1,016 research outputs found
Phase-resolved spectral analysis of 4U 1901+03 during its outburst
The high mass X-ray binary 4U 1901+03 was reported to have the pulse profile
evolving with the X-ray luminosity and energy during its outburst in
February-July 2003: the pulse peak changed from double to single along with the
decreasing luminosity. We have carried out a detailed analysis on the
contemporary phase-resolved energy spectrum of 4U 1901+03 as observed by Rossi
X-ray Timing Explorer (RXTE). We find that, both the continuum and the pulse
spectra are phase dependent. The optical depth derived from the pulse spectrum
is in general larger than that from the continuum. Fe Ka emission line is only
detected in the spectrum of the continuum and is missing in the pulse spectrum.
This suggests an origin of Fe emission from the accretion disk but not the
surface of the neutron star.Comment: 18 pages, 8 figures, accepted by Ap
Nontargeted screening using gas chromatography– atmospheric pressure ionization mass spectrometry: Recent trends and emerging potential
Gas chromatography–high-resolution mass spectrometry (GC–HRMS) is a powerful non-targeted screening technique that promises to accelerate the identification of environmental pollu-tants. Currently, most GC–HRMS instruments are equipped with electron ionization (EI), but atmospheric pressure ionization (API) ion sources have attracted renewed interest because: (i) colli-sional cooling at atmospheric pressure minimizes fragmentation, resulting in an increased yield of molecular ions for elemental composition determination and improved detection limits; (ii) a wide range of sophisticated tandem (ion mobility) mass spectrometers can be easily adapted for opera-tion with GC–API; and (iii) the conditions of an atmospheric pressure ion source can promote structure diagnostic ion–molecule reactions that are otherwise difficult to perform using conventional GC–MS instrumentation. This literature review addresses the merits of GC–API for nontargeted screening while summarizing recent applications using various GC–API techniques. One perceived drawback of GC–API is the paucity of spectral libraries that can be used to guide structure elucida-tion. Herein, novel data acquisition, deconvolution and spectral prediction tools will be reviewed. With continued development, it is anticipated that API may eventually supplant EI as the de facto GC–MS ion source used to identify unknowns
Detection of accretion X-rays from QS Vir: cataclysmic or a lot of hot air?
An XMM-Newton observation of the nearby "pre-cataclysmic" short-period (P_orb
= 3.62 hr) binary QS Vir (EC 13471-1258) revealed regular narrow X-ray eclipses
when the white dwarf passed behind its M2-4 dwarf companion. The X-ray emission
provides a clear signature of mass transfer and accretion onto the white dwarf.
The low-resolution XMM-Newton EPIC spectra are consistent with a cooling flow
model and indicate an accretion rate of Mdot= 1.7\times10^-13M\odot/yr. At 48
pc distant, QS Vir is then the second nearest accreting cataclysmic variable
known, with one of the lowest accretion rates found to date for a non-magnetic
system. To feed this accretion through a wind would require a wind mass loss
rate of Mdot ~ 2 \times 10^-12M\odot/yr if the accretion efficiency is of the
order of 10%. Consideration of likely mass loss rates for M dwarfs suggests
this is improbably high and pure wind accretion unlikely. A lack of accretion
disk signatures also presents some difficulties for direct Roche lobe overflow.
We speculate that QS Vir is on the verge of Roche lobe overflow, and that the
observed mass transfer could be supplemented by upward chromospheric flows on
the M dwarf, analogous to spicules and mottles on the Sun, that escape the
Roche surface to be subsequently swept up into the white dwarf Roche lobe. If
so, QS Vir would be in a rare evolutionary phase lasting only a million years.
The X-ray luminosity of the M dwarf estimated during primary eclipse is L_X = 3
\times 10^28 erg/s, which is consistent with that of rapidly rotating
"saturated" K and M dwarfs.Comment: ApJ in pres
Bacterial Nucleoid-Associated Protein Uncouples Transcription Levels from Transcription Timing
The histone-like nucleoid-structuring (H-NS) protein binds to horizontally acquired genes in the bacterium Salmonella enterica serovar Typhimurium, silencing their expression. We now report that overcoming the silencing effects of H-NS imposes a delay in the expression of genes activated by the transcriptional regulator PhoP. We determine that PhoP-activated genes ancestral to Salmonella are expressed before those acquired horizontally. This expression timing reflects the in vivo occupancy of the corresponding promoters by the PhoP protein. These results are surprising because some of these horizontally acquired genes reached higher mRNA levels than ancestral genes expressed earlier and were transcribed from promoters harboring PhoP-binding sites with higher in vitro affinity for the PhoP protein. Our findings challenge the often-made assumption that for genes coregulated by a given transcription factor, early genes are transcribed to higher mRNA levels than those transcribed at later times. Moreover, they provide a singular example of how gene ancestry can impact expression timing.This work was supported, in part, by grant AI49561 from the National Institutes of Health to E.A.G., who is an Investigator of the Howard
Hughes Medical Institute
Connecting planets around horizontal branch stars with known exoplanets
We study the distribution of exoplanets around main sequence (MS) stars and
apply our results to the binary model for the formation of extreme horizontal
branch (EHB; sdO; sdB; hot subdwarfs) stars. By Binary model we refer both to
stellar and substellar companions that enhance the mass loss rate, where
substellar companions stand for both massive planets and brown dwarfs. We
conclude that sdB (EHB) stars are prime targets for planet searches. We reach
this conclusion by noticing that the bimodal distribution of planets around
stars with respect to the parameter M_p*a^2, is most prominent for stars in the
mass range 1Mo < M < 1.5Mo; 'a' is the orbital separation, 'M' is the stellar
mass and 'M_p' the planet mass. This is also the mass range of the progenitors
of EHB stars that are formed through the interaction of their progenitors with
planets (assuming the EHB formation mechanism is the binary model). In the
binary model for the formation of EHB stars interaction with a binary companion
or a substellar object (a planet or a brown dwarf), causes the progenitor to
lose most of its envelope mass during its red giant branch (RGB) phase. As a
result of that the descendant HB star is hot, i.e., an EHB (sdB) star. The
bimodal distribution suggests that even if the close-in planet that formed the
EHB star did not survive its RGB common envelope evolution, one planet or more
might survive at a>1AU. Also, if a planet or more are observed at a>1AU, it is
possible that a closer massive planet did survive the common envelope phase,
and it is orbiting the EHB with an orbital period of hours to days.Comment: MNRAS, in pres
The Unseen Population of F to K-type Companions to Hot Subdwarf Stars
We present a method to select hot subdwarf stars with A to M-type companions
using photometric selection criteria. We cover a wide range in wavelength by
combining GALEX ultraviolet data, optical photometry from the SDSS and the
Carlsberg Meridian telescope, near-infrared data from 2MASS and UKIDSS. We
construct two complimentary samples, one by matching GALEX, CMC and 2MASS, as
well as a smaller, but deeper, sample using GALEX, SDSS and UKIDSS. In both
cases, a large number of composite subdwarf plus main-sequence star candidates
were found. We fit their spectral energy distributions with a composite model
in order to estimate the subdwarf and companion star effective temperatures
along with the distance to each system. The distribution of subdwarf effective
temperature was found to primarily lie in the 20,000 - 30,000 K regime, but we
also find cooler subdwarf candidates, making up ~5-10 per cent. The most
prevalent companion spectral types were seen to be main-sequence stars between
F0 and K0, while subdwarfs with M-type companions appear much rarer. This is
clear observational confirmation that a very efficient first stable Roche-lobe
overflow channel appears to produce a large number of subdwarfs with F to
K-type companions. Our samples thus support the importance of binary evolution
for subdwarf formation.Comment: 30 pages, 10 figures, 11 tables. Accepted for publication in MNRA
Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
Our Galaxy is thought to have undergone an active evolutionary history
dominated by star formation, the accretion of cold gas, and, in particular,
mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of
these interactions in the form of stellar streams, substructures, and
chemically distinct stellar components. The impact of dwarf galaxy mergers on
the content and morphology of the Galactic disk is still being explored. Recent
studies have identified kinematically distinct stellar substructures and moving
groups, which may have extragalactic origin. However, there is mounting
evidence that stellar overdensities at the outer disk/halo interface could have
been caused by the interaction of a dwarf galaxy with the disk. Here we report
detailed spectroscopic analysis of 14 stars drawn from two stellar
overdensities, each lying about 5 kiloparsecs above and below the Galactic
plane - locations suggestive of association with the stellar halo. However, we
find that the chemical compositions of these stars are almost identical, both
within and between these groups, and closely match the abundance patterns of
the Milky Way disk stars. This study hence provides compelling evidence that
these stars originate from the disk and the overdensities they are part of were
created by tidal interactions of the disk with passing or merging dwarf
galaxies.Comment: accepted for publication in Natur
Biophysical Investigation of the Mode of Inhibition of Tetramic Acids, the Allosteric Inhibitors of Undecaprenyl Pyrophosphate Synthase
Undecaprenyl pyrophosphate synthase (UPPS) catalyzes the consecutive condensation of eight molecules of isopentenyl pyrophosphate (IPP) with farnesyl pyrophosphate (FPP) to generate the C(55) undecaprenyl pyrophosphate (UPP). It has been demonstrated that tetramic acids (TAs) are selective and potent inhibitors of UPPS, but the mode of inhibition was unclear. In this work, we used a fluorescent FPP probe to study possible TA binding at the FPP binding site. A photosensitive TA analogue was designed and synthesized for the study of the site of interaction of TA with UPPS using photo-cross-linking and mass spectrometry. The interaction of substrates with UPPS and with the UPPS.TA complex was investigated by protein fluorescence spectroscopy. Our results suggested that tetramic acid binds to UPPS at an allosteric site adjacent to the FPP binding site. TA binds to free UPPS enzyme but not to substrate-bound UPPS. Unlike Escherichia coli UPPS which follows an ordered substrate binding mechanism, Streptococcus pneumoniae UPPS appears to follow a random-sequential substrate binding mechanism. Only one substrate, FPP or IPP, is able to bind to the UPPS.TA complex, but the quaternary complex, UPPS.TA.FPP.IPP, cannot be formed. We propose that binding of TA to UPPS significantly alters the conformation of UPPS needed for proper substrate binding. As the result, substrate turnover is prevented, leading to the inhibition of UPPS catalytic activity. These probe compounds and biophysical assays also allowed us to quickly study the mode of inhibition of other UPPS inhibitors identified from a high-throughput screening and inhibitors produced from a medicinal chemistry program
Color and stellar population gradients in galaxies. Correlation with mass
We analyze the color gradients (CGs) of ~50000 nearby SDSS galaxies. From
synthetic spectral models based on a simplified star formation recipe, we
derive the mean spectral properties, and explain the observed radial trends of
the color as gradients of the stellar population age and metallicity (Z). The
most massive ETGs (M_* > 10^{11} Msun) have shallow CGs in correspondence of
shallow (negative) Z gradients. In the stellar mass range 10^(10.3-10.5) < M_*
< 10^(11) Msun, the Z gradients reach their minimum of ~ -0.5 dex^{-1}. At M_*
~ 10^{10.3-10.5} Msun, color and Z gradient slopes suddenly change. They turn
out to anti-correlate with the mass, becoming highly positive at the very low
masses. We have also found that age gradients anti-correlate with Z gradients,
as predicted by hierarchical cosmological simulations for ETGs. On the other
side, LTGs have gradients which systematically decrease with mass (and are
always more negative than in ETGs), consistently with the expectation from gas
infall and SN feedback scenarios. Z is found to be the main driver of the trend
of color gradients, especially for LTGs, but age gradients are not negligible
and seem to play a significant role too. We have been able to highlight that
older galaxies have systematically shallower age and Z gradients than younger
ones. Our results for high-mass galaxies are in perfect agreement with
predictions based on the merging scenario, while the evolution of LTGs and
younger and less massive ETGs seems to be mainly driven by infall and SN
feedback. (Abridged)Comment: 20 pages, 16 figures, accepted for publication on MNRAS. This version
includes revisions after the referee's report
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