1,117 research outputs found
A Second Luminous Blue Variable in the Quintuplet Cluster
H and K band moderate resolution and 4 m high resolution spectra have
been obtained for FMM#362, a bright star in the Quintuplet Cluster near the
Galactic Center. The spectral features in these bands closely match those of
the Pistol Star, a luminous blue variable and one of the most luminous stars
known. The new spectra and previously-obtained photometry imply a very high
luminosity for FMM#362, L \Lsun, and a temperature of 10,000 -
13,000 K. Based on its luminosity, temperature, photometric variability, and
similarities to the Pistol Star, we conclude that FMM#362 is a luminous blue
variable.Comment: Accepted for publication in The Astrophysical Journal Letters, 4
PostScript figures, 2 table
Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1
Using the data from the PCA and HEXTE on board the RXTE satellite, we
investigate the evolution of the 3-200 keV spectra of the peculiar low mass
X-ray binary (LMXB) Cir X-1 along the branches on its hardness-intensity
diagram (HID) from the vertical horizontal branch (VHB), through the horizontal
horizontal branch (HHB) and normal branch (NB), to the flaring branch (FB). We
detect a power-law hard component in the spectra. It is found that the derived
photon indices () of the power-law hard component are correlated with
the position on the HID. The power-law component dominates the X-ray emission
of Cir X-1 in the energy band higher than keV. The fluxes of the
power-law component are compared with those of the bremsstrahlung component in
the spectra. A possible origin of the power-law hard component is discussed.Comment: 14 pages, 5 figures, ApJ Letter accepte
Massive Stars in the Quintuplet Cluster
We present near-infrared photometry and K-band spectra of newly-identified
massive stars in the Quintuplet Cluster, one of the three massive clusters
projected within 50 pc of the Galactic Center. We find that the cluster
contains a variety of massive stars, including more unambiguously identified
Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in
earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly
identified star is the second ``Luminous Blue Variable'' in the cluster, after
the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the
cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The
total mass in observed stars is \sim 10^3 \Msun, and the implied mass is
\sim 10^4 \Msun, assuming a lower mass cutoff of 1 \Msun and a Salpeter
initial mass function. The implied mass density in stars is at least a few
thousand \Msun pc^{-3}. The newly-identified stars increase the estimated
ionizing flux from this cluster by about an order of magnitude with respect to
earlier estimates, to 10^{50.9} photons/s, or roughly what is required to
ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity
from the massive cluster stars is \Lsun, enough to account
for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a
picture which integrates most of the major features in this part of the sky,
excepting the non-thermal filaments. We compare the cluster to other young
massive clusters and globular clusters, finding that it is unique in stellar
content and age, except, perhaps, for the young cluster in the central parsec
of the Galaxy. In addition, we find that the cluster is comparable to small
``super star clusters.'
Variability of the X-ray P Cygni Line Profiles from Circinus X-1 Near Zero Phase
The luminous X-ray binary Circinus X-1 has been observed twice near zero
orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS)
onboard Chandra. The source was in a high-flux state during a flare for the
first observation, and it was in a low-flux state during a dip for the second.
Spectra from both flux states show clear P Cygni lines, predominantly from
H-like and He-like ion species. These indicate the presence of a high-velocity
outflow from the Cir X-1 system which we interpret as an equatorial
accretion-disk wind, and from the blueshifted resonance absorption lines we
determine outflow velocities of 200 - 1900 km/s with no clear velocity
differences between the two flux states. The line strengths and profiles,
however, are strongly variable both between the two observations as well as
within the individual observations. We characterize this variability and
suggest that it is due to both changes in the amount of absorbing material
along the line of sight as well as changes in the ionization level of the wind.
We also refine constraints on the accretion-disk wind model using improved
plasma diagnostics such as the He-like Mg XI triplet, and we consider the
possibility that the X-ray absorption features seen from superluminal jet
sources can generally be explained via high-velocity outflows.Comment: 12 pages, 8 figures, accepted by ApJ (Main
High Spectral Resolution Observations of the Massive Stars in the Galactic Center
We present high-resolution near-infrared spectra, obtained with the NIRSPEC
spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive
stars within the central 25 arcseconds of the Galactic center. We have
identified a total of twenty-one emission-line stars, seven of which are new
radial velocity detections with five of those being classified as He I
emission-line stars for the first time. These stars fall into two categories
based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He
I doublet absorption lines, and 2) those with broad 2.058 micron He I emission
lines. These data have the highest spectral resolution ever obtained for these
sources and, as a result, both components of the absorption doublet are
separately resolved for the first time. We use these spectral features to
measure radial velocities. The majority of the measured radial velocities have
relative errors of 20 kms, smaller than those previously obtained with
proper-motion or radial velocity measurements for similar stellar samples in
the Galactic center. The radial velocities estimated from the He I absorption
doublet are more robust than those previously estimated from the 2.058 micron
emission line, since they do not suffer from confusion due to emission from the
surrounding ISM. Using this velocity information, we agree that the stars are
orbiting in a somewhat coherent manner but are not as defined into a disk or
disks as previously thought. Finally, multi-epoch radial velocity measurements
for IRS 16NE show a change in its velocity presumably due to an unseen stellar
companion.Comment: ApJ accepted, 42 pages, 16 figure
Star Formation in Massive Protoclusters in the Monoceros OB1 Dark Cloud
We present far-infrared, submillimetre, and millimetre observations of bright
IRAS sources and outflows that are associated with massive CS clumps in the
Monoceros OB1 Dark Cloud. Individual star-forming cores are identified within
each clump. We show that combining submillimetre maps, obtained with SCUBA on
the JCMT, with HIRES-processed and modelled IRAS data is a powerful technique
that can be used to place better limits on individual source contributions to
the far-infrared flux in clustered regions. Three previously categorized "Class
I objects" are shown to consist of multiple sources in different evolutionary
stages. In each case, the IRAS point source dominates the flux at 12 and 25
microns. In two cases, the IRAS point source is not evident at submillimetre
wavelengths. The submillimetre sources contribute significantly to the 60 and
100 micron fluxes, dominating the flux in the 100 micron waveband. Using fluxes
derived from our technique, we present the spectral energy distribution and
physical parameters for an intermediate-mass Class 0 object in one of the
regions. Our new CO J=2-1 outflow maps of the three regions studied indicate
complex morphology suggestive of multiple driving sources. We discuss the
possible implications of our results for published correlations between outflow
momentum deposition rates and "source" luminosities, and for using these
derived properties to estimate the ratio of mass ejection rates to mass
accretion rates onto protostars.Comment: 12 pages, 11 gzipped gif figures, LaTex file and MNRAS style files,
accepted by MNRAS, v2: reference typos and author affiliation have been
correcte
Charm production in nonresonant e(+)e(-) annihilations at âs =10.55 GeV
This is the publisher's version also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.37.1719We report results on the differential and total cross sections for inclusive production of the charmed particles D*+, D*(0), D(0), D(+), D(s), and Îc in e(+)e(-) annihilations at âs=10.55 GeV. Widely used quark fragmentation models are discussed and compared with the measured charmed-particle momentum distributions. This comparison, as well as that with measurements at other center-of-mass energies, shows the need to take QCD corrections into account and their importance for a correct interpretation of the model parameters. The observed rate of D(0) and D(+) production is compared to the expected total charm production cross section. We measure the probability of a charmed meson being produced as a vector meson and the D*(+) decay branching fraction into D(0)Ï+
X-ray behaviour of Circinus X-1 - I: X-ray Dips as a diagnostic of periodic behaviour
We examine the periodic nature of detailed structure (particularly dips) in
the RXTE/ASM lightcurve of Circinus X-1. The significant phase wandering of the
X-ray maxima suggests their identification with the response on a viscous
timescale of the accretion disk to perturbation. We find that the X-ray dips
provide a more accurate system clock than the maxima, and thus use these as
indicators of the times of periastron passage. We fit a quadratic ephemeris to
these dips, and find its predictive power for the X-ray lightcurve to be
superior to ephemerides based on the radio flares and the full archival X-ray
lightcurve. Under the hypothesis that the dips are tracers of the mass transfer
rate from the donor, we use their occurrence rate as a function of orbital
phase to explore the (as yet unconstrained) nature of the donor. The high
term in the ephemeris provides another piece of evidence that Cir X-1
is in a state of dynamical evolution, and thus is a very young post-supernova
system. We further suggest that the radio ``synchrotron nebula'' immediately
surrounding Cir X-1 is in fact the remnant of the event that created the
compact object, and discuss briefly the evidence for and against such an
interpretation.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Observational Constraints on the Formation and Evolution of Binary Stars
We present a high spatial resolution UV to NIR survey of 44 young binary
stars in Taurus with separations of 10-1000 AU. The primary results include:
(1) The relative ages of binary star components are more similar than the
relative ages of randomly paired single stars, supporting coeval formation. (2)
Only one of the companion masses is substellar, and hence the apparent
overabundance of T Tauri star companions relative to main-sequence star
companions can not be explained by a wealth of substellar secondaries that
would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri
binary star components have very red NIR colors (K-L > 1.4) and unusually high
mass accretion rates. This phenomenon does not appear to be restricted to
binary systems, however, since a comparable fraction of single T Tauri stars
exhibit the same properties. (4) Although the disk lifetimes of single stars
are roughly equal to their stellar ages, the disk lifetimes of binary stars are
an order of magnitude less than their ages. (5) The accretion rates for both
single and binary T Tauri stars appear to be moderately mass dependent. (6)
Although most classical T Tauri star binaries retain both a circumprimary and a
circumsecondary disk, there are several systems with only a circumprimary disk.
Together with the relative accretion rates, this suggests that circumprimary
disks survive longer, on average, than circumsecondary disks. (7) The disk
lifetimes, mass ratios, and relative accretion signatures of the closest
binaries (10-100 AU) suggest that they are being replenished from a
circumbinary reservoir with low angular momentum. Overall, these results
support fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in
Ap
High Precision Stellar Radial Velocities in the Galactic Center
We present radial velocities for 85 cool stars projected onto the central
parsec of the Galaxy. The majority of these velocities have relative errors of
1 km/s, or a factor of 30-100 smaller than those previously
obtained with proper motion or other radial velocity measurements for a similar
stellar sample. The error in a typical individual stellar velocity, including
all sources of uncertainty, is 1.7 km/s. Two similar data sets were obtained
one month apart, and the total error in the relative velocities is 0.80 km/s\
in the case where an object is common to both data sets. The data are used to
characterize the velocity distribution of the old population in the Galctic
Center. We find that the stars have a Gaussian velocity distribution with a
mean heliocentric velocity of 11.0 km/s (blueshifted) and a standard
deviation of 100.9 km/s; the mean velocity of the sample is consistent
with no bulk line-of-sight motion with respect to the Local Standard of Rest.
At the 1 sigma level, the data are consistent with a symmetric velocity
distribution about any arbitrary axis in the plane of the sky. We find evidence
for a flattening in the distribution of late-type stars within a radius of
0.4 \pc, and infer a volume density distribution of r in this
region. Finally, we establish a first epoch of radial velocity measurements
which can be compared to subsequent epochs to measure small accelerations (1
km/s/yr), corresponding to the magnitude expected over a timespan of several
years for stars nearest to Sgr A*.Comment: retrieve full version at
http://www-int.stsci.edu/figer/papers/nirspec/vel
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