109 research outputs found
An Optical and X-ray Examination of Two Radio Supernova Remnant Candidates in 30 Doradus
The giant HII region 30 Doradus is known for its violent internal motions and
bright diffuse X-ray emission, suggesting the existence of supernova remnants
(SNRs), but no nonthermal radio emission has been detected. Recently, Lazendic
et al. compared the H-alpha/H-beta and radio/H-alpha ratios and suggested two
small radio sources to be nonthermal and thus SNR candidates; however, no
optical or X-ray counterparts were detected. We have used high-resolution
optical images and high-dispersion spectra to examine the morphological,
spectral, and kinematic properties of these two SNR candidates, and still find
no optical evidence supporting their identification as SNRs. We have also
determined the X-ray luminosities of these SNR candidates, and find them 1-3
orders of magnitude lower than those commonly seen in young SNRs. High
extinction can obscure optical and X-ray signatures of an SNR, but would
prohibit the use of a high radio/H-alpha ratio to identify nonthermal radio
emission. We suggest that the SNR candidate MCRX J053831.8-690620 is associated
with a young star forming region; while the radio emission originates from the
obscured star forming region, the observed optical emission is dominated by the
foreground. We suggest that the SNR candidate MCRX J053838.8-690730 is
associated with a dust/molecular cloud, which obscures some optical emission
but not the radio emission.Comment: 13 pages, 2 figures, accepted for publication in the ApJ, Nov 10,
200
L'acacia au Sénégal
L'intĂ©rĂȘt agronomique et Ă©cologique des rhizobiums repose essentiellement sur leurs propriĂ©tĂ©s symbiotiques. Il est donc capital de pouvoir apprĂ©cier la diversitĂ© des souches sur la base de leur pouvoir de nodulation. L'objectif de ce travail est de proposer une nouvelle approche, alternative aux tests de nodulation, permettant de classer les souches en fonction de leur spĂ©cificitĂ© symbiotique. Au cours des premiĂšres Ă©tapes de l'interaction rhizobium-lĂ©gumineuses, les rhizobiums excrĂštent des molĂ©cules signal, appelĂ©es facteurs Nod, qui jouent un rĂŽle dĂ©terminant dans l'infection et la nodulation des plantes-hĂŽtes. Leur utilisation potentielle comme marqueur de la spĂ©cificitĂ© de nodulation a Ă©tĂ© Ă©valuĂ©e Ă partir de l'Ă©tude d'une collection de souches isolĂ©es d'#Acacia et de #Sesbania au SĂ©nĂ©gal. L'analyse chromatographique des facteurs Nod de ces souches a montrĂ© que les profils chromatographiques sont parfaitement corrĂ©lĂ©s Ă la fois Ă la structure chimique des facteurs Nod et Ă la spĂ©cificitĂ© d'hĂŽte des souches, et plus particuliĂšrement Ă la plante d'isolement. Une telle mĂ©thode de caractĂ©risation globale des facteurs Nod pourrait donc ĂȘtre utilisĂ©e pour la caractĂ©risation symbiotique des rhizobiums, en particulier pour l'Ă©tude taxonomique, l'Ă©tude de la biodiversitĂ© des souches ou pour le contrĂŽle des inoculums. (RĂ©sumĂ© d'auteur
Confronting the Superbubble Model with X-ray Observations of 30 Dor C
We present an analysis of XMM-Newton observations of the superbubble 30 Dor C
and compare the results with the predictions from the standard wind-blown
bubble model. We find that the observed X-ray spectra cannot be fitted
satisfactorily with the model alone and that there is evidence for nonthermal
X-ray emission, which is particularly important at > 4 keV. The total
unabsorbed 0.1-10 keV luminosities of the eastern and western parts of the
bubble are ~3 10^36 erg/s and ~5 10^36 erg/s, respectively. The unabsorbed
0.1-10 keV luminosity of the bubble model is 4 10^36 erg/s and so the power-law
component contributes between 1/3 and 1/2 to the total unabsorbed luminosity in
this energy band. The nature of the hard nonthermal emission is not clear,
although recent supernovae in the bubble may be responsible. We expect that
about one or two core-collapse supernovae could have occured and are required
to explain the enrichment of the hot gas, as evidenced by the overabundance of
alpha-elements by a factor of 3, compared to the mean value of 0.5 solar for
the interstellar medium in the Large Magellanic Cloud. As in previous studies
of various superbubbles, the amount of energy currently present in 30 Dor C is
significantly less than the expected energy input from the enclosed massive
stars over their lifetime. We speculate that a substantial fraction of the
input energy may be radiated in far-infrared by dust grains, which are mixed
with the hot gas because of the thermal conduction and/or dynamic mixing.Comment: 25 pages, 4 figures. To appear in The Astrophysical Journal, August
20, 2004 issu
Thermal and Non-thermal X-Rays from the LMC Super Bubble 30 Dor C
We report on the discovery of thermal and non-thermal X-rays from the shells
of the super bubble (SB) 30 Dor C in the Large Magellanic Cloud (LMC). The
X-ray morphology is a nearly circular shell with a radius of about 40 pc, which
is bright on the northern and western sides. The spectra of the shells are
different from region to region. The southern shell shows clear emission lines,
and is well fitted with a model of a thin-thermal plasma (kT = 0.21keV) in
non-equilibrium ionization (NEI) plus a power-law component. This thermal
plasma is located inside of the H alpha emission, which is the outer edge of
the shell of the SB. The northern and western sides of the SB are dim in H
alpha emission, but are bright in non-thermal (power-law) X-rays with a photon
index of 2.1-2.9. The non-thermal X-ray shell traces the outer boundary of the
radio shell. These features of thin-thermal and non-thermal X-rays are similar
to those of SN 1006, a prototype of synchrotron X-ray shell, but the
non-thermal component of 30 Dor C is about ten-times brighter than that of SN
1006. 30 Dor C is the first candidate of an extragalactic SB, in which
energetic electrons are accelerating in the shell. The age is much older than
that of SN 1006, and hence the particle acceleration time in this SB may be
longer than those in normal shell-like SNRs. We found point-like sources
associated with some of tight star clusters. The X-ray luminosity and spectrum
are consistent with those of young clusters of massive stars. Point-like
sources with non-thermal spectra are also found in the SB. These may be
background objects (AGNs) or stellar remnants (neutron stars or black holes).Comment: 11 pages, 6 figures, Accepted for publication in ApJ, the paper with
full resolution images in
http://www-cr.scphys.kyoto-u.ac.jp/member/bamba/Paper/30DorC.pd
Young Stellar Objects and Triggered Star Formation in the Vulpecula OB Association
The Vulpecula OB association, VulOB1, is a region of active star formation
located in the Galactic plane at 2.3 kpc from the Sun. Previous studies suggest
that sequential star formation is propagating along this 100 pc long molecular
complex. In this paper, we use Spitzer MIPSGAL and GLIMPSE data to reconstruct
the star formation history of VulOB1, and search for signatures of past
triggering events. We make a census of Young Stellar Objects (YSO) in VulOB1
based on IR color and magnitude criteria, and we rely on the properties and
nature of these YSOs to trace recent episodes of massive star formation. We
find 856 YSO candidates, and show that the evolutionary stage of the YSO
population in VulOB1 is rather homogeneous - ruling out the scenario of
propagating star formation. We estimate the current star formation efficiency
to be ~8 %. We also report the discovery of a dozen pillar-like structures,
which are confirmed to be sites of small scale triggered star formation.Comment: 30 pages, 11 figures, accepted for publication in Ap
VLT/NACO observations of the High-Magnetic field radio pulsar PSR J1119-6127
Recent radio observations have unveiled the existence of a number of radio
pulsars with spin-down derived magnetic fields in the magnetar range. However,
their observational properties appears to be more similar to classical radio
pulsars than to magnetars. To shed light on this puzzle we first have to
determine whether the spin-down derived magnetic field values for these radio
pulsars are indeed representative of the actual neutron star magnetic field or
if they are polluted, e.g. by the effects of a torque from a fallback disk. To
investigate this possibility, we have performed deep IR observations of one of
these high magnetic field radio pulsars (PSR J1119-6127) with the ESO VLT to
search for IR emission which can be associated with a disk. No IR emission is
detected from the pulsar position down to J=24, H=23, Ks=22. By comparing our
flux upper limits with the predictions of fallback disk models, we have found
that we can only exclude the presence of a disk with accretion rate dot M
>3x10^16 g/s. This lower limit cannot rule out the presence of a substantial
disk torque on the pulsar, which would then lead to overestimate the value of
the magnetic field inferred from P and dot P.Comment: 8 pages, 4 figures, A&A, in pres
An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357
We present the first high spatial resolution X-ray study of the massive star
forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside
the brightest constituent of this large HII region complex is the massive open
cluster Pismis 24. It contains two of the brightest and bluest stars known, yet
remains poorly studied; only a handful of optically bright stellar members have
been identified. We investigate the cluster extent and Initial Mass Function
and detect ~800 X-ray sources with a limiting sensitivity of 10^{30} ergs
s^{-1}; this provides the first reliable probe of the rich intermediate-mass
and low-mass population of this massive cluster, increasing the number of known
members from optical study by a factor of ~50. The high luminosity end (log
L_h[2-8 keV]\ge 30.3 ergs s^{-1}) of the observed X-ray luminosity function in
NGC 6357 is clearly consistent with a power law relation as seen in the Orion
Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total
cluster population, a few times the known Orion population. We investigate the
structure of the cluster, finding small-scale substructures superposed on a
spherical cluster with 6 pc extent, and discuss its relationship to the nebular
morphology. The long-standing Lx - 10^{-7}L_{bol} correlation for O stars is
confirmed. Twenty-four candidate O stars and one possible new obscured massive
YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be
intermediate-mass stars from available infrared photometry (assuming an age of
1 Myr), but only a few exhibit K-band excess. We report the first detection of
X-ray emission from an Evaporating Gaseous Globule at the tip of a molecular
pillar; this source is likely a B0-B2 protostar.Comment: 64 pages (double columns), 9 table, 17 figures (reduced resolution),
ApJ accepted. Please contact J. Wang for full table
The Eta Chamaeleontis Cluster: Origin in the Sco-Cen OB Association
A young, nearby compact aggregate of X-ray emitting pre-main sequence stars
was recently discovered in the vicinity of eta Cha (B8V). In this paper, we
further investigate this cluster: its membership, its environs and origins.
ROSAT HRI X-ray data for the cluster's T Tauri stars show high levels of
magnetic activity and variability. The cluster has an anomalous X-ray
luminosity function compared to other young clusters, deficient in stars with
low, but detectable X-ray luminosities. This suggests that many low-mass
members have escaped the surveyed core region. Photographic photometry from the
USNO-A2.0 catalog indicates that additional, X-ray-quiet members exist in the
cluster core region. The components of the eclipsing binary RS Cha, previously
modeled in the literature as post-MS with discordant ages, are shown to be
consistent with being coeval pre-MS stars. We compute the Galactic motion of
the cluster from Hipparcos data, and compare it to other young stars and
associations in the fourth Galactic quadrant. The kinematic study shows that
the eta Cha cluster, the TW Hya association, and a new group near epsilon Cha,
probably originated near the giant molecular cloud complex that formed the two
oldest subgroups of the Sco-Cen OB association roughly 10-15 Myr ago. Their
dispersal is consistent with the velocity dispersions seen in giant molecular
clouds. A large H I filament and dust lane located near eta Cha has been
identified as part of a superbubble formed by Sco-Cen OB winds and supernova
remnants. The passage of the superbubble may have terminated star-formation in
the eta Cha cluster and dispersed its natal molecular gas.Comment: 26 pages, 9 figures, LaTex2.09, ApJ, in press,
http://etacha.as.arizona.edu/~eem/etacha/MLF00/index.htm
Spitzer 70 and 160-micron Observations of the Extragalactic First Look Survey
We present Spitzer 70um and 160um observations of the Spitzer extragalactic
First Look Survey (xFLS). The data reduction techniques and the methods for
producing co-added mosaics and source catalogs are discussed. Currently, 26% of
the 70um sample and 49% of the 160um-selected sources have redshifts. The
majority of sources with redshifts are star-forming galaxies at z<0.5, while
about 5% have infrared colors consistent with AGN. The observed infrared colors
agree with the spectral energy distribution (SEDs) of local galaxies previously
determined from IRAS and ISO data. The average 160um/70um color temperature for
the dust is Td~= 30+/-5 K, and the average 70um/24um spectral index is alpha~=
2.4+/-0.4. The observed infrared to radio correlation varies with redshift as
expected out to z~1 based on the SEDs of local galaxies. The xFLS number counts
at 70um and 160um are consistent within uncertainties with the models of galaxy
evolution, but there are indications that the current models may require slight
modifications. Deeper 70um observations are needed to constrain the models, and
redshifts for the faint sources are required to measure the evolution of the
infrared luminosity function.Comment: 16 pages including 11 figures. Accepted A
Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region
Dynamical few-body encounters in the dense cores of young massive star
clusters are responsible for the loss of a significant fraction of their
massive stellar content. Some of the escaping (runaway) stars move through the
ambient medium supersonically and can be revealed via detection of their bow
shocks (visible in the infrared, optical or radio). In this paper, which is the
second of a series of papers devoted to the search for OB stars running away
from young (several Myr) Galactic clusters and OB associations, we present the
results of the search for bow shocks around the star-forming region NGC 6357.
Using the archival data of the Midcourse Space Experiment (MSX) satellite and
the Spitzer Space Telescope, and the preliminary data release of the Wide-Field
Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry
is consistent with the possibility that they are generated by stars expelled
from the young star clusters, Pismis 24 and AH03 J1725-34.4, associated with
NGC 6357. Two of the seven bow shocks are driven by the already known O stars.
Follow-up spectroscopy of three other bow shock-producing stars showed that
they are O-type stars as well, while the 2MASS photometry of the remaining two
stars suggests that they could be B0 V stars, provided that both are located at
the same distance as NGC 6357. Detection of numerous massive stars ejected from
the very young clusters is consistent with the theoretical expectation that
star clusters can effectively lose massive stars at the very beginning of their
dynamical evolution and lends strong support to the idea that probably all
field OB stars have been dynamically ejected from their birth clusters. A
by-product of our search for bow shocks around NGC 6357 is the detection of
three circular shells typical of luminous blue variable and late WN-type
Wolf-Rayet stars.Comment: 13 pages, 16 figures, accepted for publication in A&
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