1,234 research outputs found
The Double-Lined Spectrum of LBV 1806-20
Despite much theoretical and observational progress, there is no known firm
upper limit to the masses of stars. Our understanding of the interplay between
the immense radiation pressure produced by massive stars in formation and the
opacity of infalling material is subject to theoretical uncertainties, and many
observational claims of ``the most massive star'' have failed the singularity
test. LBV 1806-20 is a particularly luminous object, L~10^6 Lsun, for which
some have claimed very high mass estimates (M_initial>200 Msun), based, in
part, on its similarity to the Pistol Star. We present high-resolution
near-infrared spectroscopy of LBV 1806-20, showing that it is possibly a binary
system with components separated in velocity by ~70 kms. If correct, then this
system is not the most massive star known, yet it is a massive binary system.
We argue that a binary, or merged, system is more consistent with the ages of
nearby stars in the LBV 1806-20 cluster. In addition, we find that the velocity
of V_LSR=36 kms is consistent with a distance of 11.8 kpc, a luminosity of
10^6.3 Lsun, and a system mass of ~130 Msun.Comment: ApJL, accepte
Metallicity in the Galactic Center: The Arches cluster
We present a quantitative spectral analysis of five very massive stars in the
Arches cluster, located near the Galactic center, to determine stellar
parameters, stellar wind properties and, most importantly, metallicity content.
The analysis uses a new technique, presented here for the first time, and uses
line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared
spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the
cluster. It relies on the fact that massive stars reach a maximum nitrogen
abundance that is related to initial metallicity when they are in the WNL
phase. We determine the present-day nitrogen abundance of the WNL stars in the
Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity
in the cluster to be solar. This result is invariant to assumptions about the
mass-luminosity relationship, the mass-loss rates, and rotation speeds. In
addition, from this analysis, we find the age of the Arches cluster to be
2-2.5Myr, assuming coeval formation
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
High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection
We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um)
spectroscopy of the Pistol Star and its associated nebula, finding strong
evidence to support the hypothesis that the Pistol Nebula was ejected from the
Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely
surrounds the Pistol Star, although the line intensity is much stronger on its
northern and western edges. The Br-alpha spectra show the classical ring-like
signature of quasi-spherical expansion, with weak blueshifted emission (V_max
approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where
the velocities are with respect to the velocity of the Pistol Star; further,
the redshifted emission appears to be "flattened" in the position-velocity
diagram. These data suggest that the nebula was ejected from the star several
thousand years ago, with a velocity between the current terminal velocity of
the stellar wind (95 km/s) and the present expansion velocity of gas in the
outer shell of the nebula (60 km/s). The Pa-alpha image reveals several
emission-line stars in the region, including two newly-identified emission-line
stars north of the Pistol Star with spectral types earlier than WC8 (T_eff >
50,000 K). The presence of these stars, the morphology of the Pa-alpha
emission, and the velocity field in the gas suggest that the side of the nebula
furthest from us is approaching, and being ionized by, the hot stars of the
Quintuplet, and that the highest velocity redshifted gas has been decelerated
by winds from the Quintuplet stars. We also discuss the possibility that the
nebular gas might be magnetically confined by the ambient magnetic field
delineated by the nearby nonthermal filaments.Comment: Figure 1 is included as a JPG file. Figure 1 and 2 also available at
ftp://quintup.astro.ucla.edu/pistol2
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.'
Detection of hard X-rays from a Class I protostar in the HH24-26 region in the Orion Molecular Cloud
We observed the HH24-26 region in the L1630 Orion molecular cloud complex
with the X-ray observatory ASCA in the 0.510 keV band. X-ray emission was
detected from the T Tauri star SSV61 and from the region where the Class I
protostars
SSV63E and SSV63W are located (hereafter SSV63E+W). The spectra of both
SSV63E+W and SSV61 are well explained by an optically thin thermal plasma
model. The spectrum of the T Tauri star SSV61 has a low temperature of
(0.71.2) keV and a moderate absorption of (0.91.7)
cm, while that of the protostar SSV63E+W has a high
temperature of (3.37.9) keV and a heavy absorption of
(1.21.8) cm. The X-ray light curve
of SSV63E+W showed a flare during the observation. The peak flux reached about
9 times that of the quiescent flux. The temperature and the absorption column
density do not change conspicuously during the flare. The 0.510 keV
luminosity of SSV63E+W was about erg s in the quiescent
state. The present detection of hard X-rays from SSV63E+W is remarkable,
because this is the first X-ray detection of a protostar in Orion.Comment: 14 pages, 3 postscript figures, uses aasms4.st
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+
Detection of the Sgr A* activity at 3.8 and 4.8 microns with NACO
L'-band (lambda=3.8 microns) and M'-band (lambda=4.8 microns) observations of
the Galactic Center region, performed in 2003 at VLT (ESO) with the adaptive
optics imager NACO, have lead to the detection of an infrared counterpart of
the radio source Sgr A* at both wavelengths. The measured fluxes confirm that
the Sgr A* infrared spectrum is dominated by the synchrotron emission of
nonthermal electrons. The infrared counterpart exhibits no significant short
term variability but demonstrates flux variations on daily and yearly scales.
The observed emission arises away from the position of the dynamical center of
the S2 orbit and would then not originate from the closest regions of the black
hole.Comment: 5 pages, 3 figures, accepted in Astronomy & Astrophysic
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
Results from a Near Infrared Search for Emission-line Stars in the Inner Galaxy: Spectra of New Wolf-Rayet Stars
We present follow-up spectroscopy of emission line candidates detected on
near-infrared narrow band images in the inner Galaxy (Homeier et al. 2003). The
filters are optimized for the detection of Wolf-Rayet stars and other objects
which exhibit emission--lines in the 2 m region. Approximately three
square degrees along the Galactic plane have been analyzed in seven
narrow--filters (four emission--lines and three continuum). We have discovered
4 new Wolf-Rayet stars and present coordinates, finding charts, and K-band
spectra.Comment: To appear in Astronomy & Astrophysic
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