95 research outputs found
Low-Frequency Radio Transients in the Galactic Center
We report the detection of a new radio transient source, GCRT J1746-2757,
located only 1.1 degrees north of the Galactic center. Consistent with other
radio transients toward the Galactic center, this source brightened and faded
on a time scale of a few months. No X-ray counterpart was detected. We also
report new 0.33 GHz measurements of the radio counterpart to the X-ray
transient source, XTE J1748-288, previously detected and monitored at higher
radio frequencies. We show that the spectrum of XTE J1748-288 steepened
considerably during a period of a few months after its peak. We also discuss
the need for a more efficient means of finding additional radio transients
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
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
A VLA Search for Water Masers in Six HII Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated with young
low-mass protostars in six HII regions -- M16, M20, NGC 2264, NGC 6357, S125,
and S140. The survey was conducted with the NRAO Very Large Array from 2000 to
2002. For several of these HII regions, ours are the first high-resolution
observations of water masers. We detected 16 water masers: eight in M16, four
in M20, three in S140, and one in NGC 2264. All but one of these were
previously undetected. No maser emission was detected from NGC 6357 or S125.
There are two principle results to our study. (1) The distribution of water
masers in M16 and M20 does not appear to be random but instead is concentrated
in a layer of compressed gas within a few tenths of a parsec of the ionization
front. (2) Significantly fewer masers are seen in the observed fields than
expected based on other indications of ongoing star formation, indicating that
the maser-exciting lifetime of protostars is much shorter in HII regions than
in regions of isolated star formation. Both of these results confirm
predictions of a scenario in which star formation is first triggered by shocks
driven in advance of ionization fronts, and then truncated approximately 10^5
years later when the region is overrun by the ionization front.Comment: 30 pages, 20 figures, 3 tables. Accepted for publication by ApJ. Full
resolution figures and PS and PDF versions with full-res figures available at
http://eagle.la.asu.edu/healy/preprints/hhc0
Convergence of decreasing male and increasing female incidence rates in major tobacco-related cancers in Europe in 1988-2010
Introduction: Smoking prevalence has been declining in men all over Europe, while the trend varies in European regions among women. To study the impact of past smoking prevalence, we present a comprehensive overview of the most recent trends in incidence, during 1988-2010, in 26 countries, of four of the major cancers in the respiratory and upper gastro-intestinal tract associated with tobacco smoking. Methods: Data from 47 population-based cancer registries for lung, laryngeal, oral cavity and pharyngeal, and oesophageal cancer cases were obtained from the newly developed data repository within the European Cancer Observatory (http://eco.iarc.fr/). Truncated age-standardised incidence rates (35-74 years) by calendar year, average annual percentage change in incidence over 1998-2007 were calculated. Smoking prevalence in selected countries was extracted from the Organisation for Economic Co-operation and Development and the World Health Organization databases. Results:
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
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
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
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
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