651 research outputs found
A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions
Spherically symmetric equal mass star clusters containing a large amount of
primordial binaries are studied using a hybrid method, consisting of a gas
dynamical model for single stars and a Monte Carlo treatment for relaxation of
binaries and the setup of close resonant and fly-by encounters of single stars
with binaries and binaries with each other (three- and four-body encounters).
What differs from our previous work is that each encounter is being integrated
using a highly accurate direct few-body integrator which uses regularized
variables. Hence we can study the systematic evolution of individual binary
orbital parameters (eccentricity, semi-major axis) and differential and total
cross sections for hardening, dissolution or merging of binaries (minimum
distance) from a sampling of several ten thousands of scattering events as they
occur in real cluster evolution including mass segregation of binaries,
gravothermal collapse and reexpansion, binary burning phase and ultimately
gravothermal oscillations. For the first time we are able to present empirical
cross sections for eccentricity variation of binaries in close three- and
four-body encounters. It is found that a large fraction of three-body and
four-body encounters results in merging. Previous cross sections obtained by
Spitzer and Gao for strong encounters can be reproduced, while for weak
encounters non-standard processes like formation of hierarchical triples occur.Comment: 16 pages, 19 figures, Latex in the MN style, submitted to MNRA
The X-ray Globular Cluster Population in NGC 1399
We report on the {\it Chandra} observations of the elliptical galaxy NGC
1399, concentrating on the X-ray sources identified with globular clusters
(GCs). A large fraction of the 2-10 keV X-ray emission in the
{\it Chandra} image is resolved into point sources with luminosities \ergsec. These sources are most likely Low Mass X-ray Binaries
(LMXBs). In a region imaged by {\it HST} about 70% of the X-ray sources are
located within GCs. This association suggests that in giant elliptical galaxies
luminous X-ray binaries preferentially form in GCs. Many of the GC sources have
super-Eddington luminosities (for an accreting neutron star) and their average
luminosity is higher than the non-GC sources. The X-ray spectral properties of
both GC and non-GC sources are similar to those of LMXBs in our Galaxy. Two of
the brightest sources, one of which is in a GC, have an ultra-soft spectrum,
similar to that seen in the high state of black hole candidates. The
``apparent'' super-Eddington luminosity in many cases may be due to multiple
LMXB systems within individual GCs, but with some of the most extremely
luminous systems containing massive black holes.Comment: accepted in ApJ letter. 10 pages 5 figure
Predictions for Triple Stars with and without a Pulsar in Star Clusters
Though about 80 pulsar binaries have been detected in globular clusters so
far, no pulsar has been found in a triple system in which all three objects are
of comparable mass. Here we present predictions for the abundance of such
triple systems, and for the most likely characteristics of these systems. Our
predictions are based on an extensive set of more than 500 direct simulations
of star clusters with primordial binaries, and a number of additional runs
containing primordial triples. Our simulations employ a number N_{tot} of equal
mass stars from N_{tot}=512 to N_{tot}=19661 and a primordial binary fraction
from 0-50%. In addition, we validate our results against simulations with
N=19661 that include a mass spectrum with a turn-off mass at 0.8 M_{sun},
appropriate to describe the old stellar populations of galactic globular
clusters. Based on our simulations, we expect that typical triple abundances in
the core of a dense cluster are two orders of magnitude lower than the binary
abundances, which in itself already suggests that we don't have to wait too
long for the first comparable-mass triple with a pulsar to be detected.Comment: 11 pages, minor changes to match MNRAS accepted versio
Long Term Evolution of Close Planets Including the Effects of Secular Interactions
This paper studies the long term evolution of planetary systems containing
short-period planets, including the effects of tidal circularization, secular
excitation of eccentricity by companion planets, and stellar damping. For
planetary systems subject to all of these effects, analytic solutions (or
approximations) are presented for the time evolution of the semi-major axes and
eccentricities. Secular interactions enhance the inward migration and accretion
of hot Jupiters, while general relativity tends to act in opposition by
reducing the effectiveness of the secular perturbations. The analytic solutions
presented herein allow us to understand these effects over a wide range of
parameter space and to isolate the effects of general relativity in these
planetary systems.Comment: 14 pages, 2 figures, accepted to Ap
The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation
We assess models for the assembly of supermassive black holes (SMBHs) at the
center of galaxies that trace their hierarchical build-up far up in the dark
halo `merger tree'. We assume that the first `seed' black holes (BHs) formed in
(mini)halos collapsing at z=20 from high-sigma density fluctuations. As these
pregalactic holes become incorporated through a series of mergers into larger
and larger halos, they sink to the center owing to dynamical friction, accrete
a fraction of the gas in the merger remnant to become supermassive, form a
binary system, and eventually coalesce. The merger history of dark matter halos
and associated BHs is followed by cosmological Monte Carlo realizations of the
merger hierarchy. A simple model, where quasar activity is driven by major
mergers and SMBHs accrete at the Eddington rate a mass that scales with the
fifth power of the velocity dispersion, is shown to reproduce the optical LF of
quasars in the redshift range 1<z<4. Binary and triple BH interactions are
followed in our merger tree. The assumptions underlying our scenario lead to
the prediction of a population of massive BHs wandering in galaxy halos and the
intergalactic medium at the present epoch, and contributing <10% to the total
BH mass density. At all epochs the fraction of binary SMBHs in galaxy nuclei is
of order 10%, while the fraction of binary quasars is less than 0.3%Comment: revised version, accepted for publication in the ApJ, emulateapj, 15
pages, 16 figure
Massive Star Mergers: Induced Mixing and Nucleosynthesis
We study the nucleosynthesis and the induced mixing during the merging of
massive stars inside a common envelope. The systems of interest are close
binaries, initially consisting of a massive red supergiant and a main-sequence
companion of a few solar masses. We apply parameterized results based on
hydrodynamical simulations to model the stream-core interaction and the
response of the star in a standard stellar-evolution code. Preliminary results
are presented illustrating the possibility of unusual nucleosynthesis and
post-merging dredge-up which can cause composition anomalies in the
supergiant's envelope.Comment: 4 pages. To be published in Ap&S
A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
The newly discovered brown-dwarf eclipsing binary 2MASS J05352184-0546085
provides a unique laboratory for testing the predictions of theoretical models
of brown-dwarf formation and evolution. The finding that the lower-mass brown
dwarf in this system is hotter than its higher-mass companion represents a
challenge to brown-dwarf evolutionary models, none of which predict this
behavior. Here we present updated determinations of the basic physical
properties of 2M0535-05, bolstering the surprising reversal of temperatures
with mass in this system. We compare these measurements with widely used
brown-dwarf evolutionary tracks, and find that the temperature reversal can be
explained by some models if the components of 2M0535-05 are mildly non-coeval,
possibly consistent with dynamical simulations of brown-dwarf formation.
Alternatively, a strong magnetic field on the higher-mass brown dwarf might
explain its anomalously low surface temperature, consistent with emerging
evidence that convection is suppressed in magnetically active, low-mass stars.
Finally, we discuss future observational and theoretical work needed to further
characterize and understand this benchmark system.Comment: 31 pages, 7 figures, accepted by Ap
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