2,091 research outputs found
Analysis of Spitzer Spectra of Irradiated Planets: Evidence for Water Vapor?
Published mid infrared spectra of transiting planets HD 209458b and HD
189733b, obtained during secondary eclipse by the InfraRed Spectrograph (IRS)
aboard the Spitzer Space Telescope, are predominantly featureless. In
particular these flux ratio spectra do not exhibit an expected feature arising
from water vapor absorption short-ward of 10 um. Here we suggest that, in the
absence of flux variability, the spectral data for HD 189733b are inconsistent
with 8 um-photometry obtained with Spitzer's InfraRed Array Camera (IRAC),
perhaps an indication of problems with the challenging reduction of the IRS
spectra. The IRAC point, along with previously published secondary eclipse
photometry for HD 189733b, are in good agreement with a one-dimensional model
of HD 189733b that clearly shows absorption due to water vapor in the emergent
spectrum. We are not able to draw firm conclusions regarding the IRS data for
HD 209458b, but spectra predicted by 1D and 3D atmosphere models fit the data
adequately, without adjustment of the water abundance or reliance on cloud
opacity. We argue that the generally good agreement between model spectra and
IRS spectra of brown dwarfs with atmospheric temperatures similar to these
highly irradiated planets lends confidence in the modeling procedure.Comment: Revised, Accepted to ApJ Letter
Comparative Planetary Atmospheres: Models of TrES-1 and HD209458b
We present new self-consistent atmosphere models for transiting planets
TrES-1 and HD209458b. The planets were recently observed with the Spitzer Space
Telescope in bands centered on 4.5 and 8.0 m, for TrES-1, and 24 m,
for HD209458b. We find that standard solar metallicity models fit the
observations for HD209458b. For TrES-1, which has an T_eff ~300 K cooler, we
find that models with a metallicity 3-5 times enhanced over solar abundances
can match the 1 error bar at 4.5 m and 2 at 8.0m.
Models with solar abundances that included energy deposition into the
stratosphere give fluxes that fall within the 2 error bars in both
bands. The best-fit models for both planets assume that reradiation of absorbed
stellar flux occurs over the entire planet. For all models of both planets we
predict planet/star flux ratios in other Spitzer bandpasses.Comment: Accepted to the Astrophysical Journal Letters, May 17, 200
Microlensing Events from Measurements of the Deflection Angle
Microlensing events are now regularly being detected by monitoring the flux
of a large number of potential sources and measuring the combined magnification
of the images. This phenomenon could also be detected directly from the
gravitational deflection, by means of high precision astrometry using
interferometry. Relative astrometry at the level of 10\muas may become
possible in the near future. The gravitational deflection can be measured by
astrometric monitoring of a bright star having a background star within a small
angular separation. This type of monitoring program will be carried out for the
independent reasons of discovering planets from the angular motion they induce
on the nearby star around which they are orbiting, and for measuring
parallaxes, proper motions and orbits of binary stars. We discuss three
applications of the measurement of gravitational deflections by astrometric
monitoring: measuring the mass of the bright stars that are monitored,
measuring the mass of brown dwarfs or giant planets around the bright stars,
and detecting microlensing events by unrelated objects near the line of sight
to the two stars. We discuss the number of stars whose mass could be measured
by this procedure. We also give expressions for the number of expected
microlensing events by unrelated objects, which could be stars, brown dwarfs,
or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter
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
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
Atmosphere, Interior, and Evolution of the Metal-Rich Transiting Planet HD 149026b
We investigate the atmosphere and interior of the new transiting planet HD
149026b, which appears to be very rich in heavy elements. We first compute
model atmospheres at metallicities ranging from solar to ten times solar, and
show how for cases with high metallicity or inefficient redistribution of
energy from the day side, the planet may develop a hot stratosphere due to
absorption of stellar flux by TiO and VO. The spectra predicted by these models
are very different than cooler atmosphere models without stratospheres. The
spectral effects are potentially detectable with the Spitzer Space Telescope.
In addition the models with hot stratospheres lead to a large limb brightening,
rather than darkening. We compare the atmosphere of HD 149026b to other
well-known transiting planets, including the recently discovered HD 189733b,
which we show have planet-to-star flux ratios twice that of HD 209458 and
TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive
indicator of atmospheric temperature and metallicity and can be constrained
with Spitzer IRAC observations. We then turn to interior studies of HD 149026b
and use a grid of self-consistent model atmospheres and high-pressure equations
of state for all components to compute thermal evolution models of the planet.
We estimate that the mass of heavy elements within the planet is in the range
of 60 to 93 M_earth. Finally, we discuss trends in the radii of transiting
planets with metallicity in light of this new member of the class.Comment: Accepted to the Astrophysical Journal. 18 pages, including 10
figures. New section on the atmosphere of planet HD 189733b. Enhanced
discussion of atmospheric Ti chemistry and core mass for HD 149026
Spitzer Phase Curves of KELT-1b and the Signatures of Nightside Clouds in Thermal Phase Observations
We observed two full orbital phase curves of the transiting brown dwarf
KELT-1b, at 3.6um and 4.5um, using the Spitzer Space Telescope. Combined with
previous eclipse data from Beatty et al. (2014), we strongly detect KELT-1b's
phase variation as a single sinusoid in both bands, with amplitudes of
ppm at 3.6um and ppm at 4.5um, and confirm the secondary
eclipse depths measured by Beatty et al. (2014). We also measure noticeable
Eastward hotspot offsets of degrees at 3.6um and
degrees at 4.5um. Both the day-night temperature contrasts and the hotspot
offsets we measure are in line with the trends seen in hot Jupiters (e.g.,
Crossfield 2015), though we disagree with the recent suggestion of an offset
trend by Zhang et al. (2018). Using an ensemble analysis of Spitzer phase
curves, we argue that nightside clouds are playing a noticeable role in
modulating the thermal emission from these objects, based on: 1) the lack of a
clear trend in phase offsets with equilibrium temperature, 2) the sharp
day-night transitions required to have non-negative intensity maps, which also
resolves the inversion issues raised by Keating & Cowan (2017), 3) the fact
that all the nightsides of these objects appear to be at roughly the same
temperature of 1000K, while the dayside temperatures increase linearly with
equilibrium temperature, and 4) the trajectories of these objects on a Spitzer
color-magnitude diagram, which suggest colors only explainable via nightside
clouds.Comment: AJ in press. Updated to reflect the accepted versio
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
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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