171 research outputs found
Eccentric double white dwarfs as LISA sources in globular clusters
We consider the formation of double white dwarfs (DWDs) through dynamical
interactions in globular clusters. Such interactions can give rise to eccentric
DWDs, in contrast to the exclusively circular population expected to form in
the Galactic disk. We show that for a 5-year Laser Interferometer Space Antenna
(LISA) mission and distances as far as the Large Magellanic Cloud, multiple
harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher
than 8 for at least a handful of eccentric DWDs, given their formation rate and
typical lifetimes estimated from current cluster simulations. Consequently the
association of eccentricity with stellar-mass LISA sources does not uniquely
involve neutron stars, as is usually assumed. Due to the difficulty of
detecting (eccentric) DWDs with present and planned electromagnetic
observatories, LISA could provide unique dynamical identifications of these
systems in globular clusters.Comment: Published in ApJ 665, L5
On the origin of planets at very wide orbits from the re-capture of free floating planets
In recent years several planets have been discovered at wide orbits (>100 AU)
around their host stars. Theoretical studies encounter difficulties in
explaining their formation and origin. Here we propose a novel scenario for the
production of planetary systems at such orbits, through the dynamical recapture
of free floating planets (FFPs) in dispersing stellar clusters. This process is
a natural extension of the recently suggested scenario for the formation of
wide stellar binaries. We use N-body simulations of dispersing clusters with
10-1000 stars and comparable numbers of FFPs to study this process. We find
that planets are captured into wide orbits in the typical range ~100-10^6 AU,
and have a wide range of eccentricities (thermal distribution). Typically, 3-6
x (f_FFP/1) % of all stars capture a planetary companion with such properties
(where f_FFP is the number of FFP per star). The planetary capture efficiency
is comparable to that of capture-formed stellar-binaries, and shows a similar
dependence on the cluster size and structure. It is almost independent of the
specific planetary mass; planets as well as sub-stellar companions of any mass
can be captured. The capture efficiency decreases with increasing cluster size,
and for a given cluster size the it increases with the host/primary mass. More
than one planet can be captured around the same host and planets can be
captured into binary systems. Planets can also be captured into pre-existing
planetary and into orbits around black holes and massive white dwarfs, if these
formed early enough before the cluster dispersal. In particular, stellar black
holes have a high capture efficiency (>50 % and 5-10 x (f_FFP/1) % for capture
of stars and planetary companions, respectively) due to their large mass.
Finally, although rare, two FFPs or brown dwarfs can become bound and form a
FFP-binary system with no stellar host.Comment: ApJ, in press. Added two figure
Mass Segregation in Globular Clusters
We present the results of a new study of mass segregation in two-component
star clusters, based on a large number of numerical N-body simulations using
our recently developed dynamical Monte Carlo code. Specifically, we follow the
dynamical evolution of clusters containing stars with individual masses m_1 as
well as a tracer population of objects with individual masses m_2=\mu m_1,
using N=10^5 total stars. For heavy tracers, which could represent stellar
remnants such as neutron stars or black holes in a globular cluster, we
characterize in a variety of ways the tendency for these objects to concentrate
in or near the cluster core. In agreement with simple theoretical arguments, we
find that the characteristic time for this mass segregation process varies as
1/\mu. For models with very light tracers (\mu <~ 10^-2), which could represent
free-floating planets or brown dwarfs, we find the expected depletion of light
objects in the cluster core, but also sometimes a significant enhancement in
the halo. Using these results we estimate the optical depth to gravitational
microlensing by planetary mass objects or brown dwarfs in typical globular
clusters. For some initial conditions, the optical depth in the halo due to
very low-mass objects could be much greater than that of luminous stars. If we
apply our results to M22, using the recent null detection of Sahu, Anderson, &
King (2001), we find an upper limit of ~25% at the 63% confidence level for the
current mass fraction of M22 in the form of very low-mass objects.Comment: Accepted for publication in ApJ. Minor revisions reflecting the new
results of Sahu et al. on M22. 13 pages in emulateapj style, including 9
figures and 3 table
The binary fraction in the globular cluster M10 (NGC 6254): comparing core and outer regions
We study the binary fraction of the globular cluster M10 (NGC 6254) as a
function of radius from the cluster core to the outskirts, by means of a quan-
titative analysis of the color distribution of stars relative to the fiducial
main sequence. By taking advantage of two data-sets, acquired with the Advanced
Camera for Survey and the Wide Field Planetary Camera 2 on board the Hubble
Space Telescope, we have studied both the core and the external regions of the
cluster. The binary fraction is found to decrease from 14% within the core, to
1.5% in a region between 1 and 2 half-mass radii from the cluster centre. Such
a trend and the derived values are in agreement with previous results ob-
tained in clusters of comparable total magnitude. The estimated binary fraction
is sufficient to account for the suppression of mass segregation observed in
M10, without any need to invoke the presence of an intermediate-mass black hole
in its centre.Comment: Accepted for publication in ApJ (22 pages, 7 figures, 3 tables
The blue stragglers formed via mass transfer in old open clusters
In this paper, we present the simulations for the primordial blue stragglers
in the old open cluster M67 based on detailed modelling of the evolutionary
processes. The principal aim is to discuss the contribution of mass transfer
between the components of close binaries to the blue straggler population in
M67. First, we followed the evolution of a binary of 1.4M+0.9M.
The synthetic evolutionary track of the binary system revealed that a
primordial blue straggler had a long lifetime in the observed blue straggler
region of color-magnitude diagram. Second, a grid of models for close binary
systems experiencing mass exchange were computed from 1Gyr to 6Gyr in order to
account for primordial blue-straggler formation in a time sequence. Based on
such a grid, Monte-Carlo simulations were applied for the old open cluster M67.
Adopting appropriate orbital parameters, 4 primordial blue stragglers were
predicted by our simulations. This was consistent with the observational fact
that only a few blue stragglers in M67 were binaries with short orbital
periods. An upper boundary of the primordial blue stragglers in the
color-magnitude diagram (CMD) was defined and could be used to distinguish blue
stragglers that were not formed via mass exchange. Using the grid of binary
models, the orbital periods of the primordial BSs could be predicted. Compared
with the observations, it is clear that the mechanism discussed in this work
alone cannot fully predict the blue straggler population in M67. There must be
several other processes also involved in the formation of the observed blue
stragglers in M67.Comment: 11 pages, 6 figures, A&A accepte
The optical companion to the binary millisecond pulsar J1824-2452H in the globular cluster M28
We report on the optical identification of the companion star to the
eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster
M28 (NGC 6626). This star is at only 0.2" from the nominal position of the
pulsar and it shows optical variability (~ 0.25 mag) that nicely correlates
with the pulsar orbital period. It is located on the blue side of the cluster
main sequence, ~1.5 mag fainter than the turn-off point. The observed light
curve shows two distinct and asymmetric minima, suggesting that the companion
star is suffering tidal distortion from the pulsar. This discovery increases
the number of non-degenerate MSP companions optically identified so far in
globular clusters (4 out of 7), suggesting that these systems could be a common
outcome of the pulsar recycling process, at least in dense environments where
they can be originated by exchange interactions.Comment: accepted for publication on ApJ, 17 pages, 5 figure
Formation, Survival, and Detectability of Planets Beyond 100 AU
Direct imaging searches have begun to detect planetary and brown dwarf
companions and to place constraints on the presence of giant planets at large
separations from their host star. This work helps to motivate such planet
searches by predicting a population of young giant planets that could be
detectable by direct imaging campaigns. Both the classical core accretion and
the gravitational instability model for planet formation are hard-pressed to
form long-period planets in situ. Here, we show that dynamical instabilities
among planetary systems that originally formed multiple giant planets much
closer to the host star could produce a population of giant planets at large
(~100 AU - 100000 AU) separations. We estimate the limits within which these
planets may survive, quantify the efficiency of gravitational scattering into
both stable and unstable wide orbits, and demonstrate that population analyses
must take into account the age of the system. We predict that planet scattering
creates a population of detectable giant planets on wide orbits that decreases
in number on timescales of ~10 Myr. We demonstrate that several members of such
populations should be detectable with current technology, quantify the
prospects for future instruments, and suggest how they could place interesting
constraints on planet formation models.Comment: 13 pages (emulateapj format), 10 figures, accepted for publication in
Ap
Intermediate mass black holes in AGN disks: I. Production & Growth
Here we propose a mechanism for efficiently growing intermediate mass black
holes (IMBH) in disks around supermassive black holes. Stellar mass objects can
efficiently agglomerate when facilitated by the gas disk. Stars, compact
objects and binaries can migrate, accrete and merge within disks around
supermassive black holes. While dynamical heating by cusp stars excites the
velocity dispersion of nuclear cluster objects (NCOs) in the disk, gas in the
disk damps NCO orbits. If gas damping dominates, NCOs remain in the disk with
circularized orbits and large collision cross-sections. IMBH seeds can grow
extremely rapidly by collisions with disk NCOs at low relative velocities,
allowing for super-Eddington growth rates. Once an IMBH seed has cleared out
its feeding zone of disk NCOs, growth of IMBH seeds can become dominated by gas
accretion from the AGN disk. However, the IMBH can migrate in the disk and
expand its feeding zone, permitting a super-Eddington accretion rate to
continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of
migrators.
We highlight the remarkable parallel between the growth of IMBH in AGN disks
with models of giant planet growth in protoplanetary disks. If an IMBH becomes
massive enough it can open a gap in the AGN disk. IMBH migration in AGN disks
may stall, allowing them to survive the end of the AGN phase and remain in
galactic nuclei. Our proposed mechanisms should be more efficient at growing
IMBH in AGN disks than the standard model of IMBH growth in stellar clusters.
Dynamical heating of disk NCOs by cusp stars is transferred to the gas in a AGN
disk helping to maintain the outer disk against gravitational instability.
Model predictions, observational constraints and implications are discussed in
a companion paper (Paper II).Comment: 11 pages, 4 figures, MNRAS (accepted
The effects of fly-bys on planetary systems
Most of the observed extrasolar planets are found on tight and often
eccentric orbits. The high eccentricities are not easily explained by
planet-formation models, which predict that planets should be on rather
circular orbits. Here we explore whether fly-bys involving planetary systems
with properties similar to those of the gas giants in the solar system, can
produce planets with properties similar to the observed planets. Using
numerical simulations, we show that fly-bys can cause the immediate ejection of
planets, and sometimes also lead to the capture of one or more planets by the
intruder. More common, however, is that fly-bys only perturb the orbits of
planets, sometimes leaving the system in an unstable state. Over time-scales of
a few million to several hundred million years after the fly-by, this
perturbation can trigger planet-planet scatterings, leading to the ejection of
one or more planets. For example, in the case of the four gas giants of the
solar system, the fraction of systems from which at least one planet is ejected
more than doubles in 10^8 years after the fly-by. The remaining planets are
often left on more eccentric orbits, similar to the eccentricities of the
observed extrasolar planets. We combine our results of how fly-bys effect
solar-system-like planetary systems, with the rate at which encounters in young
stellar clusters occur. For example, we measure the effects of fly-bys on the
four gas giants in the solar system. We find, that for such systems, between 5
and 15 per cent suffer ejections of planets in 10^8 years after fly-bys in
typical open clusters. Thus, encounters in young stellar clusters can
significantly alter the properties of any planets orbiting stars in clusters.
As a large fraction of stars which populate the solar neighbourhood form in
stellar clusters, encounters can significantly affect the properties of the
observed extrasolar planets.Comment: 22 pages, 15 figures, 5 tables. Accepted for publication in MNRA
Compact Binaries in Star Clusters I - Black Hole Binaries Inside Globular Clusters
We study the compact binary population in star clusters, focusing on binaries
containing black holes, using a self-consistent Monte Carlo treatment of
dynamics and full stellar evolution. We find that the black holes experience
strong mass segregation and become centrally concentrated. In the core the
black holes interact strongly with each other and black hole-black hole
binaries are formed very efficiently. The strong interactions, however, also
destroy or eject the black hole-black hole binaries. We find no black
hole-black hole mergers within our simulations but produce many hard escapers
that will merge in the galactic field within a Hubble time. We also find
several highly eccentric black hole-black hole binaries that are potential LISA
sources, suggesting that star clusters are interesting targets for space-based
detectors. We conclude that star clusters must be taken into account when
predicting compact binary population statistics.Comment: 19 pages, 5 Tables, 12 Figures, updated in response to referee
report, accepted for publication in MNRA
- …