475 research outputs found
Chandra X-ray observation of the HII region Gum 31 in the Carina Nebula complex
(abridged) We used the Chandra observatory to perform a deep (70 ksec) X-ray
observation of the Gum 31 region and detected 679 X-ray point sources. This
extends and complements the X-ray survey of the central Carina nebula regions
performed in the Chandra Carina Complex Project. Using deep near-infrared
images from our recent VISTA survey of the Carina nebula complex, our Spitzer
point-source catalog, and optical archive data, we identify counterparts for
75% of these X-ray sources. Their spatial distribution shows two major
concentrations, the central cluster NGC 3324 and a partly embedded cluster in
the southern rim of the HII region, but majority of X-ray sources constitute a
rather homogeneously distributed population of young stars. Our color-magnitude
diagram analysis suggests ages of ~1-2 Myr for the two clusters, whereas the
distributed population shows a wider age range up to ~10 Myr. We also identify
previously unknown companions to two of the three O-type members of NGC 3324
and detect diffuse X-ray emission in the region. Our results suggests that the
observed region contains about 4000 young stars in total. The distributed
population is probably part of the widely distributed population of ~ 1-10 Myr
old stars, that was identified in the CCCP area. This implies that the global
stellar configuration of the Carina nebula complex is a very extended stellar
association, in which the (optically prominent) clusters contain only a
minority of the stellar population.Comment: Accepted for publication in Astronomy & Astrophysics. A high quality
preprint is available at
http://www.usm.uni-muenchen.de/people/preibisch/publications.htm
Chandra X-ray observation of the young stellar cluster NGC 3293 in the Carina Nebula Complex
We characterize the stellar population of the poorly explored young stellar
cluster NGC 3293 at the northwestern periphery of the Carina Nebula Complex, in
order to evaluate the cluster age and the mass function, and to test claims of
an abnormal IMF and a deficit of M <= 2.5 M_sun stars. We performed a deep (70
ksec) X-ray observation of NGC 3293 with Chandra and detected 1026 individual
X-ray point sources. We identify counterparts for 74% of the X-ray sources in
deep near-infrared images. Our data clearly show that NGC 3293 hosts a large
population of solar-mass stars, refuting claims of a lack of M <= 2.5 M_sun
stars. The analysis of the color magnitude diagram suggests an age of ~8-10 Myr
for the low-mass population of the cluster. There are at least 511 X-ray
detected stars with color magnitude positions that are consistent with young
stellar members within 7 arcmin of the cluster center. The number ratio of
X-ray detected stars in the 1-2 M_sun range versus the M >= 5 M_sun stars
(known from optical spectroscopy) is consistent with the expectation from a
normal field initial mass function. Most of the early B-type stars and 20% of
the later B-type stars are detected as X-ray sources. Our data shows that NGC
3293 is one of the most populous stellar clusters in the entire Carina Nebula
Complex. The cluster probably harbored several O-type stars, whose supernova
explosions may have had an important impact on the early evolution of the
Carina Nebula Complex.Comment: accepted for Astronomy & Astrophysic
Parsec-scale X-ray Flows in High-mass Star-forming Regions
The Chandra X-ray Observatory is providing remarkable new views of massive
star-forming regions, revealing all stages in the life cycle of high-mass stars
and their effects on their surroundings. We present a Chandra tour of several
high-mass star-forming regions, highlighting physical processes that
characterize the life of a cluster of high-mass stars, from deeply-embedded
cores too young to have established an HII region to superbubbles so large that
they shape our views of galaxies. Along the way we see that X-ray observations
reveal hundreds of stellar sources powering great HII region complexes,
suffused by both hard and soft diffuse X-ray structures caused by fast O-star
winds thermalized in wind-wind collisions or by termination shocks against the
surrounding media. Finally, we examine the effects of the deaths of high-mass
stars that remained close to their birthplaces, exploding as supernovae within
the superbubbles that these clusters created. We present new X-ray results on
W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and
the W3 HII region complexes W3 Main and W3(OH).Comment: 6 pages, 3 figures, to appear in the proceedings of IAU Symposium
227,"Massive Star Birth - A Crossroads of Astrophysics," eds. R. Cesaroni, E.
Churchwell, M. Felli, and C.M. Walmsle
Mitigating Charge Transfer Inefficiency in the Chandra X-ray Observatory's ACIS Instrument
The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory were
damaged in the extreme environment of the Earth's radiation belts, resulting in
enhanced charge transfer inefficiency (CTI). This produces a row dependence in
gain, event grade, and energy resolution. We model the CTI as a function of
input photon energy, including the effects of de-trapping (charge trailing),
shielding within an event (charge in the leading pixels of the 3X3 event island
protect the rest of the island by filling traps), and non-uniform spatial
distribution of traps. This technique cannot fully recover the degraded energy
resolution, but it reduces the position dependence of gain and grade
distributions. By correcting the grade distributions as well as the event
amplitudes, we can improve the instrument's quantum efficiency. We outline our
model for CTI correction and discuss how the corrector can improve
astrophysical results derived from ACIS data.Comment: Accepted by ApJ Letters; see
http://www.astro.psu.edu/users/townsley/cti
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Structure of the Cytoplasmic Loop between Putative Helices II and III of the Mannitol Permease of Escherichia coli: A Tryptophan and 5-Fluorotryptophan Spectroscopy Study
In this work, four single tryptophan (Trp) mutants of the dimeric mannitol transporter of Escherichia coli, EIImtl, are characterized using Trp and 5-fluoroTrp (5-FTrp) fluorescence spectroscopy. The four positions, 97, 114, 126, and 133, are located in a region shown by recent studies to be involved in the mannitol translocation process. To spectroscopically distinguish between the Trp positions in each subunit of dimeric EIImtl, 5-FTrp was biosynthetically incorporated because of its much simpler photophysics compared to those of Trp. The steady-state and time-resolved fluorescence methodologies used point out that all four positions are in structured environments, both in the absence and in the presence of a saturating concentration of mannitol. The fluorescence decay of all 5-FTrp-containing mutants was highly homogeneous, suggesting similar microenvironments for both probes per dimer. However, Stern-Volmer quenching experiments using potassium iodide indicate different solvent accessibilities for the two probes at positions 97 and 133. A 5 Ã… two-dimensional (2D) projection map of the membrane-embedded IICmtl dimer showing 2-fold symmetry is available. The results of this work are in better agreement with a 7 Ã… projection map from a single 2D crystal on which no symmetry was imposed.
Mapping of health technology assessment in selected countries
Objectives: The aim of this study was to develop and apply an instrument to map the level of health technology assessment (HTA) development at country level in selected countries. We examined middle-income countries (Argentina, Brazil, India, Indonesia, Malaysia, Mexico, and Russia) and countries well-known for their comprehensive HTA programs (Australia, Canada, and United Kingdom).
Methods: A review of relevant key documents regarding the HTA process was performed to develop the instrument which was then reviewed by selected HTAi members and revised. We identified and collected relevant information to map the level of HTA in the selected countries. This was supplemented by information from a structured survey among HTA experts in the selected countries (response rate: 65/385).
Results: Mapping of HTA in a country can be done by focusing on the level of institutionalization and the HTA process (identification, priority setting, assessment, appraisal, reporting, dissemination, and implementation in policy and practice). Although HTA is most advanced in industrialized countries, there is a growing community in middle-income countries that uses HTA. For example, Brazil is rapidly developing effective HTA programs. India and Russia are at the very beginning of introducing HTA. The other middle-income countries show intermediate levels of HTA development compared with the reference countries.
Conclusions: This study presents a set of indicators for documenting the current level and trends in HTA at country level. The findings can be used as a baseline measurement for future monitoring and evaluation. This will allow a variety of stakeholders to assess the development of HTA in their country, help inform strategies, and justify expenditure for HTA
Chandra observations of SN 1987A: the soft X-ray light curve revisited
We report on the present stage of SN 1987A as observed by the Chandra X-ray
Observatory. We reanalyze published Chandra observations and add three more
epochs of Chandra data to get a consistent picture of the evolution of the
X-ray fluxes in several energy bands. We discuss the implications of several
calibration issues for Chandra data. Using the most recent Chandra calibration
files, we find that the 0.5-2.0 keV band fluxes of SN 1987A have increased by
~6 x 10 ^-13 erg s^-1 cm^-2 per year since 2009. This is in contrast with our
previous result that the 0.5-2.0 keV light curve showed a sudden flattening in
2009. Based on our new analysis, we conclude that the forward shock is still in
full interaction with the equatorial ring.Comment: Accepted for publication by ApJ, 7 pages, 5 figure
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