761 research outputs found
The lost siblings of the Sun
The anomalous chemical abundances and the structure of the Edgewood-Kuiper
belt observed in the solar system constrain the initial mass and radius of the
star cluster in which the sun was born to to 3000 \msun and
to 3 pc. When the cluster dissolved the siblings of the sun
dispersed through the galaxy, but they remained on a similar orbit around the
Galactic center. Today these stars hide among the field stars, but 10 to 60 of
them are still present within a distance of pc. These siblings of
the sun can be identified by accurate measurements of their chemical
abundances, positions and their velocities. Finding even a few will strongly
constrain the parameters of the parental star cluster and the location in the
Galaxy where we were born.Comment: Submitted to ApJ Letter
The Origin of OB Runaway Stars
About 20% of all massive stars in the Milky Way have unusually high
velocities, the origin of which has puzzled astronomers for half a century. We
argue that these velocities originate from strong gravitational interactions
between single stars and binaries in the centers of star clusters. The ejecting
binary forms naturally during the collapse of a young (\aplt 1\,Myr) star
cluster. This model replicates the key characteristics of OB runaways in our
galaxy and it explains the \apgt 100\,\Msun\, runaway stars around young star
clusters, e.g. R136 and Westerlund~2. The high proportion and the distributions
in mass and velocity of runaways in the Milky Way is reproduced if the majority
of massive stars are born in dense and relatively low-mass (5000-10000 \Msun)
clusters.Comment: to appear in Scienc
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
Reconstructing the Arches I: Constraining the Initial Conditions
We have performed a series of N-body simulations to model the Arches cluster.
Our aim is to find the best fitting model for the Arches cluster by comparing
our simulations with observational data and to constrain the parameters for the
initial conditions of the cluster. By neglecting the Galactic potential and
stellar evolution, we are able to efficiently search through a large parameter
space to determine e.g. the IMF, size, and mass of the cluster. We find, that
the cluster's observed present-day mass function can be well explained with an
initial Salpeter IMF. The lower mass-limit of the IMF cannot be well
constrained from our models. In our best models, the total mass and the virial
radius of the cluster are initially (5.1 +/- 0.8) 10^4 Msun and 0.76 +/- 0.12
pc, respectively. The concentration parameter of the initial King model is w0 =
3-5.Comment: 12 pages, 14 Figures, revised and accepted for publication in MNRA
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,
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