448 research outputs found
Black hole mergers in the universe
Mergers of black-hole binaries are expected to release large amounts of
energy in the form of gravitational radiation. However, binary evolution models
predict merger rates too low to be of observational interest. In this paper we
explore the possibility that black holes become members of close binaries via
dynamical interactions with other stars in dense stellar systems. In star
clusters, black holes become the most massive objects within a few tens of
millions of years; dynamical relaxation then causes them to sink to the cluster
core, where they form binaries. These black-hole binaries become more tightly
bound by superelastic encounters with other cluster members, and are ultimately
ejected from the cluster. The majority of escaping black-hole binaries have
orbital periods short enough and eccentricities high enough that the emission
of gravitational radiation causes them to coalesce within a few billion years.
We predict a black-hole merger rate of about per year per
cubic megaparsec, implying gravity wave detection rates substantially greater
than the corresponding rates from neutron star mergers. For the first
generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), we
expect about one detection during the first two years of operation. For its
successor LIGO-II, the rate rises to roughly one detection per day. The
uncertainties in these numbers are large. Event rates may drop by about an
order of magnitude if the most massive clusters eject their black hole binaries
early in their evolution.Comment: 12 pages, ApJL in pres
Catching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I
The (yet-to-be confirmed) discovery of a Neptune-sized moon around the ~3.2
Jupiter-mass planet in Kepler 1625 puts interesting constraints on the
formation of the system. In particular, the relatively wide orbit of the moon
around the planet, at ~40 planetary radii, is hard to reconcile with planet
formation theories. We demonstrate that the observed characteristics of the
system can be explained from the tidal capture of a secondary planet in the
young system. After a quick phase of tidal circularization, the lunar orbit,
initially much tighter than 40 planetary radii, subsequently gradually widened
due to tidal synchronization of the spin of the planet with the orbit,
resulting in a synchronous planet-moon system. Interestingly, in our scenario
the captured object was originally a Neptune-like planet, turned into a moon by
its capture.Comment: Accepted for publication in ApJL. 7 pages, 5 figure
Gravitational waves from double white dwarfs
Double white dwarfs could be important sources for space based gravitational
wave detectors like OMEGA and LISA. We use population synthesis to predict the
current population of double white dwarfs in the Galaxy and the gravitational
waves produced by this population. We simulate a detailed power spectrum for an
observation with an integration time of 10^6 s. At frequencies below ~3 mHz
confusion limited noise dominates. At higher frequencies a few thousand double
white dwarfs are resolved individually. Including compact binaries containing
neutron stars and black holes in our calculations yields a further few hundred
resolved binaries and some tens which can be detected above the double white
dwarf noise at low frequencies. We find that binaries in which one white dwarf
transfers matter to another white dwarf are rare, and thus unimportant for
gravitational wave detectors. We discuss the uncertainties and compare our
results with other authors.Comment: 6 pages, to appear in the proceedings of the XXXIVth Rencontres de
Moriond on "Gravitational Waves and Experimental Gravity", January 23-30,
199
Fun for Two
We performed populations synthesis calculations of single stars and binaries
and show that binary evolution is extremely important for Galactic astronomy.
We review several binary evolution models and conclude that they give quite
different results. These differences can be understood from the assumptions
related to how mass is transfered in the binary systems. Most important are 1)
the fraction of mass that is accreted by the companion star during mass
transfer, 2) the amount of specific angular momentum which is carried away with
the mass that leaves the binary system.Comment: 7 pages, 0 figures to appear in the proceeding of the IAU Symposium
200, "The Formation of Binary Stars" eds. H. Zinnecker and R. Mathie
A triple origin for the lack of tight coplanar circumbinary planets around short-period binaries
Transiting circumbinary planets are more easily detected around short-period
than long-period binaries, but none have yet been observed by {\it Kepler}
orbiting binaries with periods shorter than seven days. In triple systems,
secular Kozai-Lidov cycles and tidal friction (KLCTF) have been shown to reduce
the inner orbital period from to a few days. Indeed, the majority
of short-period binaries are observed to possess a third stellar companion.
Using secular evolution analysis and population synthesis, we show that KLCTF
makes it unlikely for circumbinary transiting planets to exist around
short-period binaries. We find the following outcomes. (1) Sufficiently massive
planets in tight and/or coplanar orbits around the inner binary can quench the
KL evolution because they induce precession in the inner binary. The KLCTF
process does not take place, preventing the formation of a short-period binary.
(2) Secular evolution is not quenched and it drives the planetary orbit into a
high eccentricity, giving rise to an unstable configuration, in which the
planet is most likely ejected from the system. (3) Secular evolution is not
quenched but the planet survives the KLCTF evolution. Its orbit is likely to be
much wider than the currently observed inner binary orbit, and is likely to be
eccentric and inclined with respect to the inner binary. These outcomes lead to
two main conclusions: (1) it is unlikely to find a massive planet on a tight
and coplanar orbit around a short-period binary, and (2) the properties of
circumbinary planets in short-period binaries are constrained by secular
evolution.Comment: Revised to match MNRAS publication. 24 pages, 22 figure
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