5,292 research outputs found
Possible Solutions to the Radius Anomalies of Transiting Giant Planets
We calculate the theoretical evolution of the radii of all fourteen of the
known transiting extrasolar giant planets (EGPs) for a variety of assumptions
concerning atmospheric opacity, dense inner core masses, and possible internal
power sources. We incorporate the effects of stellar irradiation and customize
such effects for each EGP and star. Looking collectively at the family as a
whole, we find that there are in fact two radius anomalies to be explained. Not
only are the radii of a subset of the known transiting EGPs larger than
expected from previous theory, but many of the other objects are smaller than
the default theory would allow. We suggest that the larger EGPs can be
explained by invoking enhanced atmospheric opacities that naturally retain
internal heat. This explanation might obviate the necessity for an extra
internal power source. We explain the smaller radii by the presence in perhaps
all the known transiting EGPs of dense cores, such as have been inferred for
Saturn and Jupiter. Importantly, we derive a rough correlation between the
masses of our "best-fit" cores and the stellar metallicity that seems to
buttress the core-accretion model of their formation. Though many caveats and
uncertainties remain, the resulting comprehensive theory that incorporates
enhanced-opacity atmospheres and dense cores is in reasonable accord with all
the current structural data for the known transiting giant planets.Comment: 22 pages in emulateapj format, including 10 figures (mostly in
color), accepted to the Astrophysical Journal (February 9, 2007); to appear
in volume 661, June 200
Distances to the high galactic latitude molecular clouds G192-67 and MBM 23-24
We report on distance determinations for two high Galactic latitude cloud
complexes, G192-67 and MBM 23-24. No distance determination exists in the
literature for either cloud. Thirty-four early type stars were observed towards
the two clouds, more than half of which have parallaxes measured by the
Hipparcos satellite. For the remaining stars we have made spectroscopic
distance estimates. The data consist of high resolution echelle spectra
centered on the Na I D lines, and were obtained over six nights at the Coude
Feed telescope at Kitt Peak National Observatory. Interstellar absorption lines
were detected towards some of the stars, enabling estimates of the distances to
the clouds of 109 +/- 14 pc for G192-67, and of 139 +/- 33 pc for MBM 23-24. We
discuss the relationship of these clouds to other ISM features such as the
Local Hot Bubble and the local cavity in neutral hydrogen.Comment: 15 pages, 6 embedded figures, to be published in the ApJ Vol. 516,
No.
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
The very nearby M/T dwarf binary SCR 1845-6357
The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary
discovered. SCR 1845 is a particular object due to its tight orbit (currently
around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially
resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and
K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to
the primary SCR 1845A, orbital motion is evident even within a year. Following
the orbital motion, the binary's mass can be measured accurately within a
decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR
spectra allow for accurate determination of spectral type and also for rough
estimates of the object's physical parameters. The spectral type of SCR 1845B
is determined by direct comparison of the flux calibrated JHK spectra with T
dwarf standard template spectra and also by NIR spectral indices obtained from
synthetic photometry. Constrained values for surface gravity, effective
temperature and metallicity are derived by comparison with model spectra. Our
data prove that SCR 1845B is a brown dwarf of spectral type T6 that is
co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting
the NIR spectrum of SCR 1845B to model spectra yields an effective temperature
of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar
metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses
and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
CASTRO: A New Compressible Astrophysical Solver. III. Multigroup Radiation Hydrodynamics
We present a formulation for multigroup radiation hydrodynamics that is
correct to order using the comoving-frame approach and the
flux-limited diffusion approximation. We describe a numerical algorithm for
solving the system, implemented in the compressible astrophysics code, CASTRO.
CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement
based on a nested hierarchy of logically-rectangular variable-sized grids with
simultaneous refinement in both space and time. In our multigroup radiation
solver, the system is split into three parts, one part that couples the
radiation and fluid in a hyperbolic subsystem, another part that advects the
radiation in frequency space, and a parabolic part that evolves radiation
diffusion and source-sink terms. The hyperbolic subsystem and the frequency
space advection are solved explicitly with high-order Godunov schemes, whereas
the parabolic part is solved implicitly with a first-order backward Euler
method. Our multigroup radiation solver works for both neutrino and photon
radiation.Comment: accepted by ApJS, 27 pages, 20 figures, high-resolution version
available at https://ccse.lbl.gov/Publications/wqzhang/castro3.pd
Science-based restoration monitoring of coastal habitats, Volume Two: Tools for monitoring coastal habitats
Healthy coastal habitats are not only important ecologically; they also support healthy coastal communities and improve the quality of people’s lives. Despite their many benefits and values, coastal habitats have been systematically modified, degraded, and destroyed throughout the United States and its protectorates beginning with European colonization in the 1600’s (Dahl 1990). As a result, many coastal habitats around the United States are in desperate need of restoration. The monitoring of restoration projects, the focus of this document, is necessary to ensure that restoration efforts are successful, to further the science, and to increase the efficiency of future restoration efforts
Cloud and surface classification using SCIAMACHY polarization measurement devices
International audienceA simple scheme has been developed to discriminate surface, sun glint and cloud properties in satellite based spectrometer data utilizing visible and near infrared information. It has been designed for the use with data measured by SCIAMACHY's (SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY) Polarization Measurement Devices but the applicability is not strictly limited to this instrument. The scheme is governed by a set of constraints and thresholds developed by using satellite imagery and meteorological data. Classification targets are ice, water and generic clouds, sun glint and surface parameters, such as water, snow/ice, desert and vegetation. The validation is done using MERIS (MEdium Resolution Imaging Spectrometer) and meteorological data from METAR (MÉTéorologique Aviation Régulière ? a network for the provision of meteorological data for aviation). Qualitative and quantitative validation using MERIS satellite imagery shows good agreement. The comparison with METAR data exhibits very good agreement
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