1,419 research outputs found
Generation and reduction of the data for the Ulysses gravitational wave experiment
A procedure for the generation and reduction of the radiometric data known as REGRES is described. The software is implemented on a HP-1000F computer and was tested on REGRES data relative to the Voyager I spacecraft. The REGRES data are a current output of NASA's Orbit Determination Program. The software package was developed in view of the data analysis of the gravitational wave experiment planned for the European spacecraft Ulysses
The Protective Effect of SRI 63-441 on Ischemic Liver Injury Using the Isolated Perfused Rat Liver: Combined Protocol With Superoxide Dismutase.
Pharmacologic modulation of experimental postischemic hepatic function
The present study, evaluated and compared the effects of SRI 63-441, a potent platelet activating factor antagonist, superoxide dismutase (SOD), an oxygen free radical scavenger, and ibuprofen, a cyclooxygenase inhibitor on hepatic function after 90 minutes of warm ischemia. After warm ischemia, livers were harvested and underwent 90 minutes of warm, oxygenated, sanguinous perfusion on an isolated liver perfusion apparatus. Pretreatment of donor animals with 20 mg/kg intravenous (I.V.) SRI 63-441 5 minutes before induction of total hepatic ischemia resulted in significantly increased bile production, a significant decrease in transaminase release, and a higher tissue adenosine triphosphate (ATP) content when compared with ischemic non-treated controls. SOD resulted in improved bile production and decreased transaminase liberation only when present in the perfusate at the time of in vitro reperfusion. Ibuprofen did not improve postischemic hepatic function in this model. Electron microscopy revealed patchy hepatocellular vacuolization with an intact sinusoidal endothelium in all ischemic livers. However, the degree of damage was less severe in the livers from those rats pretreated with 20 mg/kg SRI 63-441. This study demonstrates that SRI 63-441 pretreatment significantly reduces hepatic warm ischemic injury, and in the present model, appears superior to two other agents that have been advanced in the treatment of ischemic injury. The use of such agents singly or in combinations have important implications as regards gaining a better understanding of he basic mechanisms in organ ischemia, and moreover, for therapeutic applications in organ ischemia and preservation
Integral D-Finite Functions
We propose a differential analog of the notion of integral closure of
algebraic function fields. We present an algorithm for computing the integral
closure of the algebra defined by a linear differential operator. Our algorithm
is a direct analog of van Hoeij's algorithm for computing integral bases of
algebraic function fields
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
RR Lyrae Variables in Two Fields in the Spheroid of M31
We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32—between 4 and 6 kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is = 25.29 ± 0.05, where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are = 0.557 ± 0.003 and = 0.327 ± 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at = –1.50 ± 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B – V) = 0.08 ± 0.03, we find a distance modulus of (m – M)_0 = 24.46 ± 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center
The Crystal and Molecular Structure and Absolute Configuration of (—)-(\u3ci\u3eS\u3c/i\u3e)-Warfarin
The crystal and molecular structure and the absolute configuration of (—)-(S)-warfarin, C19H1604, have been determined by X-ray crystallographic techniques. Crystals of (—)-(S)-warfarin are orthorhombic, space group P212121, with a=10·883 (3), b=9·562 (3), and c=14·902 (5) Å. Solution of the structure was by direct methods, and refinement by least-squares calculations led to a conventional R of 0·053 (Mo Kα data). The molecule crystallizes as the intramolecular hemiketal and thus may be described as (2S,4S)-2,3H-2-methyl-4-phenyl-5-oxobenzopyrano[3,4-e]dihydropyran-2-ol. The absolute configuration was confirmed by recollecting with Cu Kα radiation a group of reflect ions predicted to have the greatest observable Bijvoet differences based on the anomalous scattering of oxygen and the parameters from the refinement with Mo Kα data. A group of 51 Friedel pairs, 86% of which indicate the S enantiomer, gave a 17% decrease in the residual over the R enantiomer. Refinement of the imaginary part of the anomalous dispersion of oxygen gave a value of 0·037 for Δf
A hierarchical Bayesian approach for reconstructing the initial mass function of single stellar populations
Recent studies based on the integrated light of distant galaxies suggest that
the initial mass function (IMF) might not be universal. Variations of the IMF
with galaxy type and/or formation time may have important consequences for our
understanding of galaxy evolution. We have developed a new stellar population
synthesis (SPS) code specifically designed to reconstruct the IMF. We implement
a novel approach combining regularization with hierarchical Bayesian inference.
Within this approach we use a parametrized IMF prior to regulate a direct
inference of the IMF. This direct inference gives more freedom to the IMF and
allows the model to deviate from parametrized models when demanded by the data.
We use Markov Chain Monte Carlo sampling techniques to reconstruct the best
parameters for the IMF prior, the age, and the metallicity of a single stellar
population. We present our code and apply our model to a number of mock single
stellar populations with different ages, metallicities, and IMFs. When
systematic uncertainties are not significant, we are able to reconstruct the
input parameters that were used to create the mock populations. Our results
show that if systematic uncertainties do play a role, this may introduce a bias
on the results. Therefore, it is important to objectively compare different
ingredients of SPS models. Through its Bayesian framework, our model is
well-suited for this.Comment: Accepted for publication in MNRAS, 30 pages, 28 figure
The Star Formation History of M32
We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31
background field (F2) to determine the star formation history (SFH) of M32 from
its resolved stellar population. We find that 2-5Gyr old stars contribute
\som40%+/- 17% of M32's mass, while 55%+/-21% of M32's mass comes from stars
older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are
=6.8+/-1.5 Gyr and =-0.01+/-0.08 dex. The SFH additionally
indicates the presence of young (<2 Gyr old), metal-poor ([M/H]\sim-0.7) stars,
suggesting that blue straggler stars contribute ~2% of the mass at F1; the
remaining \sim3% of the mass is in young metal-rich stars. Line-strength
indices computed from the SFH imply a light-weighted mean age and metallicity
of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent
parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This
contradicts spectroscopic studies that show a steep age gradient from M32's
center to 1re. The inferred SFH of the M31 background field F2 reveals that the
majority of its stars are old, with \sim95% of its mass already acquired 5-14
Gyr ago. It is composed of two dominant populations; \sim30%+/-7.5% of its mass
is in a 5-8 Gyr old population, and \sim65%+/-9% of the mass is in a 8-14 Gyr
old population. The mass-weighted mean age and metallicity of F2 are
=9.2+/-1.2 Gyr and =-0.10+/-0.10 dex, respectively. Our results
suggest that the inner disk and spheroid populations of M31 are
indistinguishable from those of the outer disk and spheroid. Assuming the mean
age of M31's disk at F2 (\sim1 disk scale length) to be 5-9 Gyr, our results
agree with an inside-out disk formation scenario for M31's disk.Comment: Accepted to ApJ. 24 pages, 18 figures. A high-resolution version can
be downloaded from http://www.astro.rug.nl/~monachesi/monachesi-sfh.pd
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