1,270 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
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
Explanatory Supplement of the ISOGAL-DENIS Point Source Catalogue
We present version 1.0 of the ISOGAL-DENIS Point Source Catalogue (PSC),
containing more than 100,000 point sources detected at 7 and/or 15 micron in
the ISOGAL survey of the inner Galaxy with the ISOCAM instrument on board the
Infrared Space Observatory (ISO). These sources are cross-identified, wherever
possible, with near-infrared (0.8-2.2 micron) data from the DENIS survey. The
overall surface covered by the ISOGAL survey is about 16 square degrees, mostly
(95%) distributed near the Galactic plane (|b| < 1 deg), where the source
extraction can become confusion limited and perturbed by the high background
emission. Therefore, special care has been taken aimed at limiting the
photometric error to ~0.2 magnitude down to a sensitivity limit of typically 10
mJy. The present paper gives a complete description of the entries and the
information which can be found in this catalogue, as well as a detailed
discussion of the data processing and the quality checks which have been
completed. The catalogue is available via the VizieR Service at the Centre de
Donn\'ees Astronomiques de Strasbourg (CDS,
http://vizier.u-strasbg.fr/viz-bin/VizieR/) and also via the server at the
Institut d'Astrophysique de Paris (http://www-isogal.iap.fr/). A more complete
version of this paper, including a detailed description of the data processing,
is available in electronic form through the ADS service.Comment: 21 pages, 7 figures. A&A in press. Full length version with 32
figures and detailed description of the data processing is available here:
http://www-isogal.iap.fr/Publications/ExplSupplFull.ps.g
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
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
A layered edge-on circumstellar disk around HK Tau B
We present the first high angular resolution 1.4mm and 2.7mm continuum maps
of the T Tauri binary system HK Tau obtained with the Plateau de Bure
Interferometer. The contributions of both components are well disentangled at
1.4mm and the star previously known to host an edge-on circumstellar disk, HK
Tau B, is elongated along the disk's major axis. The optically bright primary
dominates the thermal emission from the system at both wavelengths, confirming
that it also has its own circumstellar disk. Its non-detection in scattered
light images indicates that the two disks in this binary system are not
parallel. Our data further indicate that the circumprimary disk is probably
significantly smaller than the circumsecondary disk.
We model the millimeter thermal emission from the circumstellar disk
surrounding HK Tau B. We show that the disk mass derived from scattered light
images cannot reproduce the 1.4mm emission using opacities of the same
population of submicron dust grains. However, grain growth alone cannot match
all the observed properties of this disk. We propose that this disk contains
three separate layers: two thin outer surfaces which contain dust grains that
are very similar to those of the ISM, and a disk interior which is relatively
massive and/or has experienced limited grain growth with the largest grains
significantly smaller than 1mm. Such a structure could naturally result from
dust settling in a protoplanetary disk.Comment: Accepted fopr publication in A&A, 8 pages, 1 embedded figur
L-band (3.5 micron) IR-excess in massive star formation, II. RCW 57/NGC 3576
We present a JHKL survey of the massive star forming region RCW 57 (NGC 3576)
based on L-band data at 3.5 micron taken with SPIREX (South Pole Infrared
Explorer), and 2MASS JHK data at 1.25-2.2 micron. This is the second of two
papers, the first one concerning a similar JHKL survey of 30 Doradus.
Colour-colour and colour-magnitude diagrams are used to detect sources with
infrared excess. This excess emission is interpreted as coming from
circumstellar disks, and hence gives the cluster disk fraction (CDF). Based on
the CDF and the age of RCW 57, it is possible to draw conclusions on the
formation and early evolution of massive stars. The infrared excess is detected
by comparing the locations of sources in JHKL colour-colour and L vs. (K-L)
colour-magnitude diagrams to the reddening band due to interstellar extinction.
A total of 251 sources were detected. More than 50% of the 209 sources included
in the diagrams have an infrared excess. Comparison with other JHKL surveys,
including the results on 30 Doradus from the first paper, support a very high
initial disk fraction (>80%) even for massive stars, although there is an
indication of a possible faster evolution of circumstellar disks around high
mass stars. 33 sources only found in the L-band indicate the presence of
heavily embedded, massive Class I protostars. We also report the detection of
diffuse PAHs emission throughout the RCW 57 region.Comment: 15 pages, 13 figure
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
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