6,147 research outputs found
Bimodal Infrared Colors of the M87 Globular Cluster System: Peaks in the Metallicity Distribution
The globular cluster (GC) systems of many galaxies reveal bimodal optical
color distributions. Based on stellar evolutionary models and the bimodal
colors and metallicities of Galactic GCs this is thought to reflect an
underlying bimodal metallicity distribution. However, stars at many different
phases of stellar evolution contribute to optical light. The I-H color is a
much cleaner tracer of metallicity because it primarily samples the metallicity
sensitive giant branch. Therefore, we use deep HST-NICMOS H, and WFPC2 optical
observations, of M87 GCs to study their metallicity distribution. The M87
clusters are bimodal in I-H, for which there is no known physical explanation
other than a bimodal metallicity distribution. Moreover, the two modes defined
by the B-I and I-H colors are comprised of roughly the same two sets of
objects, confirming that optical colors also primarily trace the metallicity.
This is inconsistent with a recent suggestion based on one model of metallicity
effects on the horizontal branch that bimodality arises from an underlying
unimodal metallicity distribution due to a specific color-metallicity relation.
We also find no discernable variation in the peak colors of the M87 GCs out to
roughly 75 kpc due to the declining ratio of red-to-blue GCs, as implied by
this model. Similarly, there is no evidence that the bimodal peaks are bluer
for systems with large blue-to-red GC ratio. Our observations confirm that the
primary cause of bimodality in cluster systems is an underlying bimodal
metallicity distribution, and not the specific color-metallicity relationship
defined by this horizontal branch model.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figs. Version 2
is identical to version
Stabilizing Scheduling Policies for Networked Control Systems
This paper deals with the problem of allocating communication resources for
Networked Control Systems (NCSs). We consider an NCS consisting of a set of
discrete-time LTI plants whose stabilizing feedback loops are closed through a
shared communication channel. Due to a limited communication capacity of the
channel, not all plants can exchange information with their controllers at any
instant of time. We propose a method to find periodic scheduling policies under
which global asymptotic stability of each plant in the NCS is preserved. The
individual plants are represented as switched systems, and the NCS is expressed
as a weighted directed graph. We construct stabilizing scheduling policies by
employing cycles on the underlying weighted directed graph of the NCS that
satisfy appropriate contractivity conditions. We also discuss algorithmic
design of these cycles
Yang-Baxter algebra and generation of quantum integrable models
An operator deformed quantum algebra is discovered exploiting the quantum
Yang-Baxter equation with trigonometric R-matrix. This novel Hopf algebra along
with its limit appear to be the most general Yang-Baxter algebra
underlying quantum integrable systems. Three different directions of
application of this algebra in integrable systems depending on different sets
of values of deforming operators are identified. Fixed values on the whole
lattice yield subalgebras linked to standard quantum integrable models, while
the associated Lax operators generate and classify them in an unified way.
Variable values construct a new series of quantum integrable inhomogeneous
models. Fixed but different values at different lattice sites can produce a
novel class of integrable hybrid models including integrable matter-radiation
models and quantum field models with defects, in particular, a new quantum
integrable sine-Gordon model with defect.Comment: 13 pages, revised and bit expanded with additional explanations,
accepted for publication in Theor. Math. Phy
Electron transport and thermoelectric properties of layered perovskite LaBaCo2O5.5
We have investigated the systematic transport properties of the layered
112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity,
magnetoresistance, electroresistance and thermoelectric measurements in various
conditions. In order to understand the complex conduction mechanism of
LaBaCo2O5.5, the transport data have been analyzed using different theoretical
models. The system shows semiconductor-semiconductor like transition (TSC)
around 326K, corresponding to ferromagnetic transition and in the low
temperature region resistivity data follows the Motts variable range hopping
model. Interestingly, near and below the room temperature this compound depicts
significant change in electro- and magnetoresistance behavior, the latter one
is noteworthy near the magnetic phase boundary. The temperature dependence of
thermopower, S(T), exhibits p-type polaronic conductivity in the temperature
range of 60-320K and reaches a maximum value of 303 uV/K (at 120K). In the low
temperature AFM region, the unusual S(T) behavior, generally observed for the
cobaltite series LnBaCo2O5.5 (Ln = Rare Earth), is explained by the electron
magnon scattering mechanism as previously described for perovskite manganites.Comment: 18 pages including fig
Wide-Field Imaging from Space of Early-Type Galaxies and Their Globular Clusters
Wide-field imaging from space will reveal a wealth of information about the
globular cluster systems of any galaxies in the local universe that are
observed by such a mission. Individual globular clusters around galaxies in the
local universe have compact sizes that are ideal for the excellent spatial
resolution afforded by space-based imaging, while systems of these globular
clusters have large spatial extent that can only be fully explored by
wide-field imaging. One example of the science return from such a study is the
determination of the major formation epoch(s) of galaxies from the ages of
their globular clusters determined via their optical to near-infrared colors. A
second example is determining the sites of metal-poor globular cluster
formation from their cosmological bias, which constrains the formation of
structures early in the universe.Comment: To appear in the conference proceedings of "Wide Field Imaging from
Space" published in New Astronomy Reviews, eds. T. McKay, A. Fruchter, & E.
Linde
A Variable Black Hole X-Ray Source in a NGC 1399 Globular Cluster
We have discovered an accreting black hole (BH) in a spectroscopically
confirmed globular cluster (GC) in NGC 1399 through monitoring of its X-ray
activity. The source, with a peak luminosity of L_x=2x10^39 ergs/s, reveals an
order of magnitude change in the count rate within ~10 ks in a Chandra
observation. The BH resides in a metal-rich [Fe/H]~0.2 globular cluster. After
RZ2109 in NGC 4472 this is only the second black-hole X-ray source in a GC
confirmed via rapid X-ray variability. Unlike RZ2109, the X-ray spectrum of
this BH source did not change during the period of rapid variability. In
addition to the short-term variability the source also exhibits long-term
variability. After being bright for at least a decade since 1993 within a span
of 2 years it became progressively fainter, and eventually undetectable, or
marginally detectable, in deep Chandra and XMM observations. The source also
became harder as it faded. The characteristics of the long term variability in
itself provide sufficient evidence to identify the source as a BH. The long
term decline in the luminosity of this object was likely not recognized in
previous studies because the rapid variability within the bright epoch
suppressed the average luminosity in that integration. The hardening of the
spectrum accompanying the fading would also make this black hole source
indistinguishable from an accreting neutron star in some epochs. Therefore some
low mass X-ray binaries identified as NS accretors in snapshot studies of
nearby galaxies may also be BHs. Thus the discovery of the second confirmed BH
in an extragalactic GC through rapid variability at the very least suggests
that accreting BHs in GCs are not exceedingly rare occurences.Comment: 8 pages, 2 figs. Accepted for publication in Ap
Meterwave observations of a coronal hole
Meter-wave maps are presented showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in HeI lambda10830 spectroheliograms. The 73.8 MHz coronal hole, when seen near disk center, appears to coincide with the HeI footprint of the hole. At the lower frequencies, the emission comes from higher levels of the corona, and the hole appears to be displaced, probably due to the non-radial structure of the coronal hole. The contrast of the hole relative to the quiet Sun is much greater than reported previously for a coronal hole observed at 80 MHz. The higher contrast is certainly real, due to the superior dynamic range, sensitivity, and calibration of the Clark Lake instrument. Using a coronal hole model, the electron density is derived from radio observations of the brightness temperature. A very large discrepancy is found between the derived density and that determined from Skylab EUV observations of coronal holes. This discrepancy suggests that much of the physics of coronal holes has yet to be elucidated
The Globular Cluster Systems in the Coma Ellipticals. III: The Unique Case of IC 4051
Using archival \hst WFPC2 data, we derive the metallicity distribution,
luminosity function, and spatial structure of the globular cluster system
around IC 4051, a giant E galaxy on the outskirts of the Coma cluster core. The
metallicity distribution derived from the (V-I) colors has a mean [Fe/H] =
-0.3, a near-complete lack of metal-poor clusters, and only a small metallicity
gradient with radius; it may, however, have two roughly equal metallicity
subcomponents, centered at [Fe/H] ~ 0.0 and -1.0. The luminosity distribution
(GCLF) has the Gaussian-like form observed in all other giant E galaxies, with
a peak (turnover) at V = 27.8, consistent with a Coma distance of 100 Mpc. The
radial profiles of both the GCS and the halo light show an unusually steep
falloff which may indicate that the halo of this galaxy has been tidally
truncated. Lastly, the specific frequency of the GCS is remarkably large: we
find S_N = 11 +- 2, resembling the central cD-type galaxies even though IC 4051
is not a cD or brightest cluster elliptical. A formation model consistent with
most of the observations would be that this galaxy was subjected to removal of
a large fraction of its protogalactic gas shortly after its main phase of
globular cluster formation, probably by its first passage through the Coma
core. Since then, no significant additions due to accretions or mergers have
taken place.Comment: 24 pp. plus 13 Figures. Postscript file for the complete paper can
also be downloaded from http://www.physun.mcmaster.ca/~harris/WEHarris.html.
Astron.J., in pres
Green-Kubo formula for heat conduction in open systems
We obtain an exact Green-Kubo type linear response result for the heat
current in an open system. The result is derived for classical Hamiltonian
systems coupled to heat baths. Both lattice models and fluid systems are
studied and several commonly used implementations of heat baths, stochastic as
well as deterministic, are considered. The results are valid in arbitrary
dimensions and for any system sizes. Our results are useful for obtaining the
linear response transport properties of mesoscopic systems. Also we point out
that for systems with anomalous heat transport, as is the case in
low-dimensional systems, the use of the standard Green-Kubo formula is
problematic and the open system formula should be used.Comment: 4 page
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