95 research outputs found
A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C
The characterization of a physically-diverse set of transiting exoplanets is
an important and necessary step towards establishing the physical properties
linked to the production of obscuring clouds or hazes. It is those planets with
identifiable spectroscopic features that can most effectively enhance our
understanding of atmospheric chemistry and metallicity. The newly-commissioned
LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed
fringing in the red optical, thus advancing the search for the spectroscopic
signature of water in exoplanetary atmospheres from the ground. Using data
acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of
water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b.
Our measured spectrum is best explained by the presence of water vapor, a lack
of potassium, and either a high-metallicity, cloud-free atmosphere or a
solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of
multi-scale-height spectral features in our data suggests that future
observations at higher precision could break this degeneracy and reveal the
planet's atmospheric chemical abundances. We also update HAT-P-26b's transit
ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p =
4.2345023(7) days.Comment: 9 pages, 8 figures, Accepted for publication in Ap
Ground-based optical transmission spectrum of the hot Jupiter HAT-P-1b
Time-series spectrophotometric studies of exoplanets during transit using
ground-based facilities are a promising approach to characterize their
atmospheric compositions. We aim to investigate the transit spectrum of the hot
Jupiter HAT-P-1b. We compare our results to those obtained at similar
wavelengths by previous space-based observations. We observed two transits of
HAT-P-1b with the Gemini Multi-Object Spectrograph (GMOS) instrument on the
Gemini North telescope using two instrument modes covering the 320 - 800 nm and
520 - 950 nm wavelength ranges. We used time-series spectrophotometry to
construct transit light curves in individual wavelength bins and measure the
transit depths in each bin. We accounted for systematic effects. We addressed
potential photometric variability due to magnetic spots in the planet's host
star with long-term photometric monitoring. We find that the resulting transit
spectrum is consistent with previous Hubble Space Telescope (HST) observations.
We compare our observations to transit spectroscopy models that marginally
favor a clear atmosphere. However, the observations are also consistent with a
flat spectrum, indicating high-altitude clouds. We do not detect the Na
resonance absorption line (589 nm), and our observations do not have sufficient
precision to study the resonance line of K at 770 nm. We show that even a
single Gemini/GMOS transit can provide constraining power on the properties of
the atmosphere of HAT-P-1b to a level comparable to that of HST transit studies
in the optical when the observing conditions and target and reference star
combination are suitable. Our 520 - 950 nm observations reach a precision
comparable to that of HST transit spectra in a similar wavelength range of the
same hot Jupiter, HAT-P-1b. However, our GMOS transit between 320 - 800 nm
suffers from strong systematic effects and yields larger uncertainties.Comment: A&A, accepted, 16 pages, 8 figures, 5 table
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
Phase curves of WASP-33b and HD 149026b and a New Correlation Between Phase Curve Offset and Irradiation Temperature
We present new 3.6 and 4.5 Spitzer phase curves for the highly
irradiated hot Jupiter WASP-33b and the unusually dense Saturn-mass planet HD
149026b. As part of this analysis, we develop a new variant of pixel level
decorrelation that is effective at removing intrapixel sensitivity variations
for long observations (>10 hours) where the position of the star can vary by a
significant fraction of a pixel. Using this algorithm, we measure eclipse
depths, phase amplitudes, and phase offsets for both planets at 3.6 and
4.5 . We use a simple toy model to show that WASP-33b's phase offset,
albedo, and heat recirculation efficiency are largely similar to those of other
hot Jupiters despite its very high irradiation. On the other hand, our fits for
HD 149026b prefer a very high albedo and an unusually high recirculation
efficiency. We also compare our results to predictions from general circulation
models, and find that while neither are a good match to the data, the
discrepancies for HD 149026b are especially large. We speculate that this may
be related to its high bulk metallicity, which could lead to enhanced
atmospheric opacities and the formation of reflective cloud layers in localized
regions of the atmosphere. We then place these two planets in a broader context
by exploring relationships between the temperatures, albedos, heat transport
efficiencies, and phase offsets of all planets with published thermal phase
curves. We find a striking relationship between phase offset and irradiation
temperature--the former drops with increasing temperature until around 3400 K,
and rises thereafter. Although some aspects of this trend are mirrored in the
circulation models, there are notable differences that provide important clues
for future modeling efforts
A Near-Infrared Transmission Spectrum for the Warm Saturn HAT-P-12b
We present a HST WFC3 transmission spectrum for the transiting exoplanet HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a lower temperature than the hot-Jupiters that have been studied so far. We find that the planet's measured transmission spectrum lacks the expected water absorption feature for a hydrogen-dominated atmosphere, and is instead best-described by a model with high-altitude clouds. Using a frequentist hypothesis testing procedure, we can rule out a hydrogen-dominated cloud free atmosphere to 4.9σ. When combined with other recent WFC3 studies, our observations suggest that clouds may be common in exoplanetary atmospheres
Benchmark Transiting Brown Dwarf LHS 6343 C: Spitzer Secondary Eclipse Observations Yield Brightness Temperature and Mid-T Spectral Class
There are no field brown dwarf analogs with measured masses, radii, and luminosities, precluding our ability to connect the population of transiting brown dwarfs with measurable masses and radii and field brown dwarfs with measurable luminosities and atmospheric properties. LHS 6343 C, a weakly irradiated brown dwarf transiting one member of an M+M binary in the Kepler field, provides the first opportunity to probe the atmosphere of a non-inflated brown dwarf with a measured mass and radius. Here, we analyze four Spitzer observations of secondary eclipses of LHS 6343 C behind LHS 6343 A. Jointly fitting the eclipses with a Gaussian process noise model of the instrumental systematics, we measure eclipse depths of 1.06 ± 0.21 ppt at 3.6 μm and 2.09 ± 0.08 ppt at 4.5 μm, corresponding to brightness temperatures of 1026 ± 57 K and 1249 ± 36 K, respectively. We then apply brown dwarf evolutionary models to infer a bolometric luminosity log(L_*/L_☉)= -5.16 ± 0.04. Given the known physical properties of the brown dwarf and the two M dwarfs in the LHS 6343 system, these depths are consistent with models of a 1100 K T dwarf at an age of 5 Gyr and empirical observations of field T5-6 dwarfs with temperatures of 1070 ± 130 K. We investigate the possibility that the orbit of LHS 6343 C has been altered by the Kozai–Lidov mechanism and propose additional astrometric or Rossiter–McLaughlin measurements of the system to probe the dynamical history of the system
Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
HAT-P-20b is a giant exoplanet that orbits a metal-rich star. The planet
itself has a high total density, suggesting that it may also have a high
metallicity in its atmosphere. We analyze two eclipses of the planet in each of
the 3.6- and 4.5 micron bands of Warm Spitzer. These data exhibit intra-pixel
detector sensitivity fluctuations that were resistant to traditional
decorrelation methods. We have developed a simple, powerful, and radically
different method to correct the intra-pixel effect for Warm Spitzer data, which
we call pixel-level decorrelation (PLD). PLD corrects the intra-pixel effect
very effectively, but without explicitly using - or even measuring - the
fluctuations in the apparent position of the stellar image. We illustrate and
validate PLD using synthetic and real data, and comparing the results to
previous analyses. PLD can significantly reduce or eliminate red noise in
Spitzer secondary eclipse photometry, even for eclipses that have proven to be
intractable using other methods. Our successful PLD analysis of four HAT-P-20b
eclipses shows a best-fit blackbody temperature of 1134 +/-29K, indicating
inefficient longitudinal transfer of heat, but lacking evidence for strong
molecular absorption. We find sufficient evidence for variability in the 4.5
micron band that the eclipses should be monitored at that wavelength by
Spitzer, and this planet should be a high priority for JWST spectroscopy. All
four eclipses occur about 35 minutes after orbital phase 0.5, indicating a
slightly eccentric orbit. A joint fit of the eclipse and transit times with
extant RV data yields e(cos{omega}) = 0.01352 (+0.00054, -0.00057), and
establishes the small eccentricity of the orbit to high statistical confidence.
Given the existence of a bound stellar companion, HAT-P-20b is another
excellent candidate for orbital evolution via Kozai migration or other
three-body mechanism.Comment: version published in ApJ, minor text and figure revision
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