9,373 research outputs found
Chandra observations of Cygnus OB2
Cygnus OB2 is the nearest example of a massive star forming region,
containing over 50 O-type stars and hundreds of B-type stars. We have analyzed
two Chandra pointings in Cyg OB2, detecting ~1700 X-ray sources, of which ~1450
are thought to be members of the association. Optical and near-IR photometry
has been obtained for ~90% of these sources from recent deep Galactic plane
surveys. We have performed isochrone fits to the near-IR color-magnitude
diagram, deriving ages of 3.5(+0.75,-1.0) and 5.25(+1.5,-1.0) Myrs for sources
in the two fields, both with considerable spreads around the pre-MS isochrones.
The presence of a second population in the region, somewhat older than the
present-day O-type stars, has been suggested by other authors and fits with the
ages derived here. The fraction of sources with inner circumstellar disks (as
traced by the K-band excess) is found to be very low, but appropriate for a
population of age ~5 Myrs. We measure the stellar mass functions and find a
power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global
mean value estimated by Kroupa. A steepening of the mass function at high
masses is observed and we suggest this is due to the presence of the previous
generation of stars that have lost their most massive members. Finally,
combining our mass function and an estimate of the radial density profile of
the association suggests a total mass of Cyg OB2 of ~30,000 Msun, similar to
that of many of our Galaxy's most massive star forming regions.Comment: 6 pages, 4 figures, conference proceedings for JENAM 2010: Star
Clusters in the Era of Large Surveys, Editors: A.Moitinho and J. Alve
Timing and spectral changes of the Be X-ray transient EXO 0531-6609.2 through high and low state
We report on spectral and timing analysis of BeppoSAX data of the 13.6 s
period transient X-ray pulsar EXO 0531-6609.2. Observations were carried out in
March 1997 and October 1998, catching the source during a high and a low
emission state, respectively. Correspondingly, the X-ray luminosity is found at
a level of 4.2x10^37 erg/s and 1.5x10^36 erg/s in the two states. In the high
state the X-ray emission in the energy range 1-100 keV is well fitted by an
absorbed power-law with photon index Gamma ~1.7 plus a blackbody component with
a characteristic temperature of ~3.5 keV. Moreover, we find an evidence of an
iron emission at ~6.8 keV, typical feature in this class of sources but never
revealed before in the EXO 0531-6609.2 spectrum. In the low state an absorbed
power-law with Gamma ~0.4 is sufficient to fit the 1-10 keV data. During
BeppoSAX observations EXO 0531-6609.2 display variations of the pulse profile
with the X-ray flux: it showed single peaked and double peaked profiles in the
low and high state, respectively. Based on these two observations we infer a
spin-up period derivative of -(1.14+/-0.08)x10^-10 s/s. By comparing these with
other period measurements reported in literature we find an alternating spin-up
and spin-down behaviour that correlates well with the X-ray luminosity.Comment: 6 pages, 8 figures, A&
Exponential and moment inequalities for U-statistics
A Bernstein-type exponential inequality for (generalized) canonical
U-statistics of order 2 is obtained and the Rosenthal and Hoffmann-J{\o}rgensen
inequalities for sums of independent random variables are extended to
(generalized) U-statistics of any order whose kernels are either nonnegative or
canonicalComment: 22 page
Comparative genomics of bacterial and plant folate synthesis and salvage: predictions and validations
<p>Abstract</p> <p>Background</p> <p>Folate synthesis and salvage pathways are relatively well known from classical biochemistry and genetics but they have not been subjected to comparative genomic analysis. The availability of genome sequences from hundreds of diverse bacteria, and from <it>Arabidopsis thaliana</it>, enabled such an analysis using the SEED database and its tools. This study reports the results of the analysis and integrates them with new and existing experimental data.</p> <p>Results</p> <p>Based on sequence similarity and the clustering, fusion, and phylogenetic distribution of genes, several functional predictions emerged from this analysis. For bacteria, these included the existence of novel GTP cyclohydrolase I and folylpolyglutamate synthase gene families, and of a trifunctional <it>p</it>-aminobenzoate synthesis gene. For plants and bacteria, the predictions comprised the identities of a 'missing' folate synthesis gene (<it>folQ</it>) and of a folate transporter, and the absence from plants of a folate salvage enzyme. Genetic and biochemical tests bore out these predictions.</p> <p>Conclusion</p> <p>For bacteria, these results demonstrate that much can be learnt from comparative genomics, even for well-explored primary metabolic pathways. For plants, the findings particularly illustrate the potential for rapid functional assignment of unknown genes that have prokaryotic homologs, by analyzing which genes are associated with the latter. More generally, our data indicate how combined genomic analysis of both plants and prokaryotes can be more powerful than isolated examination of either group alone.</p
Parallax and Distance Estimates for Twelve Cataclysmic Variable Stars
We report parallax and distance estimates for twelve more cataclysmic
binaries and related objects observed with the 2.4m Hiltner telescope at MDM
Observatory. The final parallax accuracy is typically about 1 mas. For only one
of the twelve objects, IR Gem, do we fail to detect a significant parallax.
Notable results include distances for V396 Hya (CE 315), a helium double
degenerate with a relatively long orbital period, and for MQ Dra
(SDSSJ155331+551615), a magnetic system with a very low accretion rate. We find
that the Z Cam star KT Persei is physically paired with a K main-sequence star
lying 15 arcsec away. Several of the targets have distance estimates in the
literature that are based on the white dwarf's effective temperature and flux;
our measurements broadly corroborate these estimates, but tend to put the stars
a bit closer, indicating that the white dwarfs may have rather larger masses
than assumed. As a side note, we briefly describe radial velocity spectroscopy
that refines the orbital period of V396 Hya to 65.07 +- 0.08 min.Comment: Accepted for Astronomical Journal. 19 pages, no figure
Survey of the needs of patients with spinal cord injury: impact and priority for improvement in hand function in tetraplegics\ud
Objective: To investigate the impact of upper extremity deficit in subjects with tetraplegia.\ud
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Setting: The United Kingdom and The Netherlands.\ud
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Study design: Survey among the members of the Dutch and UK Spinal Cord Injury (SCI) Associations.\ud
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Main outcome parameter: Indication of expected improvement in quality of life (QOL) on a 5-point scale in relation to improvement in hand function and seven other SCI-related impairments.\ud
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Results: In all, 565 subjects with tetraplegia returned the questionnaire (overall response of 42%). Results in the Dutch and the UK group were comparable. A total of 77% of the tetraplegics expected an important or very important improvement in QOL if their hand function improved. This is comparable to their expectations with regard to improvement in bladder and bowel function. All other items were scored lower.\ud
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Conclusion: This is the first study in which the impact of upper extremity impairment has been assessed in a large sample of tetraplegic subjects and compared to other SCI-related impairments that have a major impact on the life of subjects with SCI. The present study indicates a high impact as well as a high priority for improvement in hand function in tetraplegics.\ud
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Duality covariant quantum field theory on noncommutative Minkowski space
We prove that a scalar quantum field theory defined on noncommutative
Minkowski spacetime with noncommuting momentum coordinates is covariant with
respect to the UV/IR duality which exchanges coordinates and momenta. The proof
is based on suitable resonance expansions of charged noncommutative scalar
fields in a background electric field, which yields an effective description of
the field theory in terms of a coupled complex two-matrix model. The two
independent matrix degrees of freedom ensure unitarity and manifest
CT-invariance of the field theory. The formalism describes an analytic
continuation of the renormalizable Grosse-Wulkenhaar models to Minkowski
signature.Comment: 32 pages; v2: Typos corrected; v3: Further typos corrected - Final
version to appear in JHE
Tonic Activation of GluN2C/GluN2D-Containing NMDA Receptors by Ambient Glutamate Facilitates Cortical Interneuron Maturation
Developing cortical GABAergic interneurons rely on genetic programs, neuronal activity, and environmental cuesto construct inhibitory
circuits during early postnatal development. Disruption of these events can cause long-term changes in cortical inhibition and may be
involved in neurological disorders associated with inhibitory circuit dysfunction. We hypothesized that tonic glutamate signaling in the
neonatal cortex contributesto, and is necessary for,the maturation of cortical interneurons. Totestthis hypothesis, we used mice of both
sexes to quantify extracellular glutamate concentrations in the cortex during development, measure ambient glutamate-mediated activation of developing cortical interneurons, and manipulatetonic glutamate signaling using subtype-specific NMDA receptor antagonists
in vitro and in vivo. We report that ambient glutamate levels are high (100 nM) in the neonatal cortex and decrease (to 50 nM) during
the first weeks of life, coincident with increases in astrocytic glutamate uptake. Consistent with elevated ambient glutamate, putative
parvalbumin-positive interneurons in the cortex (identified using G42:GAD1-eGFP reporter mice) exhibit a transient, tonic NMDA
current at the end of the first postnatal week. GluN2C/GluN2D-containing NMDA receptors mediate the majority of this current and
contribute to the resting membrane potential and intrinsic properties of developing putative parvalbumin interneurons. Pharmacological blockade of GluN2C/GluN2D-containing NMDA receptors in vivo during the period of tonic interneuron activation, but not later,
leads to lasting decreases in interneuron morphological complexity and causes deficits in cortical inhibition later in life. These results
demonstrate that dynamic ambient glutamate signaling contributes to cortical interneuron maturation via tonic activation of GluN2C/
GluN2D-containing NMDA receptor
Evidence for folate-salvage reactions in plants
Folates in vivo undergo oxidative cleavage, giving pterin and p-aminobenzoylglutamate (pABAGlu) moieties. These breakdown products are excreted in animals, but their fate is unclear in microorganisms and unknown in plants. As indirect evidence from this and previous studies strongly suggests that plants can have high folate-breakdown rates (approximately 10% per day), salvage of the cleavage products seems likely. Four sets of observations support this possibility. First, cleavage products do not normally accumulate: pools of pABAGlu (including its polyglutamyl forms) are equivalent to, at most, 4-14% of typical total folate pools in Arabidopsis thaliana, Lycopersicon esculentum and Pisum sativum tissues. Pools of the pterin oxidation end-product pterin-6-carboxylate are, likewise, fairly small (3-37%) relative to total folate pools. Second, little pABAGlu built up in A. thaliana plantlets when net folate breakdown was induced by blocking folate synthesis with sulfanilamide. Third, A. thaliana and L. esculentum tissues readily converted supplied breakdown products to folate synthesis precursors: pABAGlu was hydrolysed to p-aminobenzoate and glutamate, and dihydropterin-6-aldehyde was reduced to 6-hydroxymethyldihydropterin. Fourth, both these reactions were detected in vitro; the reduction used NADPH as cofactor. An alternative salvage route for pABAGlu, direct reincorporation into dihydrofolate via the action of dihydropteroate synthase, appears implausible from the properties of this enzyme. We conclude that plants are excellent organisms in which to explore the biochemistry of folate salvage
Constraints on the Formation of the Galactic Bulge from Na, Al, and Heavy Element Abundances in Plaut's Field
We report chemical abundances of Na, Al, Zr, La, Nd, and Eu for 39 red giant
branch (RGB) stars and 23 potential inner disk red clump stars located in
Plaut-s low extinction window. We also measure lithium for a super Li-rich RGB
star. The abundances were determined by spectrum synthesis of high resolution
(R~25,000), high signal-to-noise (S/N~50-100 pixel-1) spectra obtained with the
Blanco 4m telescope and Hydra multifiber spectrograph. For the bulge RGB stars,
we find a general increase in the [Na/Fe] and [Na/Al] ratios with increasing
metallicity, and a similar decrease in [La/Fe] and [Nd/Fe]. Additionally, the
[Al/Fe] and [Eu/Fe] abundance trends almost identically follow those of the
{\alpha}-elements, and the [Zr/Fe] ratios exhibit relatively little change with
[Fe/H]. The consistently low [La/Eu] ratios of the RGB stars indicate that at
least a majority of bulge stars formed rapidly (<1 Gyr) and before the main
s-process could become a significant pollution source. In contrast, we find
that the potential inner disk clump stars exhibit abundance patterns more
similar to those of the thin and thick disks. Comparisons between the abundance
trends at different bulge locations suggest that the inner and outer bulge
formed on similar timescales. However, we find evidence of some abundance
differences between the most metal-poor and metal-rich stars in various bulge
fields. The data also indicate that the halo may have had a more significant
impact on the outer bulge initial composition than the inner bulge composition.
The [Na/Fe] and to a lesser extent [La/Fe] abundances further indicate that the
metal-poor bulge, at least at ~1 kpc from the Galactic center, and thick disk
may not share an identical chemistry.Comment: Accepted for publication in ApJ; 66 pages, 17 figures, 3 tables;
prior to publication, data tables in electronic form will be made available
upon reques
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