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Carbon-Monoxide Emission From Nebulosity Associated With Herbig Be And Ae Type Stars
NSF GP-36548Astronom
A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?
It has been recently proposed that massive fast-rotating highly-magnetized
white dwarfs could describe the observational properties of some of Soft
Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has
also been shown that high-field magnetic (HFMWDs) can be the outcome of white
dwarf binary mergers. The products of these mergers consist of a hot central
white dwarf surrounded by a rapidly rotating disk. Here we show that the merger
of a double degenerate system can explain the characteristics of the peculiar
AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We
also show that the observed properties of 4U 0142+6 are consistent with an
approximately 1.2 M_{\sun} white dwarf, remnant of the coalescence of an
original system made of two white dwarfs of masses 0.6\, M_{\sun} and 1.0\,
M_{\sun}. Finally, we infer a post-merging age kyr,
and a magnetic field G. Evidence for such a magnetic
field may come from the possible detection of the electron cyclotron absorption
feature observed between the and bands at Hz in the
spectrum of 4U 0142+61.Comment: to appear in ApJ Letter
A target enrichment method for gathering phylogenetic information from hundreds of loci: An example from the Compositae.
UnlabelledPremise of the studyThe Compositae (Asteraceae) are a large and diverse family of plants, and the most comprehensive phylogeny to date is a meta-tree based on 10 chloroplast loci that has several major unresolved nodes. We describe the development of an approach that enables the rapid sequencing of large numbers of orthologous nuclear loci to facilitate efficient phylogenomic analyses. •Methods and resultsWe designed a set of sequence capture probes that target conserved orthologous sequences in the Compositae. We also developed a bioinformatic and phylogenetic workflow for processing and analyzing the resulting data. Application of our approach to 15 species from across the Compositae resulted in the production of phylogenetically informative sequence data from 763 loci and the successful reconstruction of known phylogenetic relationships across the family. •ConclusionsThese methods should be of great use to members of the broader Compositae community, and the general approach should also be of use to researchers studying other families
Multiple prebiotic metals mediate translation.
Today, Mg2+ is an essential cofactor with diverse structural and functional roles in life's oldest macromolecular machine, the translation system. We tested whether ancient Earth conditions (low O2, high Fe2+, and high Mn2+) can revert the ribosome to a functional ancestral state. First, SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) was used to compare the effect of Mg2+, Fe2+, and Mn2+ on the tertiary structure of rRNA. Then, we used in vitro translation reactions to test whether Fe2+ or Mn2+ could mediate protein production, and quantified ribosomal metal content. We found that (i) Mg2+, Fe2+, and Mn2+ had strikingly similar effects on rRNA folding; (ii) Fe2+ and Mn2+ can replace Mg2+ as the dominant divalent cation during translation of mRNA to functional protein; and (iii) Fe and Mn associate extensively with the ribosome. Given that the translation system originated and matured when Fe2+ and Mn2+ were abundant, these findings suggest that Fe2+ and Mn2+ played a role in early ribosomal evolution
Molecular Mechanics Simulations and Improved Tight-binding Hamiltonians for Artificial Light Harvesting Systems: Predicting Geometric Distributions, Disorder, and Spectroscopy of Chromophores in a Protein Environment
We present molecular mechanics {and spectroscopic} calculations on prototype
artificial light harvesting systems consisting of chromophores attached to a
tobacco mosaic virus (TMV) protein scaffold. These systems have been
synthesized and characterized spectroscopically, but information about the
microscopic configurations and geometry of these TMV-templated chromophore
assemblies is largely unknown. We use a Monte Carlo conformational search
algorithm to determine the preferred positions and orientations of two
chromophores, Coumarin 343 together with its linker, and Oregon Green 488, when
these are attached at two different sites (104 and 123) on the TMV protein. The
resulting geometric information shows that the extent of disorder and
aggregation properties, and therefore the optical properties of the
TMV-templated chromophore assembly, are highly dependent on the choice of
chromophores and protein site to which they are bound. We used the results of
the conformational search as geometric parameters together with an improved
tight-binding Hamiltonian to simulate the linear absorption spectra and compare
with experimental spectral measurements. The ideal dipole approximation to the
Hamiltonian is not valid since the distance between chromophores can be very
small. We found that using the geometries from the conformational search is
necessary to reproduce the features of the experimental spectral peaks
Detection of interstellar hydrogen peroxide
The molecular species hydrogen peroxide, HOOH, is likely to be a key
ingredient in the oxygen and water chemistry in the interstellar medium. Our
aim with this investigation is to determine how abundant HOOH is in the cloud
core {\rho} Oph A. By observing several transitions of HOOH in the
(sub)millimeter regime we seek to identify the molecule and also to determine
the excitation conditions through a multilevel excitation analysis. We have
detected three spectral lines toward the SM1 position of {\rho} Oph A at
velocity-corrected frequencies that coincide very closely with those measured
from laboratory spectroscopy of HOOH. A fourth line was detected at the
4{\sigma} level. We also found through mapping observations that the HOOH
emission extends (about 0.05 pc) over the densest part of the {\rho} Oph A
cloud core. We derive an abundance of HOOH relative to that of H_2 in the SM1
core of about 1\times10^(-10). To our knowledge, this is the first reported
detection of HOOH in the interstellar medium.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics, new version corrects a typo in Table 1 (and consequently in Fig
4
Velocity Field Statistics in Star-Forming Regions. I. Centroid Velocity Observations
The probability density functions (pdfs) of molecular line centroid velocity
fluctuations and fluctuation differences at different spatial lags are
estimated for several nearby molecular clouds with active internal star
formation. The data consist of over 75,000 CO line profiles divided
among twelve spatially and/or kinematically distinct regions. Although three
regions (all in Mon R2) appear nearly Gaussian, the others show strong evidence
for non-Gaussian, often nearly exponential, centroid velocity pdfs, possibly
with power law contributions in the far tails. Evidence for nearly exponential
centroid pdfs in the neutral HI component of the ISM is also presented, based
on older optical and radio observations. These results are in contrast to pdfs
found in isotropic incompressible turbulence experiments and simulations.
Furthermore, no evidence is found for the scaling of difference pdf kurtosis
with Reynolds number which is seen in incompressible turbulence, and the
spatial distribution of high-amplitude velocity differences shows little
indication of the filamentary appearance predicted by decay simulations
dominated by vortical interactions. The variation with lag of the difference
pdf moments is presented as a constraint on future simulations.Comment: LaTeX, 23 pages, with 15 Figures included separately as gif image
files. Refereed/revised version accepted to the Astrophysical Journal. A
complete (but much larger) postscript version is available from
http://ktaadn.gsfc.nasa.gov/~miesc
A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros
We report the results of a new large scale survey of the Orion-Monoceros
complex of molecular clouds made in the J = 1->0 line of CO with the
Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of
52,288 uniformly spaced spectra that cover an area of 432 square degrees on the
sky and is the most sensitive large-scale survey of the region to date.
Distances to the constituent molecular clouds of the complex, estimated from an
analysis of foreground and background stars, have provided information on the
three dimensional structure of the entire complex.Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages with
17 colour figures - 39 if you count the sub-figures separately. The figures
here have been bit-mapped with some loss of quality and beauty. The paper
version in A&A will be in greyscale with the on-line version in colour. In
the meantime the colour version can be obtained by following links at
http://www.star.bris.ac.uk/mrwm . The 9MB PostScript is recommended if you
have appropriate bandwidth or otherwise the 2.3MB PDF is usabl
O18O and C18O observations of rho Oph A
Observations of the (N_J=1_1-1_0) ground state transition of O_2 with the
Odin satellite resulted in a about 5 sigma detection toward the dense core rho
Oph A. At the frequency of the line, 119 GHz, the Odin telescope has a beam
width of 10', larger than the size of the dense core, so that the precise
nature of the emitting source and its exact location and extent are unknown.
The current investigation is intended to remedy this. Telluric absorption makes
ground based O_2 observations essentially impossible and observations had to be
done from space. mm-wave telescopes on space platforms were necessarily small,
which resulted in large, several arcminutes wide, beam patterns. Although the
Earth's atmosphere is entirely opaque to low-lying O_2 transitions, it allows
ground based observations of the much rarer O18O in favourable conditions and
at much higher angular resolution with larger telescopes. In addition, rho Oph
A exhibits both multiple radial velocity systems and considerable velocity
gradients. Extensive mapping of the region in the proxy C18O (J=3-2) line can
be expected to help identify the O_2 source on the basis of its line shape and
Doppler velocity. Line opacities were determined from observations of optically
thin 13C18O (J=3-2) at selected positions. During several observing periods,
two C18O intensity maxima in rho Oph A were searched for in the 16O18O
(2_1-0_1) line at 234 GHz with the 12m APEX telescope. Our observations
resulted in an upper limit on the integrated O18O intensity of < 0.01 K km/s (3
sigma) into the 26.5" beam. We conclude that the source of observed O_2
emission is most likely confined to the central regions of the rho Oph A cloud.
In this limited area, implied O_2 abundances could thus be higher than
previously reported, by up to two orders of magnitude.Comment: 7 pages, 6 figures (5 colour), Astronomy & Astrophysic
The metallic resistance of a dilute two-dimensional hole gas in a GaAs quantum well: two-phase separation at finite temperature?
We have studied the magnetotransport properties of a high mobility
two-dimensional hole gas (2DHG) system in a 10nm GaAs quantum well (QW) with
densities in range of 0.7-1.6*10^10 cm^-2 on the metallic side of the
zero-field 'metal-insulator transition' (MIT). In a parallel field well above
B_c that suppresses the metallic conductivity, the 2DHG exhibits a conductivity
g(T)~0.3(e^2/h)lnT reminiscent of weak localization. The experiments are
consistent with the coexistence of two phases in our system: a metallic phase
and a weakly insulating Fermi liquid phase having a percolation threshold close
to B_c
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