2,282 research outputs found
Thermal Processes Governing Hot-Jupiter Radii
There have been many proposed explanations for the larger-than-expected radii
of some transiting hot Jupiters, including either stellar or orbital energy
deposition deep in the atmosphere or deep in the interior. In this paper, we
explore the important influences on hot-Jupiter radius evolution of (i)
additional heat sources in the high atmosphere, the deep atmosphere, and deep
in the convective interior; (ii) consistent cooling of the deep interior
through the planetary dayside, nightside, and poles; (iii) the degree of heat
redistribution to the nightside; and (iv) the presence of an upper atmosphere
absorber inferred to produce anomalously hot upper atmospheres and inversions
in some close-in giant planets. In particular, we compare the radius expansion
effects of atmospheric and deep-interior heating at the same power levels and
derive the power required to achieve a given radius increase when night-side
cooling is incorporated. We find that models that include consistent day/night
cooling are more similar to isotropically irradiated models when there is more
heat redistributed from the dayside to the nightside. In addition, we consider
the efficacy of ohmic heating in the atmosphere and/or convective interior in
inflating hot Jupiters. Among our conclusions are that (i) the most highly
irradiated planets cannot stably have uB > (10 km/s Gauss) over a large
fraction of their daysides, where u is the zonal wind speed and B is the
dipolar magnetic field strength in the atmosphere, and (ii) that ohmic heating
cannot in and of itself lead to a runaway in planet radius.Comment: Accepted by ApJ., 20 pages, 11 figure
Characterization of Exoplanet Atmospheres with the Optical Coronagraph on WFIRST
WFIRST-CGI is a NASA technology demonstration mission that is charged with
demonstrating key technologies for future exo-Earth imaging missions in space.
In the process, it will obtain images and low-resolution spectra of a handful
to a dozen extrasolar planets and possibly protoplanetary disks. Its
unprecedented contrast levels in the optical will provide astronomers with
their first direct look at mature, Jupiter sized planets at moderate
separations. This paper addresses the question: what science can be done with
such data? An analytic noise model, which is informed by the ongoing
engineering developments, is used to compute maximum achievable signal-to-noise
ratios and scientifically viable integration times for hypothetical star planet
systems, as well as to investigate the constraining power of various
combinations of WFIRST-CGI photometric and spectral observations. This work
introduces two simple models for planetary geometric albedos, which are
inspired largely by the solar system's gas giants. The first planet model is a
hybrid Jupiter-Neptune model, which separately treats the short and long
wavelengths where chromophores and methane dominate absorption, respectively.
The second planet model fixes cloud and haze properties in CoolTLusty to match
Jupiter's albedo spectrum, it then perturbs only the metallicity. MCMC
retrievals performed on simulated observations are used to assess the precision
with which planet model parameters can be measured subject to different
exposure times and observing cases. Fit results for both models'
parameterizations of geometric albedo spectra demonstrate that a rough
indication of the metallicity or methane content should be possible for some
WFIRST-CGI targets. We conclude that real observations will likely be able to
differentiate between extreme cases using these models, but will lack the
precision necessary to uncover subtle trends.Comment: 29 pages, 25 figures, 2 table
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
Probing the galactic halo with ROSAT
We discuss the current status of ROSAT shadowing observations designed to search for emission from million degree gas in the halo of the Milky Way galaxy. Preliminary results indicate that million degree halo gas is observed in the 1/4 keV band in some directions, most notably toward the Draco cloud at (l,b) = (92 deg, +38 deg), but that the halo emission is patchy and highly anisotropic. Our current understanding of this halo emission is based on a small handful of observations which have been analyzed to date. Many more observations are currently being analyzed or are scheduled for observation within the next year, and we expect our understanding of this component of the galactic halo to improve dramatically in the near future
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