2,132 research outputs found
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Comparing key compositional indicators in Jupiter with those in extra-solar giant planets
Spectroscopic transiting observations of the atmospheres of hot Jupiters
around other stars, first with Hubble Space Telescope and then Spitzer, opened
the door to compositional studies of exoplanets. The James Webb Space Telescope
will provide such a profound improvement in signal-to-noise ratio that it will
enable detailed analysis of molecular abundances, including but not limited to
determining abundances of all the major carbon- and oxygen-bearing species in
hot Jupiter atmospheres. This will allow determination of the carbon-to-oxygen
ratio, an essential number for planet formation models and a motivating goal of
the Juno mission currently around JupiterComment: Submitted to the Astro2020 Decadal Survey as a white paper; thematic
areas "Planetary Systems" and "Star and Planet Formation
Energy Efficiency Ratios of Surface Mining Systems for a Small Iowa Coal Mine
The normal criterion used to determine if a mining venture should be started or continued is its financial profitability. Energy efficiency is another criterion for dealing with public energy decisions. The energy efficiency ratios (output/input) of a large bulldozer, a scraper ripper, a small dragline, and a large dragline system were calculated for the Iowa Coal Project Demonstration Mine #1 (ICPDM #1) located between Oskaloosa and Bussey, Iowa. Even though the large dragline had the lowest cost and highest energy efficiency, it would not be used on this site without having other nearby sites available to allow long-term use
The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs
We identify the pressure--broadened red wings of the saturated potassium
resonance lines at 7700 \AA as the source of anomalous absorption seen in the
near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in
general. This conclusion is supported by the recent work of Tsuji {\it et al.}
1999, though unlike them we find that dust need not be invoked to explain the
spectra of methane dwarfs shortward of 1 micron. We find that a combination of
enhanced alkali abundances due to rainout and a more realistic non-Lorentzian
theory of resonant line shapes may be all that is needed to properly account
for these spectra from 0.5 to 1.0 microns. The WFPC2 measurement of Gliese
229B is also consistent with this theory. Furthermore, a combination of the
blue wings of this K I resonance doublet, the red wings of the Na D lines at
5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural way
the observed WFPC2 band flux of Gliese 229B. Hence, we conclude that the
neutral alkali metals play a central role in the near-infrared and optical
spectra of methane dwarfs and that their lines have the potential to provide
crucial diagnostics of brown dwarfs. We speculate on the systematics of the
near-infrared and optical spectra of methane dwarfs, for a given mass and
composition, that stems from the progressive burial with decreasing \teff of
the alkali metal atoms to larger pressures and depths.Comment: Revised and accepted to Ap.J. volume 531, March 1, 2000, also
available at http://jupiter.as.arizona.edu/~burrows/papers/BMS.p
Atmospheric, Evolutionary, and Spectral Models of the Brown Dwarf Gliese 229 B
Theoretical spectra and evolutionary models that span the giant planet--brown
dwarf continuum have been computed based on the recent discovery of the brown
dwarf, Gliese 229 B. A flux enhancement in the 4--5 micron window is a
universal feature from Jovian planets to brown dwarfs. We confirm the existence
of methane and water in Gl 229 B's spectrum and find its mass to be 30 to 55
Jovian masses. Although these calculations focus on Gliese 229 B, they are also
meant to guide future searches for extra-solar giant planets and brown dwarfs.Comment: 8 pages, plain TeX, plus four postscript figures, gzipped and
uuencoded, accepted for Scienc
On the Radii of Close-in Giant Planets
The recent discovery that the close-in extrasolar giant planet, HD209458b,
transits its star has provided a first-of-its-kind measurement of the planet's
radius and mass. In addition, there is a provocative detection of the light
reflected off of the giant planet, Boo b. Including the effects of
stellar irradiation, we estimate the general behavior of radius/age
trajectories for such planets and interpret the large measured radii of
HD209458b and Boo b in that context. We find that HD209458b must be a
hydrogen-rich gas giant. Furthermore, the large radius of close-in gas giant is
not due to the thermal expansion of its atmosphere, but to the high residual
entropy that remains throughout its bulk by dint of its early proximity to a
luminous primary. The large stellar flux does not inflate the planet, but
retards its otherwise inexorable contraction from a more extended configuration
at birth. This implies either that such a planet was formed near its current
orbital distance or that it migrated in from larger distances (0.5 A.U.),
no later than a few times years of birth.Comment: aasms4 LaTeX, 1 figure, accepted to Ap.J. Letter
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