3,110 research outputs found
Blue Straggler Stars: The Spectacular Population in M80
Using HST-WFPC2 observations in two ultraviolet (UV) filters (F225W and
F336W) of the central region of the high density Galactic Globular cluster
(GGC) M80 we have identified 305 Blue Straggler Stars (BSS) which represents
the largest and most concentrated population of BSS ever observed in a GGC. We
also identify the largest, clean sample of evolved BSS yet found. The high
stellar density alone cannot explain the BSS, and we suggest that in M80 we are
witnessing a transient dynamical state, during which stellar interactions are
delaying the core-collapse process leading to an exceptionally large population
of collisional-BSS.Comment: 15 pages, 5 figures, Astrophysical Journal Letters, in pres
Another Non-segregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419
We have used a combination of ACS-HST high-resolution and wide-field SUBARU
data in order to study the Blue Straggler Star (BSS) population over the entire
extension of the remote Galactic globular cluster NGC 2419. The BSS population
presented here is among the largest ever observed in any stellar system, with
more than 230 BSS in the brightest portion of the sequence. The radial
distribution of the selected BSS is essentially the same as that of the other
cluster stars. In this sense the BSS radial distribution is similar to that of
omega Centauri and unlike that of all Galactic globular clusters studied to
date, which have highly centrally segregated distributions and, in most cases,
a pronounced upturn in the external regions. As in the case of omega Centauri,
this evidence indicates that NGC 2419 is not yet relaxed even in the central
regions. This observational fact is in agreement with estimated half-mass
relaxation time, which is of the order of the cluster age.Comment: in press in the Ap
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
Adaptive Lévy processes and area-restricted search in human foraging
A considerable amount of research has claimed that animals’ foraging behaviors display movement lengths with power-law distributed tails, characteristic of Lévy flights and Lévy walks. Though these claims have recently come into question, the proposal that many animals forage using Lévy processes nonetheless remains. A Lévy process does not consider when or where resources are encountered, and samples movement lengths independently of past experience. However, Lévy processes too have come into question based on the observation that in patchy resource environments resource-sensitive foraging strategies, like area-restricted search, perform better than Lévy flights yet can still generate heavy-tailed distributions of movement lengths. To investigate these questions further, we tracked humans as they searched for hidden resources in an open-field virtual environment, with either patchy or dispersed resource distributions. Supporting previous research, for both conditions logarithmic binning methods were consistent with Lévy flights and rank-frequency methods–comparing alternative distributions using maximum likelihood methods–showed the strongest support for bounded power-law distributions (truncated Lévy flights). However, goodness-of-fit tests found that even bounded power-law distributions only accurately characterized movement behavior for 4 (out of 32) participants. Moreover, paths in the patchy environment (but not the dispersed environment) showed a transition to intensive search following resource encounters, characteristic of area-restricted search. Transferring paths between environments revealed that paths generated in the patchy environment were adapted to that environment. Our results suggest that though power-law distributions do not accurately reflect human search, Lévy processes may still describe movement in dispersed environments, but not in patchy environments–where search was area-restricted. Furthermore, our results indicate that search strategies cannot be inferred without knowing how organisms respond to resources–as both patched and dispersed conditions led to similar Lévy-like movement distributions
The small Blue Straggler star population in the dense Galactic Globular Cluster NGC 6752
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) which spans
the entire radial extent of the globular cluster NGC 6752. The BSS sample is
the most extensive ever obtained for this cluster. Though NGC 6752 is a high
density cluster with a large binary population, we found that its BSS content
is surprisingly low: the specific number of BSS is among the lowest ever
measured in a cluster. The BSS distribution is highly peaked in the cluster
center, shows a rapid decrease at intermediate radii, and finally rises again
at larger distances. This distribution closely resembles those observed in M3
and 47Tuc by Ferraro et al. (1993), Ferraro et al. (2003c). To date, BSS
surveys covering the central regions with HST and the outer regions with wide
field CCD ground-based observations have been performed for only these three
clusters. Despite the different dynamical properties, a bimodal radial
distribution has been found in each. A detailed comparison of observed BSS
luminosity and temperature distributions with theoretical models reveals a
population of luminous, hot BSS which is not easily interpreted.Comment: 29 pages, 10 figures, in press on Ap
Blue Stragglers in Galactic Open Clusters and the Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic open clusters
are studies in this work, where twenty-seven Galactic open clusters of ages >=
1Gyr are selected as the working sample. Based on the photometric observations
of these open clusters, synthetic integrated spectrum has been made for the
stellar population of each cluster. The effects of blue straggler stars (BSSs)
on the conventional simple stellar population (SSP) model are analyzed on an
individual cluster base. It is shown that the BSSs, whose holding positions in
the color-magnitude diagrams (CMDs) cannot be predicted by the current
single-star evolution theory, present significant modifications to the
integrated properties of theoretical SSP model. The synthesized integrated
spectral energy distributions (ISEDs) of our sample clusters are dramatically
different from the SSPs based on isochrone only. The BSSs corrected ISEDs of
stellar populations show systematic enhancements towards shorter wavelength in
the spectra. When measured with wide-band colors in unresolvable conditions,
the age of a stellar population can be seriously under-estimated by the
conventional SSP model. Therefore, considering the common existence of BSS
component in real stellar populations, a considerable amount of alternations on
the conventional ISEDs should be expected when applying the technique of
evolutionary population synthesis (EPS) to more complicated stellar systems.Comment: 45 pages, 21 figures Accepted for publication in ApJ (Feburary 1,
2005 issue
Tidal Interactions of Red Giants with Environment Stars in Globular Clusters
We investigate the tidal interactions of a red giant with a main sequence in
the dense stellar core of globular clusters by Smoothed Particle Hydrodynamics
method. Two models of 0.8 \msun red giant with the surface radii 20 and 85
R_\sun are used with 0.6 or 0.8M_\sun main sequence star treated as a point
mass. We demonstrate that even for the wide encounters that two stars fly
apart, the angular momentum of orbital motion can be deposited into the red
giant envelope to such an extent as to trigger rotational mixing and to explain
the fast rotation observed for the horizontal branch stars, and also that
sufficient mass can be accreted on the main sequence stars to disguise their
surface convective zone with the matter from the red giant envelope. On the
basis of the present results, we discuss the parameter dependence of these
transfer characteristics with non-linear effects taken into account, and derive
fitting formulae to give the amounts of energy and angular momentum deposited
into the red giant and of mass accreted onto the perturber as functions of
stellar parameters and the impact parameter of encounter. These formulae are
applicable to the encounters not only of the red giants but also of the main
sequence stars, and useful in the study of the evolution of stellar systems
with the star-star interactions taken into account.Comment: 36 pages, 11 figures, accepte
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
Chaotic Amplification of Neutrino Chemical Potentials by Neutrino Oscillations in Big Bang Nucleosynthesis
We investigate in detail the parameter space of active-sterile neutrino
oscillations that amplifies neutrino chemical potentials at the epoch of Big
Bang Nucleosynthesis. We calculate the magnitude of the amplification and show
evidences of chaos in the amplification process. We also discuss the
implications of the neutrino chemical potential amplification in the Big Bang
Nucleosynthesis. It is shown that with a eV \nue, the amplification
of its chemical potential by active-sterile neutrino oscillations can lower the
effective number of neutrino species at Big Bang Nucleosynthesis to
significantly below 3.Comment: Revtex 20 pages, 7 postscript figures. Also by
ftp://astro.queensu.ca/pub/shi/ . Submitted to PR
Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has
commenced science observations of the Sun starting in late 2016. Since the Sun
is much larger than the field of view of individual ALMA dishes, the ALMA
interferometer is unable to measure the background level of solar emission when
observing the solar disk. The absolute temperature scale is a critical
measurement for much of ALMA solar science, including the understanding of
energy transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide an
absolute temperature scale, ALMA solar observing will take advantage of the
remarkable fast-scanning capabilities of the ALMA 12m dishes to make
single-dish maps of the full Sun. This article reports on the results of an
extensive commissioning effort to optimize the mapping procedure, and it
describes the nature of the resulting data. Amplitude calibration is discussed
in detail: a path that utilizes the two loads in the ALMA calibration system as
well as sky measurements is described and applied to commissioning data.
Inspection of a large number of single-dish datasets shows significant
variation in the resulting temperatures, and based on the temperature
distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3
mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of
order 100 K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods with very
different levels of solar activity. At a resolution of order 25 arcsec, the 1.3
mm wavelength images show temperatures on the disk that vary over about a 2000
K range.Comment: Solar Physics, accepted: 24 pages, 13 figure
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