1,191 research outputs found
Close Approach during Hard Binary--Binary Scattering
We report on an extensive series of numerical experiments of binary--binary
scattering, analysing the cross--section for close approach during interactions
for a range of hard binary parameters of interest in globular cluster cores. We
consider the implied rate for tidal interactions for different globular
clusters and compare our results with previous, complementary estimates of
stellar collision rates in globular clusters. We find that the collision rate
for binary--binary encounters dominates in low density clusters if the binary
fraction in the cluster is larger than for wide main--sequence binaries.
In dense clusters binary--single interactions dominate the collision rate and
the core binary fraction must be \ltorder 0.1 per decade in semi--major axis
or too many collisions take place compared to observations. The rates are
consistent if binaries with semi--major axes are overabundant in
low density clusters or if breakup and ejection substantially lowers the binary
fraction in denser clusters. Given reasonable assumptions about fractions of
binaries in the cores of low density clusters such as NGC~5053, we cannot
account for all the observed blue stragglers by stellar collisions during
binary encounters, suggesting a substantial fraction may be due to coalescence
of tight primordial binaries.Comment: 13 pages including 13 ps figures. MNRAS in pres
Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts
The merger of compact binaries, especially black holes and neutron stars, is
frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we
present three dimensional hydrodynamical simulations of the relatively
neglected mergers of white dwarfs and black holes. During the merger, the white
dwarf is tidally disrupted and sheared into an accretion disk. Nuclear
reactions are followed and the energy release is negligible. Peak accretion
rates are ~0.05 Msun/s (less for lower mass white dwarfs) lasting for
approximately a minute. Many of the disk parameters can be explained by a
simple analytic model which we derive and compare to our simulations. This
model can be used to predict accretion rates for white dwarf and black hole (or
neutron star) masses which are not simulated in this paper. Although the
mergers studied here create disks with larger radii, and longer accretion times
than those from the merger of double neutron stars, a larger fraction of the
merging star's mass becomes part of the disk. Thus the merger of a white dwarf
and a black hole could produce a long duration GRB. The event rate of these
mergers may be as high as 1/Myr per galaxy.Comment: 17 pages text + 9 figures, minor corrections to text and tables,
added references, accepted by Ap
Effects of seagulls on ecosystem respiration, soil nitrogen and vegetation cover on a pristine volcanic island, Surtsey, Iceland
When Surtsey rose from the North Atlantic Ocean south of Iceland in 1963, it became a unique natural laboratory on how organisms colonize volcanic islands and form ecosystems with contrasting structures and functions. In July, 2004, ecosystem respiration rate (<i>R</i><sub>e</sub>), soil properties and surface cover of vascular plants were measured in 21 permanent research plots distributed among the juvenile communities of the island. The plots were divided into two main groups, inside and outside a seagull (<i>Larus</i> spp.) colony established on the island. Vegetation cover of the plots was strongly related to the density of gull nests. Occurrence of nests and increased vegetation cover also coincided with significant increases in <i>R</i><sub>e</sub>, soil carbon, nitrogen and C:N ratio, and with significant reductions in soil pH and soil temperatures. Temperature sensitivity (Q10 value) of <i>R</i><sub>e</sub> was determined as 5.3. When compared at constant temperature the <i>R</i><sub>e</sub> was found to be 59 times higher within the seagull colony, similar to the highest fluxes measured in drained wetlands or agricultural fields in Iceland. The amount of soil nitrogen, mainly brought onto the island by the seagulls, was the critical factor that most influenced ecosystem fluxes and vegetation development on Surtsey. The present study shows how ecosystem activity can be enhanced by colonization of animals that transfer resources from a nearby ecosystem
Discovery of a luminous white dwarf in a young star cluster in the Large Magellanic Cloud
We have identified a candidate 1-2 x 10^5 year old luminous white dwarf in
NGC 1818, a young star cluster in the Large Magellanic Cloud. This discovery
strongly constrains the boundary mass M_c at which stars stop forming neutron
stars and start forming white dwarfs, to M_c > 7.6 Msun.Comment: 4 pages, 2 figures, greyscale image available by ftp from
[email protected]. ApJLetters, accepted 17 March 199
Tidal Disruption of a Star By a Black Hole : Observational Signature
We have modeled the time-variable profiles of the Halpha emission line from
the non-axisymmetric disk and debris tail created in the tidal disruption of a
solar-type star by a million solar mass black hole. Two tidal disruption event
simulations were carried out using a three dimensional relativistic
smooth-particle hydrodynamic code, to describe the early evolution of the
debris during the first fifty to ninety days. We have calculated the physical
conditions and radiative processes in the debris using the photoionization code
CLOUDY. We model the emission line profiles in the period immediately after the
accretion rate onto the black hole became significant. We find that the line
profiles at these very early stages of the evolution of the post-disruption
debris do not resemble the double peaked profiles expected from a rotating disk
since the debris has not yet settled into such a stable structure. As a result
of the uneven distribution of the debris and the existence of a ``tidal tail''
(the stream of returning debris), the line profiles depend sensitively on the
orientation of the tail relative to the line of sight. Moreover, the predicted
line profiles vary on fairly short time scales (of order hours to days). Given
the accretion rate onto the black hole we also model the Halpha light curve
from the debris and the evolution of the Halpha line profiles in time.Comment: 20 pages, 9 figures, to appear in ApJ, 1 August 2004 issue; mpeg
simulations of tidal disruption available at
http://www.astro.psu.edu/users/tamarab/tdmovies.htm
Double Blue Straggler sequences in GCs: the case of NGC 362
We used high-quality images acquired with the WFC3 on board the HST to probe
the blue straggler star (BSS) population of the Galactic globular cluster NGC
362. We have found two distinct sequences of BSS: this is the second case,
after M 30, where such a feature has been observed. Indeed the BSS location,
their extension in magnitude and color and their radial distribution within the
cluster nicely resemble those observed in M 30, thus suggesting that the same
interpretative scenario can be applied: the red BSS sub-population is generated
by mass transfer binaries, the blue one by collisions. The discovery of four
new W UMa stars, three of which lying along the red-BSS sequence, further
supports this scenario. We also found that the inner portion of the density
profile deviates from a King model and is well reproduced by either a mild
power-law (\alpha -0.2) or a double King profile. This feature supports the
hypothesis that the cluster is currently undergoing the core collapse phase.
Moreover, the BSS radial distribution shows a central peak and monotonically
decreases outward without any evidence of an external rising branch. This
evidence is a further indication of the advanced dynamical age of NGC 362: in
fact, together with M 30, NGC 362 belongs to the family of dynamically old
clusters (Family III) in the "dynamical clock" classification proposed by
Ferraro et al. (2012). The observational evidence presented here strengthens
the possible connection between the existence of a double BSS sequence and a
quite advanced dynamical status of the parent cluster.Comment: Accepted for publication by ApJ; 39 pages, 16 figures, 1 tabl
Estimating the detectable rate of capture of stellar mass black holes by massive central black holes in normal galaxies
The capture and subsequent inspiral of stellar mass black holes on eccentric
orbits by central massive black holes, is one of the more interesting likely
sources of gravitational radiation detectable by LISA. We estimate the rate of
observable events and the associated uncertainties. A moderately favourable
mass function could provide many detectable bursts each year, and a detection
of at least one burst per year is very likely given our current understanding
of the populations in cores of normal spiral galaxies.Comment: 3 pages 2-column revtex Latex macro. No figures. Classical and
Quantum Gravity, accepte
Flow cytometry in primary breast cancer: improving the prognostic value of the fraction of cells in the S-phase by optimal categorisation of cut-off levels.
The use of continuous prognostic variables is clinically impractical, and arbitrarily chosen cut-off points can result in a loss of prognostic information. Here we report findings from a study of primary breast cancer, showing how the prognostic value of the fraction of cells in the S-phase of the cell cycle (SPF), as measured by flow cytometry, can be affected by the SPF cut-off level(s) adopted. It was possible to evaluate the SPF in 566 (94%) of 603 consecutive cases where fresh frozen specimens were available in a tumour bank at our department. Clinically, all patients were without distant spread at the time of diagnosis, and the median duration of follow-up was 4 years. Using different survival end-points and chi 2 values for each cut-off level, two optimal cut-off points, at the 7% and 12% levels, were consistently obtained for the SPF. Furthermore, both disease-free survival and the relative risk of recurrence exhibited a non-linear relationship with SPF values; the curves implied that the prognosis was better among patients with SPF values about 2-5% than in patients with lower SPF values (parabolic shape), though the relationship with higher SPF values approached linearity. The non-linearity of the curves is incompatible with the general use of the median SPF as a prognostic cut-off value. An alternative procedure might be to use two cut-off levels, one to distinguish patients with the lowest SPF values (i.e. within the parabolic survival curve) from those with higher values (i.e. with a survival curve approaching linearity), the other to distinguish between patients with intermediate SPF values and those with high values (i.e. within the almost linear part of the survival curve). The 7% and 12% obtained here would be suitable for this purpose. We conclude that prognostic information can be gained by dividing the SPF into three prognostic categories (less than 7.0%, 7.0-11.9% and greater than or equal to 12%), instead of using the median SPF level
"Kludge" gravitational waveforms for a test-body orbiting a Kerr black hole
One of the most exciting potential sources of gravitational waves for
low-frequency, space-based gravitational wave (GW) detectors such as the
proposed Laser Interferometer Space Antenna (LISA) is the inspiral of compact
objects into massive black holes in the centers of galaxies. The detection of
waves from such "extreme mass ratio inspiral" systems (EMRIs) and extraction of
information from those waves require template waveforms. The systems' extreme
mass ratio means that their waveforms can be determined accurately using black
hole perturbation theory. Such calculations are computationally very expensive.
There is a pressing need for families of approximate waveforms that may be
generated cheaply and quickly but which still capture the main features of true
waveforms. In this paper, we introduce a family of such "kludge" waveforms and
describe ways to generate them. We assess performance of the introduced
approximations by comparing "kludge" waveforms to accurate waveforms obtained
by solving the Teukolsky equation in the adiabatic limit (neglecting GW
backreaction). We find that the kludge waveforms do extremely well at
approximating the true gravitational waveform, having overlaps with the
Teukolsky waveforms of 95% or higher over most of the parameter space for which
comparisons can currently be made. Indeed, we find these kludges to be of such
high quality (despite their ease of calculation) that it is possible they may
play some role in the final search of LISA data for EMRIs.Comment: 29 pages, 11 figures, requires subeqnarray; v2 contains minor changes
for consistency with published versio
Gravitational Waves from a Compact Star in a Circular, Inspiral Orbit, in the Equatorial Plane of a Massive, Spinning Black Hole, as Observed by LISA
Results are presented from high-precision computations of the orbital
evolution and emitted gravitational waves for a stellar-mass object spiraling
into a massive black hole in a slowly shrinking, circular, equatorial orbit.
The focus of these computations is inspiral near the innermost stable circular
orbit (isco)---more particularly, on orbits for which the angular velocity
Omega is 0.03 < Omega/Omega_{isco} < 1. The computations are based on the
Teukolsky-Sasaki-Nakamura formalism, and the results are tabulated in a set of
functions that are of order unity and represent relativistic corrections to
low-orbital-velocity formulas. These tables can form a foundation for future
design studies for the LISA space-based gravitational-wave mission. A first
survey of applications to LISA is presented: Signal to noise ratios S/N are
computed and graphed as functions of the time-evolving gravitational-wave
frequency for representative values of the hole's mass M and spin a and the
inspiraling object's mass \mu, with the distance to Earth chosen to be r_o = 1
Gpc. These S/N's show a very strong dependence on the black-hole spin, as well
as on M and \mu. A comparison with predicted event rates shows strong promise
for detecting these waves, but not beyond about 1Gpc if the inspiraling object
is a white dwarf or neutron star. This argues for a modest lowering of LISA's
noise floor. A brief discussion is given of the prospects for extracting
information from the observed wavesComment: Physical Review D, in press; 21 pages, 9 figures, 10 tables it is
present in the RevTeX fil
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