67 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 dynamics of stellar disks in live dark-matter halos
Recent developments in computer hardware and software enable researchers to
simulate the self-gravitating evolution of galaxies at a resolution comparable
to the actual number of stars. Here we present the results of a series of such
simulations. We performed -body simulations of disk galaxies with between
100 and 500 million particles over a wide range of initial conditions. Our
calculations include a live bulge, disk, and dark matter halo, each of which is
represented by self-gravitating particles in the -body code. The simulations
are performed using the gravitational -body tree-code Bonsai running on the
Piz Daint supercomputer. We find that the time scale over which the bar forms
increases exponentially with decreasing disk-mass fraction and that the bar
formation epoch exceeds a Hubble time when the disk-mass fraction is
. These results can be explained with the swing-amplification theory.
The condition for the formation of spirals is consistent with that for
the formation of the bar, which is also an phenomenon. We further argue
that the non-barred grand-design spiral galaxies are transitional, and that
they evolve to barred galaxies on a dynamical timescale. We also confirm that
the disk-mass fraction and shear rate are important parameters for the
morphology of disk galaxies. The former affects the number of spiral arms and
the bar formation epoch, and the latter determines the pitch angle of the
spiral arms.Comment: 23 pages; 29 figures. Accepted by MNRA
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
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
How many young star clusters exist in the Galactic center?
We study the evolution and observability of young compact star clusters
within about 200pc of the Galactic center. Calculations are performed using
direct N-body integration on the GRAPE-4, including the effects of both stellar
and binary evolution and the external influence of the Galaxy. The results of
these detailed calculations are used to calibrate a simplified model applicable
over a wider range of cluster initial conditions. We find that clusters within
200 pc from the Galactic center dissolve within about 70 Myr. However, their
projected densities drop below the background density in the direction of the
Galactic center within 20 Myr, effectively making these clusters undetectable
after that time. Clusters farther from the Galactic center but at the same
projected distance are more strongly affected by this selection effect, and may
go undetected for their entire lifetimes. Based on these findings, we conclude
that the region within 200 pc of the Galactic center could easily harbor some
50 clusters with properties similar to those of the Arches or the Quintuplet
systems.Comment: ApJ Letters in pres
Core Formation by a Population of Massive Remnants
Core radii of globular clusters in the Large and Small Magellanic Clouds show
an increasing trend with age. We propose that this trend is a dynamical effect
resulting from the accumulation of massive stars and stellar-mass black holes
at the cluster centers. The black holes are remnants of stars with initial
masses exceeding 20-25 solar masses; as their orbits decay by dynamical
friction, they heat the stellar background and create a core. Using analytical
estimates and N-body experiments, we show that the sizes of the cores so
produced and their growth rates are consistent with what is observed. We
propose that this mechanism is responsible for the formation of cores in all
globular clusters and possibly in other systems as well.Comment: 5 page
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