass transfer is discussed, with particular attention payed to the stability of
these processes against runaway on the thermal and dynamical timescales of the
mass donating star. Mass transfer in red giant - neutron star binary systems is
used as a specific example of such mass transfer, and is investigated.
Hjellming and Webbink's (1987) results on the dynamic timescale response of a
convective star with a core to mass loss are applied, with new results.
It is found that mass transfer is usually stable, so long as the the wind's
specific angular momentum does not exceed the angular momentum per reduced mass
of the system. This holds for both dynamical and thermal timescales. Those
systems which are not stable will usually transfer mass on the thermal
timescale. Included are graphs illustrating the variation of the Roche radius
exponent with mass ratio in the binary, for various parameters in the
non-conservative mass transfer, as well as evolutionary paths of interacting
red giant neutron star binaries.Comment: 18 pages, 11 figures, LaTeX, l-aa.sty and psfig.sty macros incl. to
appear in Astronomy and Astrophysic
The Role of Helium Stars in the Formation of Double Neutron Stars
We have calculated the evolution of 60 model binary systems consisting of
helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star
companion to investigate the formation of double neutron star systems.Orbital
periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe
overflow starting from the helium main sequence to after the ignition of carbon
burning in the core. We have also examined the evolution into a common envelope
phase via secular instability, delayed dynamical instability, and the
consequence of matter filling the neutron star's Roche lobe. The survival of
some close He-star neutron-star binaries through the last mass transfer episode
(either dynamically stable or unstable mass transfer phase) leads to the
formation of extremely short-period double neutron star systems (with
P<~0.1days). In addition, we find that systems throughout the entire calculated
mass range can evolve into a common envelope phase, depending on the orbital
period at the onset of mass transfer. The critical orbital period below which
common envelope evolution occurs generally increases with M_He. In addition, a
common envelope phase may occur during a short time for systems characterized
by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun).
The existence of a short-period population of double neutron stars increases
the predicted detection rate of inspiral events by ground-based
gravitational-wave detectors and impacts their merger location in host galaxies
and their possible role as gamma-ray burst progenitors. We use a set of
population synthesis calculations and investigate the implications of the
mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592
n1 July 20, 200
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
Type Ia Supernovae: An Examination of Potential Progenitors and the Redshift Distribution
We examine the possibility that supernovae type Ia (SN Ia) are produced by
white dwarfs accreting from Roche-lobe filling evolved companions, under the
assumption that a strong optically thick stellar wind from accretor is able to
stabilize the mass transfer. We show that if a mass transfer phase on a thermal
timescale precedes a nuclear burning driven phase, then such systems (of which
the supersoft X-ray sources are a subgroup) can account for about 10% of the
inferred SN Ia rate.
In addition, we examine the cosmic history of the supernova rate, and we show
that the ratio of the rate of SN Ia to the rate of supernovae produced by
massive stars (supernovae of types II, Ib, Ic) should increase from about z = 1
towards lower redshifts.Comment: 29 pages, Latex, 6 figures, aasms4.sty, psfig.sty, to appear in The
Astrophysical Journa
Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries
Hydrodynamical simulations of semi-detached, polytropic binary stars are
presented in an effort to study the onset and stability of dynamical mass
transfer events. Initial, synchronously rotating equilibrium models are
constructed using a self-consistent-field technique and then evolved with an
Eulerian hydrodynamics code in a fully self-consistent manner. We describe code
improvements introduced over the past few years that permit us to follow
dynamical mass-transfer events through more than 30 orbits. Mass-transfer
evolutions are presented for two different initial configurations: A
dynamically unstable binary with initial mass ratio (donor/accretor) that leads to a complete merger in orbits; and a
double-degenerate binary with initial mass ratio that, after some
initial unstable growth of mass transfer, tends to separate as the
mass-transfer rate levels off.Comment: 47 pages, 11 figures, submitted to the Astrophysical Journal. See
http://www.phys.lsu.edu/faculty/tohline/astroph/dmtf05 for high resolution
figures and mpeg animation
Prevalence and Predictors of Abnormal Cardiovascular Responses to Exercise Testing Among Individuals With Type 2 Diabetes: The Look AHEAD (Action for Health in Diabetes) study
OBJECTIVE We examined maximal graded exercise test (GXT) results in 5,783 overweight/obese men and women, aged 45–76 years, with type 2 diabetes, who were entering the Look AHEAD (Action for Health in Diabetes) study, to determine the prevalence and correlates of exercise-induced cardiac abnormalities. RESEARCH DESIGN AND METHODS Participants underwent symptom-limited maximal GXTs. Questionnaires and physical examinations were used to determine demographic, anthropometric, metabolic, and health status predictors of abnormal GXT results, which were defined as an ST segment depression ≥1.0 mm, ventricular arrhythmia, angina pectoris, poor postexercise heart rate recovery (<22 bpm reduction 2 min after exercise), or maximal exercise capacity less than 5.0 METs. Systolic blood pressure response to exercise was examined as a continuous variable, without a threshold to define abnormality. RESULTS Exercise-induced abnormalities were present in 1,303 (22.5%) participants, of which 693 (12.0%) consisted of impaired exercise capacity. ST segment depression occurred in 440 (7.6%), abnormal heart rate recovery in 206 (5.0%), angina in 63 (1.1%), and arrhythmia in 41 (0.7%). Of potential predictors, only greater age was associated with increased prevalence of all abnormalities. Other predictors were associated with some, but not all, abnormalities. Systolic blood pressure response decreased with greater age, duration of diabetes, and history of cardiovascular disease. CONCLUSIONS We found a high rate of abnormal GXT results despite careful screening for cardiovascular disease symptoms. In this cohort of overweight and obese individuals with type 2 diabetes, greater age most consistently predicted abnormal GXT. Long-term follow-up of these participants will show whether these abnormalities are clinically significant. Cardiovascular disease (CVD) risk factors improve with exercise in individuals with diabetes (1). Similarly, individuals with diabetes who are physically active or have higher fitness levels have reduced CVD incidence and mortality (2,3). Nevertheless, participation in exercise may involve risks for individuals with diabetes because of their high prevalence of CVD, including silent ischemia, and other comorbid conditions (4). Knowledge about the typical cardiovascular responses to exercise in individuals with type 2 diabetes has come mainly from small clinic-based studies (5) and a few larger studies in the general population (4,6,7). The Look AHEAD (Action for Health in Diabetes) study is a multicenter randomized clinical trial designed to evaluate the long-term effects of an intensive lifestyle intervention program of weight loss and physical activity on morbidity and mortality from CVD in overweight and obese individuals with type 2 diabetes (8). Graded exercise tests (GXTs) with electrocardiographic monitoring were conducted at baseline in 5,783 individuals, providing the largest sample of systematic stress testing ever conducted in individuals with type 2 diabetes. The aims of this analysis are to examine the range of abnormal exercise responses and to examine the degree to which demographic factors, health characteristics, and medication use associate with abnormal exercise responses. Identifying predictors from readily available demographic and clinical data may assist risk stratification before exercise testing or exercise prescription for individuals with diabetes
On the formation of neon-enriched donor stars in ultracompact X-ray binaries
We study the formation of neon-enriched donor stars in ultracompact X-ray
binaries (orbital periods P<80 min) and show that their progenitors have to be
low-mass (0.3 - 0.4 solar mass) ``hybrid'' white dwarfs (with CO cores and
thick helium mantles). Stable mass transfer is possible if in the initial
stages of mass exchange mass is lost from the system, taking away the specific
orbital angular momentum of the accretor (``isotropic re-emission''). The
excess of neon in the transferred matter is due to chemical fractionation of
the white dwarf which has to occur prior to the Roche lobe overflow by the
donor. The estimated lower limit of the orbital periods of the systems with
neon-enriched donors is close to 10 min. We show that the X-ray pulsar 4U
1626-67, which likely also has a neon-enriched companion, may have been formed
via accretion induced collapse of an oxygen-neon white dwarf accretor if the
donor was a hybrid white dwarf.Comment: 6 pages, 3 figures, uses aa.cls 5.1 version class file, accepted for
publication in Astronomy and Astrophysic
Orbital Parameters for the Soft X-ray Transient 4U 1543-47: Evidence for a Black Hole
(shortened) Spectroscopic observations of the soft X-ray transient 4U 1543-47
reveal a radial velocity curve with a period of P=1.123 +/- 0.008 days and a
semi-amplitude of K_2 = 124 +/- 4 km/sec. The mass function is f(M) = 0.22 +/-
0.02 solar masses. We derive a distance of d = 9.1 +/-1.1 kpc if the secondary
is on the main sequence. The V and I light curves exhibit two waves per orbital
cycle with amplitudes of about 0.08 mag. We modeled the light curves as
ellipsoidal variations in the secondary star and derive extreme inclination
limits of 20 <= i <= 40 deg and formal 3 sigma limits of 24 <= i <= 36 deg for
a mass ratio Q = M_1/M_2 > 1. However, there are systematic effects in the data
that the model does not account for, so the above constraints should be treated
with caution. We argue that the secondary star is still on the main sequence
and if the secondary star has a mass near the main sequence values for early
A-stars (2.3 <= M_2 <= 2.6 solar masses), then the best fits for the 3 sigma
inclination range (24 <= i <= 36 deg) and the 3 sigma mass function range (0.16
<= f(M) <= 0.28 solar masses) imply a primary mass in the range 2.7 <= M_1 <=
7.5 solar masses. Thus the mass of the compact object in 4U 1543-47 is likely
to be in excess of approximately 3 solar masses and we conclude 4U 1543-47 most
likely contains a black hole.Comment: 18 pages, 9 figures, LaTeX (uses the standard AAS style file
aas2pp4.sty), accepted for publication in the Ap
Coalescing Binary Neutron Stars
Coalescing compact binaries with neutron star or black hole components
provide the most promising sources of gravitational radiation for detection by
the LIGO/VIRGO/GEO/TAMA laser interferometers now under construction. This fact
has motivated several different theoretical studies of the inspiral and
hydrodynamic merging of compact binaries. Analytic analyses of the inspiral
waveforms have been performed in the Post-Newtonian approximation. Analytic and
numerical treatments of the coalescence waveforms from binary neutron stars
have been performed using Newtonian hydrodynamics and the quadrupole radiation
approximation. Numerical simulations of coalescing black hole and neutron star
binaries are also underway in full general relativity. Recent results from each
of these approaches will be described and their virtues and limitations
summarized.Comment: Invited Topical Review paper to appear in Classical and Quantum
Gravity, 35 pages, including 5 figure
Cygnus X-3 and the problem of the missing Wolf-Rayet X-ray binaries
Cygnus X-3 is a strong X-ray source (L_X about 10^38 erg/s) which is thought
to consist of a compact object, accreting matter from a helium star. We find
analytically that the estimated ranges of mass-loss rate and orbital-period
derivative for Cyg X-3 are consistent with two models: i) the system is
detached and the mass loss from the system comes from the stellar wind of a
massive helium star, of which only a fraction that allows for the observed
X-ray luminosity is accreted, or ii) the system is semidetached and a
Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the
compact object, followed by ejection of its excess over the Eddington rate from
the system. These analytical results appear to be consistent with evolutionary
calculations. By means of population synthesis we find that currently in the
Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a
>~ 7 MSun Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes
matter from a Roche-lobe-overflowing helium star with mass <~ 1.5 MSun. Cyg X-3
is probably one of these systems.Comment: 12 pages, 6 figures, accepted by A&
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