40 research outputs found
Interacting Binaries with Eccentric Orbits. III. Orbital Evolution due to Direct Impact and Self-Accretion
The rapid circularization and synchronization of the stellar components in an
eccentric binary system at the onset of Roche lobe overflow (RLO) is a
fundamental assumption common to all binary stellar evolution and population
synthesis codes, even though the validity of this assumption is questionable
both theoretically and observationally. Here we calculate the evolution of the
orbital elements of an eccentric binary through the direct three-body
integration of a massive particle ejected through the inner Lagrangian point of
the donor star at periastron. The trajectory of this particle leads to three
possible outcomes: direct accretion (DA) onto the companion star within a
single orbit, self-accretion (SA) back onto the donor star within a single
orbit, or a quasi-periodic orbit around the companion star. We calculate the
secular evolution of the binary orbit in the first two cases and conclude that
DA can increase or decrease the orbital semi-major axis and eccentricity, while
SA always decreases the orbital both orbital elements. In cases where mass
overflow contributes to circularizing the orbit, circularization can set in on
timescales as short as a few per cent of the mass transfer timescale. In cases
where mass overflow increases the eccentricity, the orbital evolution is
governed by competition between mass overflow and tidal torques. In the absence
of tidal torques, mass overflow resulting in DI can lead to substantially
subsynchronously rotating donor stars. Contrary to common assumptions, DI
furthermore does not always provide a strong sink of orbital angular momentum
in close mass-transferring binaries; in fact we instead find that a significant
part can be returned to the orbit during the particle orbit. The formulation
presented here can be combined with stellar and binary evolution codes to
generate a better picture of the evolution of eccentric, RLO binary star
systems.Comment: 15 pages, 10 figures, Accepted for publication in Ap
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch and Turn-Off Stars in Four Globular Clusters
We present a spectroscopic analysis of HST/STIS and FOS low- and
intermediate-resolution spectroscopy of 55 stars (turn-off stars, horizontal
branch stars and blue stragglers) in four globular clusters (47 Tucanae, M3,
NGC6752, and NGC6397). Stars were analyzed with non-Local Thermodynamic
Equilibrium model atmospheres, and values for their effective temperatures and
gravities and some rotation rates were obtained. Using photometric fluxes, we
also obtained radii, luminosities and spectroscopic masses.Comment: 71 pages, 28 figures. Electronic figures only in the published
versio
Probing Electron-Capture Supernovae: X-Ray Binaries in Starbursts
Presenting population models of high-mass X-ray binaries (HMXBs) formed after
bursts of star formation, we investigate the effect of electron-capture
supernovae (ECS) of massive ONeMg white dwarfs and the hypothesis that ECS
events are associated with typically low supernova kicks imparted to the
nascent neutron stars. We identify an interesting ECS bump in the time
evolution of HMXB numbers; this bump is caused by significantly increased
production of wind-fed HMXBs 20-60 Myr post starburst. The amplitude and age
extent of the ECS bump depend on the strength of ECS kicks and the mass range
of ECS progenitors. We also find that ECS-HMXBs form through a specific
evolutionary channel that is expected to lead to binaries with Be donors in
wide orbits. These characteristics, along with their sensitivity to ECS
properties, provide us with an intriguing opportunity to probe ECS physics and
progenitors through studies of starbursts of different ages. Specifically, the
case of the Small Magellanic Cloud, with a significant observed population of
Be HMXBs and starburst activity 30-60 Myr ago, arises as a promising laboratory
for understanding the role of electron-capture supernovae in neutron star
formation.Comment: 5 pages, 3 figures, Published by ApJ in 07/0
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
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways
We investigate the characteristics of young ( 1e36
erg/s) High-Mass X-ray Binaries (HMXBs) and find the population to be strongly
metallicity-dependent. We separate the model populations among two distinct
formation pathways: (1) systems undergoing active Roche Lobe Overflow (RLO),
and (2) wind accretion systems with donors in the (super)giant (SG) stage,
which we find to dominate the HMXB population. We find metallicity to primarily
affect the number of systems which move through each formation pathway, rather
than the observable parameters of systems which move through each individual
pathway. We discuss the most important model parameters affecting the HMXB
population at both low and high metallicities. Using these results, we show
that (1) the population of ultra-luminous X-Ray sources can be consistently
described by very bright HMXBs which undergo stable Roche Lobe overflow with
mild super-Eddington accretion and (2) the HMXB population of the bright
starburst galaxy NGC~1569 is likely dominated by one extremely metal-poor
starburst cluster.Comment: 12 pages, 10 figures, Accepted by Ap
Tidally-Induced Apsidal Precession in Double White Dwarfs: a new mass measurement tool with LISA
Galactic interacting double white dwarfs (DWD) are guaranteed gravitational
wave (GW) sources for the GW detector LISA, with more than 10^4 binaries
expected to be detected over the mission's lifetime. Part of this population is
expected to be eccentric, and here we investigate the potential for
constraining the white dwarf (WD) properties through apsidal precession in
these binaries. We analyze the tidal, rotational, and general relativistic
contributions to apsidal precession by using detailed He WD models, where the
evolution of the star's interior is followed throughout the cooling phase. In
agreement with previous studies of zero-temperature WDs, we find that apsidal
precession in eccentric DWDs can lead to a detectable shift in the emitted GW
signal when binaries with cool (old) components are considered. This shift
increases significantly for hot (young) WDs. We find that apsidal motion in hot
(cool) DWDs is dominated by tides at orbital frequencies above ~10^{-4}Hz
(10^{- 3}$Hz). The analysis of apsidal precession in these sources while
ignoring the tidal component would lead to an extreme bias in the mass
determination, and could lead us to misidentify WDs as neutron stars or black
holes. We use the detailed WD models to show that for older, cold WDs, there is
a unique relationship that ties the radius and apsidal precession constant to
the WD masses, therefore allowing tides to be used as a tool to constrain the
source masses.Comment: 23 pages, 7 figures, revised to match accepted ApJ versio
Elliptical motions of stars in close binary systems
Motions of stars in close binary systems with a conservative mass exchange
are examined. It is shown that Paczynski-Huang model widely used now for
obtaining the semi-major axis variation of a relative stars orbit is incorrect,
because it brings about large mistakes. A new model suitable for elliptical
orbits of stars is proposed. Both of reactive and attractive forces between
stars and a substance of the flowing jet are taken into account. A possibility
of a mass exchange at presence of accretion disk is considere