467 research outputs found
Angular momentum losses and the orbital period distribution of cataclysmic variables below the period gap: effects of circumbinary disks
The population synthesis of cataclysmic variables below the period is
investigated. A grid of detailed binary evolutionary sequences has been
calculated and included in the simulations to take account of additional
angular momentum losses beyond that associated with gravitational radiation and
mass loss, due to nova outbursts, from the system. As a specific example, we
consider the effect of a circumbinary disk to gain insight into the ingredients
necessary to reproduce the observed orbital period distribution. The resulting
distributions show that the period minimum lies at about 80 minutes with the
number of systems monotonically increasing with increasing orbital period to a
maximum near 90 minutes. There is no evidence for an accumulation of systems at
the period minimum which is a common feature of simulations in which only
gravitational radiation losses are considered. The period distribution is found
to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The
steepness of the lower edge of the period gap can be reproduced, for example,
by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic
variable binary systems from orbital periods longer than 2.75 hrs. The good
agreement with the cumulated distribution function of observed systems within
the framework of our model indicates that the angular momentum loss by a
circumbinary disk or a mechanism which mimics its features coupled with a
weighting factor to account for selection effects in the discovery of such
systems and a flow of systems from above the period gap to below the period gap
are important ingredients for understanding the overall period distribution of
cataclysmic variable binary systems.Comment: Accepted for publication in Ap
Time-Series Ensemble Photometry and the Search for Variable Stars in the Open Cluster M11
This work presents the first large-scale photometric variability survey of
the intermediate age (~200 Myr) open cluster M11. Thirteen nights of data over
two observing seasons were analyzed (using crowded field and ensemble
photometry techniques) to obtain high relative precision photometry. In this
study we focus on the detection of candidate member variable stars for
follow-up studies. A total of 39 variable stars were detected and can be
categorized as follows: 1 irregular (probably pulsating) variable, 6 delta
Scuti variables, 14 detached eclipsing binary systems, 17 W UMa variables, and
1 unidentified/candidate variable. While previous proper motion studies allow
for cluster membership determination for the brightest stars, we find that
membership determination is significantly hampered below V=15,R=15.5 by the
large population of field stars overlapping the cluster MS. Of the brightest
detected variables that have a high likelihood of cluster membership, we find
five systems where further work could help constrain theoretical stellar
models, including one potential W UMa member of this young cluster.Comment: 38 pages, 13 figures, accepted for December 2005 AJ, high-resolution
version available upon reques
An AMR Study of the Common Envelope Phase of Binary Evolution
The hydrodynamic evolution of the common envelope phase of a low mass binary
composed of a 1.05 Msun red giant and a 0.6 Msun companion has been followed
for five orbits of the system using a high resolution method in three spatial
dimensions. During the rapid inspiral phase, the interaction of the companion
with the red giant's extended atmosphere causes about 25% of the common
envelope to be ejected from the system, with mass continuing to be lost at the
end of the simulation at a rate ~ 2 Msun/yr. In the process the resulting loss
of angular momentum and energy reduces the orbital separation by a factor of
seven. After this inspiral phase the eccentricity of the orbit rapidly
decreases with time. The gravitational drag dominates hydrodynamic drag at all
times in the evolution, and the commonly-used Bondi-Hoyle-Lyttleton
prescription for estimating the accretion rate onto the companion significantly
overestimates the true rate. On scales comparable to the orbital separation,
the gas flow in the orbital plane in the vicinity of the two cores is subsonic
with the gas nearly corotating with the red giant core and circulating about
the red giant companion. On larger scales, 90% of the outflow is contained
within 30 degrees of the orbital plane, and the spiral shocks in this material
leave an imprint on the density and velocity structure. Of the energy released
by the inspiral of the cores, only about 25% goes toward ejection of the
envelope.Comment: 18 pages, 11 figures, submitted to ApJ; accepted versio
CCD Photometry of the Globular Cluster M5. I. The Color-Magnitude Diagram and Luminosity Functions
We present new BVI photometry for the halo globular cluster M5, and examine
the B- and I-band luminosity functions (LFs), based on over 20,000 stars. We do
not see evidence in the LF of a ``subgiant excess'' or of a discrepancy in the
relative numbers of stars on the red-giant branch and main sequence, both of
which have been claimed in more metal-poor clusters. Inclusion of alpha-element
enhancements improves the agreement between the observed and predicted
positions of the ``red-giant bump''. Using the \Delta V_{TO}^{HB} method, we
conclude that the clusters M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC
1851 and NGC 2808 are the same age at the level of about 1.5 Gyr, with the
possible exception of NGC 288 (older by 3.5\pm 1.5 if the reddest NGC 288 HB
stars are on the zero-age horizontal branch). Even with NGC 288 set aside, the
large range in HB morphology in the remaining clusters appears to eliminate age
as the sole second parameter determining HB morphology in the case of constant
mass loss between RGB and HB. We are unable to chose between the two competing
values for M5's (absolute) metallicity: [Fe/H] = -1.40 (Zinn & West 1984) and
-1.17 (Sneden et al. 1992). This level of discrepancy has a signifcant effect
on the derivation of the distance modulus and absolute age of M5. From
theoretical isochrones and luminosity functions, we find an absolute age for M5
of 13.5 \pm 1 Gyr (internal error, assuming perfect models and no [M/H] error)
for the Zinn & West abundance scale and 11 \pm 1 Gyr for the higher abundance
value.Comment: AASTeX, 44 pages, uses amssym.sty, figures and tables only available
from http://ucowww.ucsc.edu/~erics/paper.html, ApJ, in pres
Close binary stars in the solar-age Galactic open cluster M67
We present multi-colour time-series CCD photometry of the solar-age galactic
open cluster M67 (NGC 2682). About 3600 frames spread over 28 nights were
obtained with the 1.5 m Russian-Turkish and 1.2 m Mercator telescopes.
High-precision observations of the close binary stars AH Cnc, EV Cnc, ES Cnc,
the Scuti type systems EX Cnc and EW Cnc, and some long-period
variables belonging to M67 are presented. Three full multi-colour light curves
of the overcontact binary AH Cnc were obtained during three observing seasons.
Likewise we gathered three light curves of EV Cnc, an EB-type binary, and two
light curves of ES Cnc, a blue straggler binary. Parts of the light change of
long-term variables S1024, S1040, S1045, S1063, S1242, and S1264 are obtained.
Period variation analysis of AH Cnc, EV Cnc, and ES Cnc were done using all
times of mid-eclipse available in the literature and those obtained in this
study. In addition, we analyzed multi-colour light curves of the close binaries
and also determined new frequencies for the Scuti systems. The
physical parameters of the close binary stars were determined with simultaneous
solutions of multi-colour light and radial velocity curves. Finally we
determined the distance of M67 as 857(33) pc via binary star parameters, which
is consistent with an independent method from earlier studies.Comment: 12 pages, 9 Figures, 13 Table
Modelling Collision Products of Triple-Star Mergers
In dense stellar clusters, binary-single and binary-binary encounters can
ultimately lead to collisions involving two or more stars. A comprehensive
survey of multi-star collisions would need to explore an enormous amount of
parameter space, but here we focus on a number of representative cases
involving low-mass main-sequence stars. Using both Smoothed Particle
Hydrodynamics (SPH) calculations and a much faster fluid sorting software
package (MMAS), we study scenarios in which a newly formed product from an
initial collision collides with a third parent star. By varying the order in
which the parent stars collide, as well as the orbital parameters of the
collision trajectories, we investigate how factors such as shock heating affect
the chemical composition and structure profiles of the collision product. Our
simulations and models indicate that the distribution of most chemical elements
within the final product is not significantly affected by the order in which
the stars collide, the direction of approach of the third parent star, or the
periastron separations of the collisions. We find that the sizes of the
products, and hence their collisional cross sections for subsequent encounters,
are sensitive to the order and geometry of the collisions. For the cases that
we consider, the radius of the product formed in the first (single-single star)
collision ranges anywhere from roughly 2 to 30 times the sum of the radii of
its parent stars. The final product formed in our triple-star collisions can
easily be as large or larger than a typical red giant. We therefore expect the
collisional cross section of a newly formed product to be greatly enhanced over
that of a thermally relaxed star of the same mass.Comment: 20 pages, submitted to MNRA
Population Synthesis of Common Envelope Mergers: I. Giant Stars with Stellar or Substellar Companions
Using a population synthesis technique, we have calculated detailed models of
the present-day field population of objects that have resulted from the merger
of a giant primary and a main-sequence or brown dwarf secondary during
common-envelope evolution. We used a grid of 116 stellar and 32 low-mass/brown
dwarf models, a crude model of the merger process, and followed the angular
momentum evolution of the binary orbit and the primary's rotation prior to
merger, as well as the merged object's rotation after the merger. We find that
present-day merged objects that are observable as giant stars or core-helium
burning stars in our model population constitute between 0.24% and 0.33% of the
initial population of ZAMS binaries, depending upon the input parameters
chosen. The median projected rotational velocity of these merged objects is ~16
km/sec, an order of magnitude higher than the median projected rotational
velocity in a model population of normal single stars calculated using the same
stellar models and initial mass function. The masses of the merged objects are
typically less than ~2 solar masses, with a median mass of 1.28 solar masses,
which is slightly more than, but not significantly different from, their normal
single star counterparts. The luminosities in our merged object population
range from ~10-100 solar luminosities, with a strong peak in the luminosity
distribution at ~60 solar luminosities, since the majority of the merged
objects (57%) lie on the horizontal branch at the present epoch. The results of
our population synthesis study are discussed in terms of possible observational
counterparts either directly involving the high rotational velocity of the
merger product or indirectly, via the effect of rotation on envelope abundances
and on the amount and distribution of circumstellar matter.Comment: 16 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Composition Mixing during Blue Straggler Formation and Evolution
We use smoothed-particle hydrodynamics to examine differences between direct
collisions of single stars and binary star mergers in their roles as possible
blue straggler star formation mechanisms. We find in all cases that core helium
in the progenitor stars is largely retained in the core of the remnant, almost
independent of the type of interaction or the central concentration of the
progenitor stars.
We have also modelled the subsequent evolution of the hydrostatic remnants,
including mass loss and energy input from the hydrodynamical interaction. The
combination of the hydrodynamical and hydrostatic models enables us to predict
that little mixing will occur during the merger of two globular cluster stars
of equal mass. In contrast to the results of Proctor Sills, Bailyn, & Demarque
(1995), we find that neither completely mixed nor unmixed models can match the
absolute colors of observed blue stragglers in NGC 6397 at all luminosity
levels. We also find that the color distribution is probably the crucial test
for explanations of BSS formation - if stellar collisions or mergers are the
correct mechanisms, a large fraction of the lifetime of the straggler must be
spent away from the main sequence. This constraint appears to rule out the
possibility of completely mixed models. For NGC 6397, unmixed models predict
blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely
mixed models predict a range from about 0.6 to 4 Gyr.Comment: AASTeX, 28 pg., accepted for ApJ, also available at
http://ucowww.ucsc.edu/~erics/bspaper.htm
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