348 research outputs found
A new Search for Carbon Monoxide Absorption in the Transmission Spectrum of the Extrasolar Planet HD 209458b
We have revisited the search for carbon monoxide absorption features in
transmission during the transit of the extrasolar planet HD 209458b. We
acquired 1077 high resolution spectra at 2 microns using NIRSPEC on Keck II
during three transits. Our sensitivity is sufficient to test the degree of CO
absorption in the first overtone bands during transit, based on plausible
models of the planetary atmosphere. We compare to theoretical tangent geometry
absorption spectra, computed by adding height-invariant ad hoc temperature
pertubations to the model atmosphere of Sudarsky et al., and by treating cloud
height as an adjustable parameter. We do not detect CO absorption. Our analysis
indicates a weakening similar to the case of sodium, suggesting that a general
masking mechanism is at work in the planetary atmosphere. If this masking is
provided by high clouds, our analysis defines the maximum cloud top pressure
(i.e., minimum height) as a function of the model atmospheric temperature. For
the relatively hot model used by Charbonneau et al. to interpret their sodium
detection, our CO limit requires cloud tops at or above 3.3 mbar, and these
clouds must be opaque at a wavelength of 2 microns. High clouds comprised of
submicron-sized particles are already present in some models, but may not
provide sufficient opacity to account for our CO result. Cooler model
atmospheres, having smaller atmospheric scale heights and lower CO mixing
ratios, may alleviate this problem to some extent. However, even models 500K
cooler that the Sudarsky et al. model require clouds above the 100 mbar level
to be consistent with our observations. Our null result therefore requires that
clouds exist at an observable level in the atmosphere of HD 209458b, unless
this planet is dramatically colder than current belief.Comment: 29 pages, 8 figures, accepted for Ap
The X-ray Size-Temperature Relation for Intermediate Redshift Galaxy Clusters
We present the first measurements of the X-ray size-temperature (ST) relation
in intermediate redshift (z~0.30) galaxy clusters. We interpret the local ST
relation (z~0.06) in terms of underlying scaling relations in the cluster dark
matter properties, and then we use standard models for the redshift evolution
of those dark matter properties to show that the ST relation does not evolve
with redshift. We then use ROSAT HRI observations of 11 clusters to examine the
intermediate redshift ST relation; for currently favored cosmological
parameters, the intermediate redshift ST relation is consistent with that of
local clusters. Finally, we use the ST relation and our evolution model to
measure angular diameter distances; with these 11 distances we evaluate
constraints on Omega_M and Omega_L which are consistent with those derived from
studies of Type Ia supernovae. The data rule out a model with Omega_M=1 and
Omega_L=0 with 2.5 sigma confidence. When limited to models where
Omega_M+Omega_L=1, these data are inconsistent with Omega_M=1 with 3 sigma
confidence.Comment: ApJ: submitted April 7, accepted June 28, to appear Dec 1 (vol 544
SuperMassive Black Holes in Bulges
We present spatially extended gas kinematics at parsec-scale resolution for
the nuclear regions of four nearby disk galaxies, and model them as rotation of
a gas disk in the joint potential of the stellar bulge and a putative central
black hole. The targets were selected from a larger set of long-slit spectra
obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei
with STIS (SUNNS). They represents the 4 galaxies (of 24) that display
symmetric gas velocity curves consistent with a rotating disk. We derive the
stellar mass distribution from the STIS acquisition images adopting the stellar
mass-to-light ratio normalized so as to match ground-based velocity dispersion
measurements over a large aperture. Subsequently, we constrain the mass of a
putative black hole by matching the gas rotation curve, following two distinct
approaches. In the most general case we explore all the possible disk
orientations, alternatively we constrain the gas disk orientation from the
dust-lane morphology at similar radii. In the latter case the kinematic data
indicate the presence of a central black hole for three of the four objects,
with masses of 10^7 - 10^8 solar masses, representing up to 0.025 % of the host
bulge mass. For one object (NGC2787) the kinematic data alone provide clear
evidence for the presence of a central black hole even without external
constraints on the disk orientation. These results illustrate directly the need
to determine black-hole masses by differing methods for a large number of
objects, demonstrate that the variance in black hole/bulge mass is much larger
than previously claimed, and reinforce the recent finding that the black-hole
mass is tightly correlated with the bulge stellar velocity dispersion.Comment: 26 pages, 11 Postscript figures, accepted for publication on Ap
Probing the evolution of early-type cluster galaxies through chemical enrichment
A simple chemical enrichment model for cluster early-type galaxies is
described in which the mechanisms considered in the evolutionary model are
infall of primordial gas, outflows and a possible variation in the star
formation efficiency. We find that - within the framework of our models - only
outflows can generate a suitable range of metallicities. The chemical
enrichment tracks can be combined with the latest population synthesis models
to simulate clusters over a wide redshift range, for a set of toy models. The
color-magnitude relation of local clusters is used as a constraint, fixing the
correlation between absolute luminosity and ejected fraction of gas from
outflows. It is found that the correlations between color or mass-to-light
ratios and absolute luminosity are degenerate with respect to most of the input
parameters. However, a significant change between monolithic and hierarchical
models is predicted for redshifts z\simgt 1. The comparison between predicted
and observed mass-to-light ratios yield an approximate linear bias between
total and stellar masses: in
early-type galaxies. If we assume that outflows constitute the driving
mechanism for the colors observed in cluster early type galaxies, the
metallicity of the intracluster medium (ICM) can be linked to outflows. The
color-magnitude constraint requires faint galaxies to eject 85%
of their gas, which means that most of the metals in the ICM may have
originated in these dwarf galaxies.Comment: Accepted for publication in ApJ. Uses emulateapj.sty. 12 pages with
10 embedded EPS figure
Structure based inhibitor design targeting glycogen phosphorylase b. Virtual screening, synthesis, biochemical and biological assessment of novel N-acyl-ÎČ-d-glucopyranosylamines
Glycogen phosphorylase (GP) is a validated target for the development of new type 2 diabetes treatments. Exploiting the Zinc docking database, we report the in silico screening of 1888 ÎČ- D-glucopyranose-NH-CO-R putative GP inhibitors differing only in their R groups. CombiGlide and GOLD docking programs with different scoring functions were employed with the best performing methods combined in a âconsensus scoringâ approach to ranking of ligand binding affinities for the active site. Six selected candidates from the screening were then synthesized and their inhibitory potency was assessed both in vitro and ex vivo. Their inhibition constantsâ values, in vitro, ranged from 5 to 377 ”M while two of them were effective at causing inactivation of GP in rat hepatocytes at low ”M concentrations. The crystal structures of GP in complex with the inhibitors were defined and provided the structural basis for their inhibitory potency and data for further structure based design of more potent inhibitors
The Sloan Digital Sky Survey Quasar Lens Search. III. Constraints on Dark Energy from the Third Data Release Quasar Lens Catalog
We present cosmological results from the statistics of lensed quasars in the
Sloan Digital Sky Survey (SDSS) Quasar Lens Search. By taking proper account of
the selection function, we compute the expected number of quasars lensed by
early-type galaxies and their image separation distribution assuming a flat
universe, which is then compared with 7 lenses found in the SDSS Data Release 3
to derive constraints on dark energy under strictly controlled criteria. For a
cosmological constant model (w=-1) we obtain
\Omega_\Lambda=0.74^{+0.11}_{-0.15}(stat.)^{+0.13}_{-0.06}(syst.). Allowing w
to be a free parameter we find
\Omega_M=0.26^{+0.07}_{-0.06}(stat.)^{+0.03}_{-0.05}(syst.) and
w=-1.1\pm0.6(stat.)^{+0.3}_{-0.5}(syst.) when combined with the constraint from
the measurement of baryon acoustic oscillations in the SDSS luminous red galaxy
sample. Our results are in good agreement with earlier lensing constraints
obtained using radio lenses, and provide additional confirmation of the
presence of dark energy consistent with a cosmological constant, derived
independently of type Ia supernovae.Comment: 9 pages, 3 figures, 2 tables, accepted for publication in A
Ecosystem biogeochemistry considered as a distributed metabolic network ordered by maximum entropy production
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of The Royal Society for personal use, not for redistribution. The definitive version was published in Philosophical Transactions of the Royal Society B 365 (2010): 1417-1427, doi:10.1098/rstb.2009.0272.We examine the application of the maximum entropy production principle for describing ecosystem biogeochemistry. Since ecosystems can be functionally stable despite changes in species composition, we utilize a distributed metabolic network for describing biogeochemistry, which synthesizes generic biological structures that catalyze reaction pathways, but is otherwise organism independent. Allocation of biological structure and regulation of biogeochemical reactions is determined via solution of an optimal control problem in which entropy production is maximized. However, because synthesis of biological structures cannot occur if entropy production is maximized instantaneously, we propose that information stored within the metagenome allows biological systems to maximize entropy production when averaged over time. This differs from abiotic systems that maximize entropy production at a point in space-time, which we refer to as the steepest descent pathway. It is the spatiotemporal averaging that allows biological systems to outperform abiotic processes in entropy production, at least in many situations. A simulation of a methanotrophic system is used to demonstrate the approach. We conclude with a brief discussion on the implications of viewing ecosystems as self organizing molecular machines that function to maximize entropy production at the ecosystem level of organization.The work presented here was funded by the PIE-LTER program (NSF OCE-0423565), as well as from NSF CBET-0756562, NSF EF-0928742 and NASA Exobiology and Evolutionary Biology (NNG05GN61G)
The Seventh Data Release of the Sloan Digital Sky Survey
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey
(SDSS), marking the completion of the original goals of the SDSS and the end of
the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most
of the roughly 2000 deg^2 increment over the previous data release lying in
regions of low Galactic latitude. The catalog contains five-band photometry for
357 million distinct objects. The survey also includes repeat photometry over
250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A
coaddition of these data goes roughly two magnitudes fainter than the main
survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2
in the Northern Galactic Cap, closing the gap that was present in previous data
releases. There are over 1.6 million spectra in total, including 930,000
galaxies, 120,000 quasars, and 460,000 stars. The data release includes
improved stellar photometry at low Galactic latitude. The astrometry has all
been recalibrated with the second version of the USNO CCD Astrograph Catalog
(UCAC-2), reducing the rms statistical errors at the bright end to 45
milli-arcseconds per coordinate. A systematic error in bright galaxy photometr
is less severe than previously reported for the majority of galaxies. Finally,
we describe a series of improvements to the spectroscopic reductions, including
better flat-fielding and improved wavelength calibration at the blue end,
better processing of objects with extremely strong narrow emission lines, and
an improved determination of stellar metallicities. (Abridged)Comment: 20 pages, 10 embedded figures. Accepted to ApJS after minor
correction
Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector
The inclusive and dijet production cross-sections have been measured for jets
containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass
energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The
measurements use data corresponding to an integrated luminosity of 34 pb^-1.
The b-jets are identified using either a lifetime-based method, where secondary
decay vertices of b-hadrons in jets are reconstructed using information from
the tracking detectors, or a muon-based method where the presence of a muon is
used to identify semileptonic decays of b-hadrons inside jets. The inclusive
b-jet cross-section is measured as a function of transverse momentum in the
range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet
cross-section is measured as a function of the dijet invariant mass in the
range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets
and the angular variable chi in two dijet mass regions. The results are
compared with next-to-leading-order QCD predictions. Good agreement is observed
between the measured cross-sections and the predictions obtained using POWHEG +
Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet
cross-section. However, it does not reproduce the measured inclusive
cross-section well, particularly for central b-jets with large transverse
momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final
version published in European Physical Journal
- âŠ