349 research outputs found
The Formation of the First Stars II. Radiative Feedback Processes and Implications for the Initial Mass Function
We consider the radiative feedback processes that operate during the
formation of the first stars, including the photodissociation of H_2, Ly-alpha
radiation pressure, formation and expansion of an HII region, and disk
photoevaporation. These processes may inhibit continued accretion once the
stellar mass has reached a critical value, and we evaluate this mass separately
for each process. Photodissociation of H_2 in the local dark matter minihalo
occurs relatively early in the growth of the protostar, but we argue this does
not affect subsequent accretion since by this time the depth of the potential
is large enough for accretion to be mediated by atomic cooling. However,
neighboring starless minihalos can be affected. Ionization creates an HII
region in the infalling envelope above and below the accretion disk. Ly-alpha
radiation pressure acting at the boundary of the HII region is effective at
reversing infall from narrow polar directions when the star reaches ~20-30Msun,
but cannot prevent infall from other directions. Expansion of the HII region
beyond the gravitational escape radius for ionized gas occurs at masses
~50-100Msun, depending on the accretion rate and angular momentum of the
inflow. However, again, accretion from the equatorial regions can continue
since the neutral accretion disk has a finite thickness and shields a
substantial fraction of the accretion envelope from direct ionizing flux. At
higher stellar masses, ~140Msun in the fiducial case, the combination of
declining accretion rates and increasing photoevaporation-driven mass loss from
the disk act to effectively halt the increase in the protostellar mass. We
identify this process as the mechanism that terminates the growth of Population
III stars... (abridged)Comment: 31 pages, including 10 figures, accepted to Ap
Ulcerative colitis disease activity as subjectively assessed by patient-completed questionnaires following orthotopic liver transplantation for sclerosing cholangitis
To assess whether or not liver transplantation and subsequent immunosuppression with cyclosporine and prednisone affect ulcerative colitis symptomatology, we surveyed by questionnaire all 23 surviving patients with pretransplant colonoscopy-documented ulcerative colitis who were transplanted for primary sclerosing cholangitis between June 1982 and September 1985. At follow-up [89.8±7.6 weeks (mean±sem], all six patients who had had asymptomatic colonoscopy-documented ulcerative colitis reported continued ulcerative colitis quiescence. Among the 17 patients who had had symptomatic colonoscopydocumented ulcerative colitis at time of liver transplantation, 88.2% reported improvement in overall ulcerative colitis severity (P<0.001), with significant improvement in the frequency of bowel movements reported by 100%, in crampy abdominal pain by 87.5%, in bowel urgency by 75%, in the occurrence of pus or mucus in stool by 87.5%, in the incidence of ulcerative colitis flares by 81.8%, and in the number of days unable to function normally due to ulcerative colitis symptoms by 78.6% (all at least P<0.01). These data demonstrate that ulcerative colitis symptom severity significantly improves following liver transplantation with immunosuppression with cyclosporine and prednisone. © 1991 Plenum Publishing Corporation
Characterisation of the Mopra Radio Telescope at 16--50 GHz
We present the results of a programme of scanning and mapping observations of
astronomical masers and Jupiter designed to characterise the performance of the
Mopra Radio Telescope at frequencies between 16-50 GHz using the 12-mm and 7-mm
receivers. We use these observations to determine the telescope beam size, beam
shape and overall telescope beam efficiency as a function of frequency. We find
that the beam size is well fit by / over the frequency range with a
correlation coefficient of ~90%. We determine the telescope main beam
efficiencies are between ~48-64% for the 12-mm receiver and reasonably flat at
~50% for the 7-mm receiver. Beam maps of strong HO (22 GHz) and SiO masers
(43 GHz) provide a means to examine the radial beam pattern of the telescope.
At both frequencies the radial beam pattern reveals the presence of three
components, a central `core', which is well fit by a Gaussian and constitutes
the telescopes main beam, and inner and outer error beams. At both frequencies
the inner and outer error beams extend out to approximately 2 and 3.4 times the
full-width half maximum of the main beam respectively. Sources with angular
sizes a factor of two or more larger than the telescope main beam will couple
to the main and error beams, and therefore the power contributed by the error
beams needs to be considered. From measurements of the radial beam power
pattern we estimate the amount of power contained in the inner and outer error
beams is of order one-fifth at 22 GHz rising slightly to one-third at 43 GHz.Comment: Accepted for publication in PAS
Discovery of the Transiting Planet Kepler-5B
We present 44 days of high duty cycle, ultra precise photometry of the 13th magnitude star Kepler-5 (KIC 8191672, T(eff) = 6300 K, log g = 4.1), which exhibits periodic transits with a depth of 0.7%. Detailed modeling of the transit is consistent with a planetary companion with an orbital period of 3.548460 +/- 0.000032 days and a radius of 1.431(-0.052)(+0.041) R(J). Follow-up radial velocity measurements with the Keck HIRES spectrograph on nine separate nights demonstrate that the planet is more than twice as massive as Jupiter with a mass of 2.114(-0.059)(+0.056) M(J) and a mean density of 0.894 +/- 0.079 g cm(-3).NASA's Science Mission DirectorateAstronom
X-ray Astronomy in the Laboratory with a Miniature Compact Object Produced by Laser-Driven Implosion
Laboratory spectroscopy of non-thermal equilibrium plasmas photoionized by
intense radiation is a key to understanding compact objects, such as black
holes, based on astronomical observations. This paper describes an experiment
to study photoionizing plasmas in laboratory under well-defined and genuine
conditions. Photoionized plasma is here generated using a 0.5-keV Planckian
x-ray source created by means of a laser-driven implosion. The measured x-ray
spectrum from the photoionized silicon plasma resembles those observed from the
binary stars Cygnus X-3 and Vela X-1 with the Chandra x-ray satellite. This
demonstrates that an extreme radiation field was produced in the laboratory,
however, the theoretical interpretation of the laboratory spectrum
significantly contradicts the generally accepted explanations in x-ray
astronomy. This model experiment offers a novel test bed for validation and
verification of computational codes used in x-ray astronomy.Comment: 5 pages, 4 figures are included. This is the original submitted
version of the manuscript to be published in Nature Physic
Target Selection for SETI: 1. A Catalog of Nearby Habitable Stellar Systems
In preparation for the advent of the Allen Telescope Array, the SETI
Institute has the need to greatly expand its former list of ~2000 targets
compiled for Project Phoenix, a search for extraterrestrial technological
signals. In this paper we present a catalog of stellar systems that are
potentially habitable to complex life forms (including intelligent life), which
comprises the largest portion of the new SETI target list. The Catalog of
Nearby Habitable Systems (HabCat) was created from the Hipparcos Catalogue by
examining the information on distances, stellar variability, multiplicity,
kinematics and spectral classification for the 118,218 stars contained therein.
We also make use of information from several other catalogs containing data for
Hipparcos stars on X-ray luminosity, CaII H&K activity, rotation, spectral
types, kinematics, metallicity, and Stroemgren photometry. Combined with
theoretical studies on habitable zones, evolutionary tracks and third body
orbital stability, these data were used to remove unsuitable stars from HabCat,
leaving a residue of stars that, to the best of our current knowledge, are
potentially habitable hosts for complex life. While this Catalog will no doubt
need to be modified as we learn more about individual objects, the present
analysis results in 17,129 Hipparcos "habstars" near the Sun (75% within 140
pc), ~2200 of which are known or suspected to be members of binary or triple
star systems.Comment: 42 pages, 14 figures, accepted by ApJ Supp, to appear in v145, March
200
Searching for transits in the Wide Field Camera Transit Survey with difference-imaging light curves
The Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. We produce an alternative set of light curves using the difference-imaging method for the most complete field in the survey and carry out a quantitative comparison between the photometric precision achieved with both methods. The results show that differencephotometry light curves present an important improvement for stars with J > 16. We report an implementation on the box-fitting transit detection algorithm, which performs a trapezoid-fit to the folded light curve, providing more accurate results than the boxfitting model. We describe and optimize a set of selection criteria to search for transit candidates, including the V-shape parameter calculated by our detection algorithm. The optimized selection criteria are applied to the aperture photometry and difference-imaging light curves, resulting in the automatic detection of the best 200 transit candidates from a sample of ~475 000 sources. We carry out a detailed analysis in the 18 best detections and classify them as transiting planet and eclipsing binary candidates. We present one planet candidate orbiting a late G-type star. No planet candidate around M-stars has been found, confirming the null detection hypothesis and upper limits on the occurrence rate of short-period giant planets around M-dwarfs presented in a prior study. We extend the search for transiting planets to stars with J ≤ 18, which enables us to set a stricter upper limit of 1.1%. Furthermore, we present the detection of five faint extremely-short period eclipsing binaries and three M-dwarf/M-dwarf binary candidates. The detections demonstrate the benefits of using the difference-imaging light curves, especially when going to fainter magnitudes.Peer reviewe
Planetary Candidates Observed by Kepler, III: Analysis of the First 16 Months of Data
New transiting planet candidates are identified in sixteen months (May 2009 -
September 2010) of data from the Kepler spacecraft. Nearly five thousand
periodic transit-like signals are vetted against astrophysical and instrumental
false positives yielding 1,091 viable new planet candidates, bringing the total
count up to over 2,300. Improved vetting metrics are employed, contributing to
higher catalog reliability. Most notable is the noise-weighted robust averaging
of multi-quarter photo-center offsets derived from difference image analysis
which identifies likely background eclipsing binaries. Twenty-two months of
photometry are used for the purpose of characterizing each of the new
candidates. Ephemerides (transit epoch, T_0, and orbital period, P) are
tabulated as well as the products of light curve modeling: reduced radius
(Rp/R*), reduced semi-major axis (d/R*), and impact parameter (b). The largest
fractional increases are seen for the smallest planet candidates (197% for
candidates smaller than 2Re compared to 52% for candidates larger than 2Re) and
those at longer orbital periods (123% for candidates outside of 50-day orbits
versus 85% for candidates inside of 50-day orbits). The gains are larger than
expected from increasing the observing window from thirteen months (Quarter 1--
Quarter 5) to sixteen months (Quarter 1 -- Quarter 6). This demonstrates the
benefit of continued development of pipeline analysis software. The fraction of
all host stars with multiple candidates has grown from 17% to 20%, and the
paucity of short-period giant planets in multiple systems is still evident. The
progression toward smaller planets at longer orbital periods with each new
catalog release suggests that Earth-size planets in the Habitable Zone are
forthcoming if, indeed, such planets are abundant.Comment: Submitted to ApJS. Machine-readable tables are available at
http://kepler.nasa.gov, http://archive.stsci.edu/kepler/results.html, and the
NASA Exoplanet Archiv
Characteristics of Kepler Planetary Candidates Based on the First Data Set: The Majority are Found to be Neptune-Size and Smaller
In the spring of 2009, the Kepler Mission commenced high-precision photometry
on nearly 156,000 stars to determine the frequency and characteristics of small
exoplanets, conduct a guest observer program, and obtain asteroseismic data on
a wide variety of stars. On 15 June 2010 the Kepler Mission released data from
the first quarter of observations. At the time of this publication, 706 stars
from this first data set have exoplanet candidates with sizes from as small as
that of the Earth to larger than that of Jupiter. Here we give the identity and
characteristics of 306 released stars with planetary candidates. Data for the
remaining 400 stars with planetary candidates will be released in February
2011. Over half the candidates on the released list have radii less than half
that of Jupiter. The released stars include five possible multi-planet systems.
One of these has two Neptune-size (2.3 and 2.5 Earth-radius) candidates with
near-resonant periods.Comment: Paper to accompany Kepler's June 15, 2010 data release; submitted to
Astrophysical Journal Figures 1,2,& 3 revised. Improved labeling on all
figures. Slight changes to planet frequencies in result
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
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