632 research outputs found
Investigating Binary Properties with Next-Generation Microlensing Surveys
We explore the usefulness of future gravitational microlensing surveys in the
study of binary properties such as the binary fraction and the distributions of
binary separation and mass ratio by using the binary sample detectable through
a channel of repeating events. For this, we estimate the rate of repeating
microlensing eventstoward the Galactic bulge field based on standard models of
dynamical and physical distributions of Galactic matter combined with models of
binary separation and mass function. From this, we find that the total number
of repeating events expected to be detected from -year space-based
surveys will be --400, that is --50 times higher than the
rate of current surveys. We find that the high detection rate is due to the
greatly improved sensitivity to events associated with faint source stars and
low-magnification events. We find that the separation range of the binaries to
be covered by the repeating events will extend up to 100 AU. Therefore, the
future lensing surveys will provide a homogeneous sample that will allow to
investigate the statistical properties of Galactic binaries unbiased by
brightness of the binary components.Comment: total 6 pages, including 4 figures, ApJ, in pres
The GL 569 Multiple System
We report the results of high spectral and angular resolution infrared
observations of the multiple system GL 569 A and B that were intended to
measure the dynamical masses of the brown dwarf binary believed to comprise GL
569 B. Our analysis did not yield this result but, instead, revealed two
surprises. First, at age ~100 Myr, the system is younger than had been reported
earlier. Second, our spectroscopic and photometric results provide support for
earlier indications that GL 569 B is actually a hierarchical brown dwarf triple
rather than a binary. Our results suggest that the three components of GL 569 B
have roughly equal mass, ~0.04 Msun.Comment: 29 pages, 10 figures, accepted for publication in the Astrophysical
Journal; minor corrections to Section 5.1; changed typo in 6.
Binaries and the dynamical mass of star clusters
The total mass of a distant star cluster is often derived from the virial
theorem, using line-of-sight velocity dispersion measurements and half-light
radii, under the implicit assumption that all stars are single (although it is
known that most stars form part of binary systems). The components of binary
stars exhibit orbital motion, which increases the measured velocity dispersion,
resulting in a dynamical mass overestimation. In this article we quantify the
effect of neglecting the binary population on the derivation of the dynamical
mass of a star cluster. We find that the presence of binaries plays an
important role for clusters with total mass M < 10^5 Msun; the dynamical mass
can be significantly overestimated (by a factor of two or more). For the more
massive clusters, with Mcl > 10^5 Msun, binaries do not affect the dynamical
mass estimation significantly, provided that the cluster is significantly
compact (half-mass radius < 5 pc).Comment: Comments: 2 pages. Conference proceedings for IAUS246 'Dynamical
Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief Editor), M.
Giersz, A. Sills, Capri, Sept. 200
Cluster Origin of Triple Star HD 188753 and its Planet
The recent discovery by M. Konacki of a ``hot Jupiter'' in the hierarchical
triple star system HD 188753 challenges established theories of giant-planet
formation. If the orbital geometry of the triple has not changed since the
birth of the planet, then a disk around the planetary host star would probably
have been too compact and too hot for a Jovian planet to form by the
core-accretion model or gravitational collapse. This paradox is resolved if the
star was initially either single or had a much more distant companion. It is
suggested here that a close multi-star dynamical encounter transformed this
initial state into the observed triple, an idea that follows naturally if HD
188753 formed in a moderately dense stellar system--perhaps an open
cluster--that has since dissolved. Three distinct types of encounters are
investigated. The most robust scenario involves an initially single planetary
host star that changes places with the outlying member of a pre-existing
hierarchical triple.Comment: Accepted by ApJL; minor changes from origina
Orbital Stability of Planets in Binary Systems: A New Look at Old Results
About half of all known stellar systems with Sun-like stars consist of two or
more stars, significantly affecting the orbital stability of any planet in
these systems. This observational evidence has prompted a large array of
theoretical research, including the derivation of mathematically stringent
criteria for the orbital stability of planets in stellar binary systems, valid
for the "coplanar circular restricted three-body problem". In the following, we
use these criteria to explore the validity of results from previous theoretical
studies.Comment: 3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and
Dynamics, IAU Symposium 249, eds. Y.-S. Sun, S. Ferraz-Mello, and J.-L. Zhou
(Cambridge: Cambridge University Press
The Galactic disk mass-budget : II. Brown dwarf mass-function and density
In this paper, we extend the calculations conducted previously in the stellar
regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte
Carlo calculations taking into account the brown dwarf formation rate, spatial
distribution and binary fraction. Comparison with existing surveys seems to
exclude a power-law MF as steep as the one determined in the stellar regime
below 1 \msol and tends to favor a more flatish behaviour. Comparison with
methane-dwarf detections tends to favor an eventually decreasing form like the
lognormal or the more general exponential distributions determined in the
previous paper. We calculate predicting brown dwarf counts in near-infrared
color diagrams and brown dwarf discovery functions. These calculations yield
the presently most accurate determination of the brown dwarf census in the
Galactic disk. The brown dwarf number density is comparable to the stellar one,
pc. The corresponding brown dwarf mass
density, however, represents only about 10% of the stellar contribution, i.e.
\rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density
determined previously yields the density of star-like objects, stars and brown
dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ,
corrected version with correct figure
The Progeny of Stellar Dynamics and Stellar Evolution within an N-body model of NGC 188
We present a direct N-body simulation modeling the evolution of the old (7
Gyr) open cluster NGC 188. This is the first N-body open cluster simulation
whose initial binary population is directly defined by observations of a
specific open cluster: M35 (150 Myr). We compare the simulated color-magnitude
diagram at 7 Gyr to that of NGC 188, and discuss the blue stragglers produced
in the simulation. We compare the solar-type main sequence binary period and
eccentricity distributions of the simulation to detailed observations of
similar binaries in NGC 188. These results demonstrate the importance of
detailed observations in guiding N-body open cluster simulations. Finally, we
discuss the implications of a few discrepancies between the NGC 188 model and
observations and suggest a few methods for bringing N-body open cluster
simulations into better agreement with observations.Comment: Contributed talk at IAUS 266 'Star clusters: basic galactic building
blocks', R. de Grijs and J.R.D. Lepine, eds. 6 pages, 4 figure
A dual-mask coronagraph for observing faint companions to binary stars
Observations of binary stars for faint companions with conventional
coronagraphic methods are challenging, as both targets will be bright enough to
obscure any nearby faint companions if their scattered light is not suppressed.
We propose coronagraphic examination of binary stars using an apodized pupil
Lyot coronagraph and a pair of actively-controlled image plane masks to
suppress both stars simultaneously. The performance is compared to imaging with
a band-limited mask, a dual-mask Lyot coronagraph and with no coronagraph at
all. An imaging procedure and control system for the masks are also described.Comment: 17 pages, 6 figure
Close encounters of three black holes
We present the first fully relativistic longterm numerical evolutions of
three equal-mass black holes in a system consisting of a third black hole in a
close orbit about a black-hole binary. We find that these
close-three-black-hole systems have very different merger dynamics from
black-hole binaries. In particular, we see complex trajectories, a
redistribution of energy that can impart substantial kicks to one of the holes,
distinctive waveforms, and suppression of the emitted gravitational radiation.
We evolve two such configurations and find very different behaviors. In one
configuration the binary is quickly disrupted and the individual holes follow
complicated trajectories and merge with the third hole in rapid succession,
while in the other, the binary completes a half-orbit before the initial merger
of one of the members with the third black hole, and the resulting
two-black-hole system forms a highly elliptical, well separated binary that
shows no significant inspiral for (at least) the first t~1000M of evolution.Comment: 4 pages, 5 figure
Beyond Caustic Crossings: Properties of Binary Microlensing Light Curves
Binary microlensing light curves have a variety of morphologies. Many are
indistinguishable from point lens light curves. Of those that deviate from the
point lens form, caustic crossing light curves have tended to dominate
identified binary lens events. Other distinctive signatures of binary lenses
include significant asymmetry, multiple peaks, and repeating events. We have
quantified, using high resolution simulations, the theoretically expected
relative numbers of each type of binary lens event, based on its measurable
characteristics. We find that a microlensing survey with current levels of
photometric uncertainty and sampling should find at least as many non-caustic
crossing binary lens events as caustic crossing events; in future surveys with
more sensitive photometry, the contribution of distinctive non-caustic crossing
events will be even greater. To try to explain why caustic crossing light
curves appear to be so dominant among the published binary lensing events, we
investigate the influence of several physical effects, including blending,
sampling rate, and various binary populations.Comment: 17 pages, 17 figures, submitted to Ap
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