6,147 research outputs found
Chemical Abundance Study of One Red Giant Star in NGC 5694 : A Globular Cluster with Dwarf Spheroidals' Chemical Signature?
We report the abundance analysis of one red giant branch star in the
metal-poor outer halo globular cluster NGC 5694. We obtain [Fe/H] = -1.93,
based on the ionized lines, and our metallicity measurement is in good
agreement with previous estimates. We find that [Ca+Ti/2Fe] and [Cu/Fe] of NGC
5694 are about 0.3 -- 0.4 dex lower than other globular clusters with similar
metallicities, but similar to some LMC clusters and stars in some dwarf
spheroidal galaxies. Differences persist, however, in the abundances of neutron
capture elements. The unique chemical abundance pattern and the large
Galactocentric distance (30 kpc) and radial velocity (-138.6 +/- 1.0 km/sec)
indicate that NGC 5694 had an extragalactic origin.Comment: ApJL accepte
Angular momentum dependent friction slows down rotational relaxation under non-equilibrium conditions
It has recently been shown that relaxation of the rotational energy of hot
non-equlibrium photofragments (i) slows down significantly with the increase of
their initial rotational temperature and (ii) differs dramatically from the
relaxation of the equilibrium rotational energy correlation function,
manifesting thereby breakdown of the linear response description [Science 311,
1907 (2006)]. We demonstrate that this phenomenon may be caused by the angular
momentum dependence of rotational friction. We have developed the generalized
Fokker-Planck equation whose rotational friction depends upon angular momentum
algebraically. The calculated rotational correlation functions correspond well
to their counterparts obtained via molecular dynamics simulations in a broad
range of initial non-equilibrium conditions. It is suggested that the angular
momentum dependence of friction should be taken into account while describing
rotational relaxation far from equilibrium
IRAC Observations of Taurus Pre-Main Sequence Stars
We present infrared photometry obtained with the IRAC camera on the Spitzer
Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in
the Taurus star-forming region. We find a clear separation in some IRAC
color-color diagrams between objects with and without disks. A few
``transition'' objects are noted, which correspond to systems in which the
inner disk has been evacuated of small dust. Separating pure disk systems from
objects with remnant protostellar envelopes is more difficult at IRAC
wavelengths, especially for objects with infall at low rates and large angular
momenta. Our results generally confirm the IRAC color classification scheme
used in previous papers by Allen et al. and Megeath et al. to distinguish
between protostars, T Tauri stars with disks, and young stars without (inner)
disks. The observed IRAC colors are in good agreement with recent improved disk
models, and in general accord with models for protostellar envelopes derived
from analyzing a larger wavelength region. We also comment on a few Taurus
objects of special interest. Our results should be useful for interpreting IRAC
results in other, less well-studied star-forming regions.Comment: 29 pages 10 figures, to appear in Ap
Dynamical Evolution of Elliptical Galaxies with Central Singularities
We study the effect of a massive central singularity on the structure of a
triaxial galaxy using N-body simulations. Starting from a single initial model,
we grow black holes with various final masses Mh and at various rates, ranging
from impulsive to adiabatic. In all cases, the galaxy achieves a final shape
that is nearly spherical at the center and close to axisymmetric throughout.
However, the rate of change of the galaxy's shape depends strongly on the ratio
Mh/Mg of black hole mass to galaxy mass. When Mh/Mg < 0.3%, the galaxy evolves
in shape on a timescale that exceeds 100 orbital periods, or roughly a galaxy
lifetime. When Mh/Mg > 2%, the galaxy becomes axisymmetric in little more than
a crossing time. We propose that the rapid evolution toward axisymmetric shapes
that occurs when Mh/Mg > 2% provides a negative feedback mechanism which limits
the mass of central black holes by cutting off their supply of fuel.Comment: 27 Latex pages, 9 Postscript figures, uses aastex.sty. Accepted for
Publication in The Astrophysical Journal, Nov. 26, 199
Evidence of a Curved Synchrotron Spectrum in the Supernova Remnant SN 1006
A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo
Observatory Synthesis Telescope radio data was performed for 13 small regions
along the bright northeastern rim of the supernova remnant SN 1006. These data
were fitted with a synchrotron radiation model. The nonthermal electron
spectrum used to compute the photon emission spectra is the traditional
exponentially cut off power law, with one notable difference: The power-law
index is not a constant. It is a linear function of the logarithm of the
momentum. This functional form enables us to show, for the first time, that the
synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The
effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio
synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray
synchrotron-emitting momenta). This amount of change in the index is
qualitatively consistent with theoretical models of the amount of curvature in
the proton spectrum of the remnant. The evidence of spectral curvature implies
that cosmic rays are dynamically important instead of being "test" particles.
The spectral analysis also provides a means of determining the critical
frequency of the synchrotron spectrum associated with the highest-energy
electrons. The critical frequency seems to vary along the northeastern rim,
with a maximum value of 1.1e17 (0.6e17 - 2.1e17) Hz. This value implies that
the electron diffusion coefficient can be no larger than a factor of ~4.5-21
times the Bohm diffusion coefficient if the velocity of the forward shock is in
the range 2300-5000 km/s. Since the coefficient is close to the Bohm limit,
electrons are accelerated nearly as fast as possible in the regions where the
critical frequency is about 1.0e17 Hz.Comment: 41 pages, 8 figures, accepted by Ap
The 2-10 keV XRB dipole and its cosmological implications
The hard X-ray (>2 keV) emission of the local and distant Universe as
observed with the HEAO1-A2 experiment is reconsidered in the context of large
scale cosmic structure. Using all-sky X-ray samples of AGN and galaxy clusters
we remove the dominant local X-ray flux from within a redshift of ~ 0.02. We
evaluate the dipolar and higher order harmonic structure in 4 X-ray colours.
The estimated dipole anisotropy of the unresolved flux appears to be consistent
with a combination of the Compton-Getting effect due to the Local Group motion
(dipole amplitude Delta = 0.0042) and remaining large scale structure (0.0023
<~ Delta <~ 0.0085), in good agreement with the expectations of Cold Dark
Matter models. The observed anisotropy does however also suggest a
non-negligible Galactic contribution which is more complex than current, simple
models of >2 keV Galactic X-ray emission. Comparison of the soft and hard
colour maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data
qualitatively suggests that at least a third of the faint, unresolved ~ 18 deg
scale structure in the HEAO1-A2 data may be Galactic in origin. However, the
effect on measured flux dipoles is small (<~3%).
We derive an expression for dipole anisotropy and acceleration and
demonstrate how the dipole anisotropy of the distant X-ray frame can constrain
the amplitude of bulk motions of the universe. From observed bulk motions over
a local ~ 50 Mpc/h radius volume we determine 0.14 <~ Omega^0.6/b_x(0) <~ 0.59.Comment: 39 pages, Revised version accepted ApJ Main Journal, 3 new Figures +
additional tex
Models for Nonthermal Photon Spectra
We describe models of nonthermal photon emission from a homogeneous
distribution of relativistic electrons and protons. Contributions from the
synchrotron, inverse Compton, nonthermal bremsstrahlung and neutral-pion decay
processes are computed separately using a common parameterization of the
underlying distribution of nonthermal particles. The models are intended for
use in fitting spectra from multi-wavelength observations and are designed to
be accurate and efficient. Although our applications have focused on Galactic
supernova remnants, the software is modular, making it straightforward to
customize for different applications. In particular, the shapes of the particle
distribution functions and the shape of the seed photon spectrum used by the
inverse Compton model are defined in separate modules and may be customized for
specific applications. We assess the accuracy of these models by using a
recurrence relation and by comparing them with analytic results and with
previous numerical work by other authors.Comment: 14 pages, 7 figures, Accepted for publication in ApJ Supplemen
The Production of Sodium and Aluminum in Globular Cluster Red Giant Stars
We study the production of Na and Al around the hydrogen shell of two
red-giant sequences of different metallicity in order to explain the abundance
variations seen in globular cluster stars in a mixing scenario. Using detailed
stellar models together with an extensive nuclear reaction network, we have
calculated the distribution of the various isotopic abundances around the
hydrogen shell at numerous points along the red-giant branch. These
calculations allow for the variation in both temperature and density in the
shell region as well as the timescale of the nuclear processing, as governed by
the outward movement of the hydrogen shell. The reaction network uses updated
rates over those of Caughlin \& Fowler (1988). We find evidence for the
production of Na and Al occurring in the NeNa and MgAl cycles. In particular,
Na is significantly enhanced throughout the region above the hydrogen shell.
The use of the newer reaction rates causes a substantial increase in the
production of Al above the hydrogen shell through heavy leakage from the
NeNa cycle and should have an important effect on the predicted surface
abundances. We also find that the nuclear processing is considerably more
extensive at lower metallicities.Comment: 4 pages with 4 EPS figures embedded, accepted by ApJL March 28, 199
Partial suppression of the radial orbit instability in stellar systems
It is well known that the simple criterion proposed originally by Polyachenko
and Shukhman (1981) for the onset of the radial orbit instability, although
being generally a useful tool, faces significant exceptions both on the side of
mildly anisotropic systems (with some that can be proved to be unstable) and on
the side of strongly anisotropic models (with some that can be shown to be
stable). In this paper we address two issues: Are there processes of
collisionless collapse that can lead to equilibria of the exceptional type?
What is the intrinsic structural property that is responsible for the sometimes
noted exceptional stability behavior? To clarify these issues, we have
performed a series of simulations of collisionless collapse that start from
homogeneous, highly symmetrized, cold initial conditions and, because of such
special conditions, are characterized by very little mixing. For these runs,
the end-states can be associated with large values of the global pressure
anisotropy parameter up to 2K_r/K_T \approx 2.75. The highly anisotropic
equilibrium states thus constructed show no significant traces of radial
anisotropy in their central region, with a very sharp transition to a radially
anisotropic envelope occurring well inside the half-mass radius (around 0.2
r_M). To check whether the existence of such almost perfectly isotropic
"nucleus" might be responsible for the apparent suppression of the radial orbit
instability, we could not resort to equilibrium models with the above
characteristics and with analytically available distribution function; instead,
we studied and confirmed the stability of configurations with those
characteristics by initializing N-body approximate equilibria (with given
density and pressure anisotropy profiles) with the help of the Jeans equations.Comment: 26 pages, 9 figures, accepted for publication in The Astrophysical
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