1,644 research outputs found
Protocol for Future Amino Acid Analyses of Samples Returned by the Stardust Mission
We have demonstrated that LC-ToF-MS coupled with UV fluorescence detection is a powerful tool for the detection of amino acids in meteorite extracts. Using this new analytical technique we were able to identify the extraterrestrial amino acid AIB extracted from fifteen 20 micron sized Murchison meteorite grains. We found that the amino acid contamination levels in Stardust aerogels was much lower than the levels observed in the Murchison meteorite. In addition, the alpha-dialkyl amino acids AIB and isovaline which are the most abundant amino acids in Murchison were not detected in the aerogel above blank levels. We are currently integrating LIF detection capability to our existing nanoflow LC-ToF-MS for enhanced sensitivity required for the analysis of amino acids in Stardust samples
Bounding the dimensions of rational cohomology groups
Let be an algebraically closed field of characteristic , and let
be a simple simply-connected algebraic group over that is defined and
split over the prime field . In this paper we investigate
situations where the dimension of a rational cohomology group for can be
bounded by a constant times the dimension of the coefficient module. We then
demonstrate how our results can be applied to obtain effective bounds on the
first cohomology of the symmetric group. We also show how, for finite Chevalley
groups, our methods permit significant improvements over previous estimates for
the dimensions of second cohomology groups.Comment: 13 page
On the Correlated X-ray and Optical Evolution of SS Cygni
We have analyzed the variability and spectral evolution of the prototype
dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of
pointed RXTE/PCA observations allow us to trace the evolution of the X-ray
spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO
and RXTE/ASM light curves show long-term patterns. Employing a technique in
which we stack the X-ray flux over multiple outbursts, phased according to the
optical light curve, we investigate the outburst morphology. We find that the
3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen
previously, but only in a handful of cycles. The several outbursts of SS Cygni
observed with the more sensitive RXTE/PCA also show a depression of the X-rays
during optical outburst. We quantify the time lags between the optical and
X-ray outbursts, and the timescales of the X-ray recovery from outburst. The
optical light curve of SS Cygni exhibits brief anomalous outbursts. During
these events the hard X-rays and optical flux increase together. The long-term
data suggest that the X-rays decline between outburst. Our results are in
general agreement with modified disk instability models (DIM), which invoke a
two-component accretion flow consisting of a cool optically thick accretion
disk truncated at an inner radius, and a quasi-spherical hot corona-like flow
extending to the surface of the white dwarf. We discuss our results in the
framework of one such model, involving the evaporation of the inner part of the
optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister (1994).Comment: 24 pages, 8 figures, 2 tables, accepted for publication in Ap
How Many Cooks Spoil the Soup?
In this work, we study the following basic question: "How much parallelism
does a distributed task permit?" Our definition of parallelism (or symmetry)
here is not in terms of speed, but in terms of identical roles that processes
have at the same time in the execution. We initiate this study in population
protocols, a very simple model that not only allows for a straightforward
definition of what a role is, but also encloses the challenge of isolating the
properties that are due to the protocol from those that are due to the
adversary scheduler, who controls the interactions between the processes. We
(i) give a partial characterization of the set of predicates on input
assignments that can be stably computed with maximum symmetry, i.e.,
, where is the minimum multiplicity of a state in
the initial configuration, and (ii) we turn our attention to the remaining
predicates and prove a strong impossibility result for the parity predicate:
the inherent symmetry of any protocol that stably computes it is upper bounded
by a constant that depends on the size of the protocol.Comment: 19 page
AMINO ACID ANALYSES OF THE ANTARCTIC CM2 METEORITES ALH 83100 AND LEW 90500 USING LIQUID CHROMATOGRAPHY-TIME OF FLIGHT-MASS SPECTROMETRY
The investigation of organic compounds in primitive carbonaceous meteorites provides a record of the chemical processes that occurred in the early solar system. In particular, amino acids have been shown to be potential indicators in tracing the nature of carbonaceous chondrite parent bodies [ 13. The delivery of amino acids by carbonaceous chondrites to the early Earth could have been any important source of the Earth's prebiotic organic inventory [2]. Over 80 different amino acids have been detected in the Murchison CM2 meteorite, most of them completely non-existent in the terrestrial biosphere [3]. We have optimized a new liquid chromatography-time-of-flight-mass spectrometry (LC-ToF-MS) technique coupled with OPAMAC derivatization in order to detect amino acids in meteorite extracts by UV fluorescence and exact mass simultaneously. The detection limit of the LC-ToF-MS instrument for amino acids is at least 3 orders of magnitude lower than traditional GC-MS techniques. Here we report on the first analyses of amino acids and their enantiomeric abundances in the CM2 carbonaceous meteorites ALH 83100, LEW 90500, and Murchison using this new LC-ToF-MS instrument configuration. Amino acid analyses of any kind for the CM meteorite ALH 83100 have not previously been reported
A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um
with a spectral resolution of about 4 km s^-1. We have identified a forest of
lines in six bands of C2H2 involving the vibrational states from the ground to
3nu5 and in two bands of HCN, involving the vibrational states from the ground
up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We
have estimated the kinetic, vibrational, and rotational temperatures, and the
abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16
cm) by fitting about 300 of these ro-vibrational lines. The envelope can be
divided into three regions with approximate boundaries at 0.019 arcsec (the
stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4
arcsec (outer dust formation zone). Most of the lines might require a large
microturbulence broadening. The derived abundances of C2H2 and HCN increase by
factors of 10 and 4, respectively, from the innermost envelope outwards. The
derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational
states up to 3000 K above ground are populated, suggesting pumping by
near-infrared radiation from the star and innermost envelope. Low rotational
levels can be considered under LTE while those with J>20-30 are not
thermalized. A few lines require special analysis to deal with effects like
overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the
Astrophysical Journa
Random bond Ising chain in a transverse magnetic field: A finite-size scaling analysis
We investigate the zero-temperature quantum phase transition of the random
bond Ising chain in a transverse magnetic field. Its critical properties are
identical to those of the McCoy-Wu model, which is a classical Ising model in
two dimensions with layered disorder. The latter is studied via Monte Carlo
simulations and transfer matrix calculations and the critical exponents are
determined with a finite-size scaling analysis. The magnetization and
susceptibility obey conventional rather than activated scaling. We observe that
the order parameter-- and correlation function--probability distribution show a
nontrivial scaling near the critical point which implies a hierarchy of
critical exponents associated with the critical behavior of the generalized
correlation lengths.Comment: RevTeX 13 pages + 4 figures (appended as uuencoded compressed
tar-file), THP61-9
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"The dearest of our possessions": applying Floridi's information privacy concept in models of information behavior and information literacy
This conceptual paper argues for the value of an approach to privacy in the digital information environment informed by Luciano Floridi's philosophy of information and information ethics. This approach involves achieving informational privacy, through the features of anonymity and obscurity, through an optimal balance of ontological frictions. This approach may be used to modify models for information behavior and for information literacy, giving them a fuller and more effective coverage of privacy issues in the infosphere. For information behavior, the Information Seeking and Communication Model, and the Information Grounds conception, are most appropriate for this purpose. For information literacy, the metaliteracy model, using a modification a privacy literacy framework, is most suitable
Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic
We have computed, based on 17 infrared radial velocities, the first set of
orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The
giant's companion is a neutron star, the bright X-ray source GX 1+4. We find an
orbital period of 1161 days by far the longest of any known X-ray binary. The
orbit has a modest eccentricity of 0.10 with an orbital circularization time of
less than 10^6 years. The large mass function of the orbit significantly
restricts the mass of the M giant. Adopting a neutron-star mass of 1.35M(Sun),
the maximum mass of the M giant is 1.22M(Sun), making it the less massive star.
Derived abundances indicate a slightly subsolar metallicity. Carbon and
nitrogen are in the expected ratio resulting from the red-giant first dredge-up
phase. The lack of O-17 suggests that the M-giant has a mass less than
1.3M(Sun), consistent with our maximum mass. The red giant radius is 103R(Sun),
much smaller than the estimated Roche lobe radius. Thus, the mass loss of the
red giant is via a stellar wind. Although the M giant companion to the neutron
star has a mass similar to the late-type star in low-mass X-ray binaries, its
near-solar abundances and apparent runaway velocity are not fully consistent
with the properties of this class of stars.Comment: In press to The Astrophysical Journal (10 April 2006 issue). 23 page
Dynamics and Transport in Random Antiferromagnetic Spin Chains
We present the first results on the low-frequency dynamical and transport
properties of random antiferromagnetic spin chains at low temperature (). We
obtain the momentum and frequency dependent dynamic structure factor in the
Random Singlet (RS) phases of both spin-1/2 and spin-1 chains, as well as in
the Random Dimer phase of spin-1/2 chains. We also show that the RS phases are
unusual `spin-metals' with divergent low-frequency conductivity at T=0, and
follow the spin conductivity through `metal-insulator' transitions tuned by the
strength of dimerization or Ising anisotropy in the spin-1/2 case, and by the
strength of disorder in the spin-1 case.Comment: 4 pages (two-column format). Presentation substantially revised to
accomodate new result
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