686 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
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.
Sensitive monitoring of the dynamics of a membrane-bound transport protein by tryptophan phosphorescence spectroscopy
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
HIF-1 alpha-independent hypoxia-induced rapid PTK6 stabilization is associated with increased motility and invasion
© 2014 Landes Bioscience. PTK6/Brk is a non-receptor tyrosine kinase overexpressed in cancer. Here we demonstrate that cytosolic PTK6 is rapidly and robustly induced in response to hypoxic conditions in a HIF-1-independent manner. Furthermore, a proportion of hypoxic PTK6 subsequently re-localized to the cell membrane. We observed that the rapid stabilization of PTK6 is associated with a decrease in PTK6 ubiquitylation and we have identified c-Cbl as a putative PTK6 E3 ligase in normoxia. The consequences of hypoxia-induced PTK6 stabilization and subcellular re-localization to the plasma membrane include increased cell motility and invasion, suggesting PTK6 targeting as a therapeutic approach to reduce hypoxia-regulated metastatic potential. This could have particular significance for breast cancer patients with triple negative disease
Chandra/ACIS-I study of the X-ray properties of the NGC 6611 and M16 stellar population
Mechanisms regulating the origin of X-rays in YSOs and the correlation with
their evolutionary stage are under debate. Studies of the X-ray properties in
young clusters allow to understand these mechanisms. One ideal target for this
analysis is the Eagle Nebula (M16), with its central cluster NGC6611. At 1750
pc from the Sun, it harbors 93 OB stars, together with a population of low-mass
stars from embedded protostars to disk-less Class III objects, with age <=
3Myrs. We study an archival 78 ksec Chandra/ACIS-I observation of NGC6611, and
two new 80ksec observations of the outer region of M16, one centered on the
Column V, and one on a region of the molecular cloud with ongoing
star-formation. We detect 1755 point sources, with 1183 candidate cluster
members (219 disk-bearing and 964 disk-less). We study the global X-ray
properties of M16 and compare them with those of the Orion Nebula Cluster. We
also compare the level of X-ray emission of Class II and Class III stars, and
analyze the X-ray spectral properties of OB stars. Our study supports the lower
level of X-ray activity for the disk-bearing stars with respect to the
disk-less members. The X-ray Luminosity Function (XLF) of M16 is similar to
that of Orion, supporting the universality of the XLF in young clusters. 85% of
the O stars of NGC6611 have been detected in X-rays. With only one possible
exception, they show soft spectra with no hard component, indicating that
mechanisms for the production of hard X-ray emission in O stars are not
operating in NGC 6611.Comment: Accepted in Ap
The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center
We examine the X-ray spectra and variability of the sample of X-ray sources
with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the
Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We
find that the spectra of the point sources near the Galactic center are very
hard between 2--8 keV, even after accounting for absorption. When modeled as
power laws the median photon index is Gamma=0.7, while when modeled as thermal
plasma we can only obtain lower limits to the temperature of kT>8 keV. The
combined spectra of the point sources is similarly hard, with a photon index of
Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and
H-like Fe, both in the average spectra and in the brightest individual sources.
The line ratios of the highly-ionized Fe in the average spectra are consistent
with emission from a plasma in thermal equilibrium. This line emission is
observed whether average spectra are examined as a function of the count rate
from the source, or as a function of the hardness ratios of individual sources.
This suggests that the hardness of the spectra may in fact to due local
absorption that partially-covers the X-ray emitting regions in the Galactic
center systems. We suggest that most of these sources are intermediate polars,
which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely
exhibit either flares on short time scales or changes in their mean X-ray flux
on long time scales, and (3) are the most numerous hard X-ray sources with
comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004,
v613 issue. An electronic version of table 2 is on
http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data
files for each source are available on
http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 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
What Happens after the First Shock? Temporal changes in microbial ecx-values in metal contaminated field trials
Extended abstract.Kris Broos, Michael Warne, Mike McLaughli
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