153 research outputs found
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
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
The Relation Between the Globular Cluster Mass and Luminosity Functions
The relation between the globular cluster luminosity function (GCLF,
dN/dlogL) and globular cluster mass function (GCMF, dN/dlogM) is considered.
Due to low-mass star depletion, dissolving GCs have mass-to-light (M/L) ratios
that are lower than expected from their metallicities. This has been shown to
lead to an M/L ratio that increases with GC mass and luminosity. We model the
GCLF and GCMF and show that the power law slopes inherently differ (1.0 versus
0.7, respectively) when accounting for the variability of M/L. The observed
GCLF is found to be consistent with a Schechter-type initial cluster mass
function and a mass-dependent mass-loss rate.Comment: 4 pages, 2 figures. To appear in the proceedings of "Galaxy Wars:
Stellar Populations and Star Formation in Interacting Galaxies" (Tennessee,
July 2009
The distinction between star clusters and associations
In Galactic studies a distinction is made between (open) star clusters and
associations. For barely resolved objects at a distance of several Mpc this
distinction is not trivial to make. Here we provide an objective definition by
comparing the age of the stars to the crossing time of nearby stellar
agglomerates. We find that a satisfactory separation can be made where this
ratio equals unity. Stellar agglomerates for which the age of the stars exceeds
the crossing time are bound, and are referred to as star clusters.
Alternatively, those for which the crossing time exceeds the stellar age are
unbound and are referred to as associations. This definition is useful whenever
reliable measurements for the mass, radius and age are available.Comment: 2 pages, 2 figures, accepted for MNRAS Letter
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