9,075 research outputs found
Globular Clusters and Dwarf Spheroidal Galaxies
Traditionally globular clusters and dwarf spheroidal galaxies have been
distinguished by using one or more of the following criteria: (1) mass, (2)
luminosity, (3) size, (4) mass-to-light ratio and (5) spread in metallicity.
However, a few recently discovered objects show some overlap between the
domains in parameter space that are occupied by galaxies and clusters. In the
present note it is shown that ellipticity can, in some cases, be used to help
distinguish between globular clusters and dwarf spheroidal galaxies.Comment: MNRAS (Letters), in pres
Metastable states of a ferromagnet on random thin graphs
We calculate the mean number of metastable states of an Ising ferromagnet on
random thin graphs of fixed connectivity c. We find, as for mean field spin
glasses that this mean increases exponentially with the number of sites, and is
the same as that calculated for the +/- J spin glass on the same graphs. An
annealed calculation of the number <N_{MS}(E)> of metastable states of energy E
is carried out. For small c, an analytic result is obtained. The result is
compared with the one obtained for spin glasses in order to discuss the role
played by loops on thin graphs and hence the effect of real frustration on the
distribution of metastable states.Comment: 15 pages, 3 figure
Kinetic energy functional for Fermi vapors in spherical harmonic confinement
Two equations are constructed which reflect, for fermions moving
independently in a spherical harmonic potential, a differential virial theorem
and a relation between the turning points of kinetic energy and particle
densities. These equations are used to derive a differential equation for the
particle density and a non-local kinetic energy functional.Comment: 8 pages, 2 figure
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Fatalism, Social Support and Mental Health in Four Former Soviet Cultures
Research on social support has identified differences in levels of support between cultures, but has provided only a limited explanation of the role of values or beliefs in accounting for such variations. In this paper we examine the relationship between fatalism and perceived support amongst 2672 respondents in four former Soviet States (Russia, Georgia, Ukraine and Belorussia), with participants drawn from groups of manual workers, managers, civil servants, students and the retired in these four countries. We also examine the consequences of such social support for mental health across these nations. Findings indicate a small but significant moderator effect for fatalism on the relationship between social support and mental health. These results are discussed in the context of the continuing economic and social challenges facing the citizens of these nations
The shape of the initial cluster mass function: what it tells us about the local star formation efficiency
We explore how the expulsion of gas from star-cluster forming cloud-cores due
to supernova explosions affects the shape of the initial cluster mass function,
that is, the mass function of star clusters when effects of gas expulsion are
over. We demonstrate that if the radii of cluster-forming gas cores are roughly
constant over the core mass range, as supported by observations, then more
massive cores undergo slower gas expulsion. Therefore, for a given star
formation efficiency, more massive cores retain a larger fraction of stars
after gas expulsion. The initial cluster mass function may thus differ from the
core mass function substantially, with the final shape depending on the star
formation efficiency. A mass-independent star formation efficiency of about 20
per cent turns a power-law core mass function into a bell-shaped initial
cluster mass function, while mass-independent efficiencies of order 40 per cent
preserve the shape of the core mass function.Comment: accepted in Ap
Kinematic Masses of Super Star Clusters in M82 from High-Resolution Near-Infrared Spectroscopy
Using high-resolution (R~22,000) near-infrared (1.51 -- 1.75 microns) spectra
from Keck Observatory, we measure the kinematic masses of two super star
clusters in M82. Cross-correlation of the spectra with template spectra of cool
evolved stars gives stellar velocity dispersions of sigma_r=15.9 +/- 0.8 km/s
for MGG-9 and sigma_r=11.4 +/- 0.8 km/s for MGG-11. The cluster spectra are
dominated by the light of red supergiants, and correlate most closely with
template supergiants of spectral types M0 and M4.5. We fit King models to the
observed profiles of the clusters in archival HST/NICMOS images to measure the
half-light radii. Applying the virial theorem, we determine masses of 1.5 +/-
0.3 x 10^6 M_sun for MGG-9 and 3.5 +/- 0.7 x 10^5 M_sun for MGG-11. Population
synthesis modelling suggests that MGG-9 is consistent with a standard initial
mass function, whereas MGG-11 appears to be deficient in low-mass stars
relative to a standard IMF. There is, however, evidence of mass segregation in
the clusters, in which case the virial mass estimates would represent lower
limits.Comment: 16 pages, 8 figures; ApJ, in pres
Tracking and data systems support for the Helios project. Volume 2: DSN support of Project Helios April 1975 - May 1976
Deep Space Network activities in the development of the Helios B mission from planning through entry of Helios 2 into first superior conjunction (end of Mission Phase II) are summarized. Network operational support activities for Helios 1 from first superior conjunction through entry into third superior conjunction are included
Multiplicity in Early Stellar Evolution
Observations from optical to centimeter wavelengths have demonstrated that
multiple systems of two or more bodies is the norm at all stellar evolutionary
stages. Multiple systems are widely agreed to result from the collapse and
fragmentation of cloud cores, despite the inhibiting influence of magnetic
fields. Surveys of Class 0 protostars with mm interferometers have revealed a
very high multiplicity frequency of about 2/3, even though there are
observational difficulties in resolving close protobinaries, thus supporting
the possibility that all stars could be born in multiple systems. Near-infrared
adaptive optics observations of Class I protostars show a lower binary
frequency relative to the Class 0 phase, a declining trend that continues
through the Class II/III stages to the field population. This loss of
companions is a natural consequence of dynamical interplay in small multiple
systems, leading to ejection of members. We discuss observational consequences
of this dynamical evolution, and its influence on circumstellar disks, and we
review the evolution of circumbinary disks and their role in defining binary
mass ratios. Special attention is paid to eclipsing PMS binaries, which allow
for observational tests of evolutionary models of early stellar evolution. Many
stars are born in clusters and small groups, and we discuss how interactions in
dense stellar environments can significantly alter the distribution of binary
separations through dissolution of wider binaries. The binaries and multiples
we find in the field are the survivors of these internal and external
destructive processes, and we provide a detailed overview of the multiplicity
statistics of the field, which form a boundary condition for all models of
binary evolution. Finally we discuss various formation mechanisms for massive
binaries, and the properties of massive trapezia.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Evaporation of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young clusters (CYCs) near
the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5
Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they
appear to be as massive as the smallest Galactic globular clusters (~10^4
Msun). A survey of cluster lifetimes for various initial mass functions,
cluster masses, and galactocentric radii is presented. Short relaxation times
due to the compactness of CYCs, and the strong tidal fields near the GC make
clusters evaporate fairly quickly. Depending on cluster parameters, mass
segregation may occur on a time scale shorter than the lifetimes of most
massive stars, which accelerates the cluster's dynamical evolution even more.
When the difference between the upper and lower mass boundaries of the initial
mass function is large enough, strongly selective ejection of lighter stars
makes massive stars dominate even in the outer regions of the cluster, so the
dynamical evolution of those clusters is weakly dependent on the lower mass
boundary. The mass bins for Fokker-Planck simulations were carefully chosen to
properly account for a relatively small number of the most massive stars. We
find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple
calculation based on the total masses in observed CYCs and the lifetimes
obtained here indicates that the massive CYCs comprise only a fraction of the
star formation rate (SFR) in the inner bulge estimated from Lyman continuum
photons and far-IR observations.Comment: 20 pages in two-column format, accepted for publication in Ap
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