141 research outputs found
High Resolution Transmission Spectroscopy as a Diagnostic for Jovian Exoplanet Atmospheres: Constraints from Theoretical Models
We present high resolution transmission spectra of giant planet atmospheres
from a coupled 3-D atmospheric dynamics and transmission spectrum model that
includes Doppler shifts which arise from winds and planetary motion. We model
jovian planets covering more than two orders of magnitude in incident flux,
corresponding to planets with 0.9 to 55 day orbital periods around solar-type
stars. The results of our 3-D dynamical models reveal certain aspects of high
resolution transmission spectra that are not present in simple 1-D models. We
find that the hottest planets experience strong substellar to anti-stellar
(SSAS) winds, resulting in transmission spectra with net blue shifts of up to 3
km s, whereas less irradiated planets show almost no net Doppler shifts.
Compared to 1-D models, peak line strengths are significantly reduced for the
hottest atmospheres owing to Doppler broadening from a combination of rotation
(which is faster for close-in planets under the assumption of tidal locking)
and atmospheric winds. Finally, high resolution transmission spectra may be
useful in studying the atmospheres of exoplanets with optically thick clouds
since line cores for very strong transitions should remain optically thick to
very high altitude. High resolution transmission spectra are an excellent
observational test for the validity of 3-D atmospheric dynamics models, because
they provide a direct probe of wind structures and heat circulation.
Ground-based exoplanet spectroscopy is currently on the verge of being able to
verify some of our modeling predictions, most notably the dependence of SSAS
winds on insolation. We caution that interpretation of high resolution
transmission spectra based on 1-D atmospheric models may be inadequate, as 3-D
atmospheric motions can produce a noticeable effect on the absorption
signatures.Comment: Accepted to ApJ; 34 pages, 6 figures, 1 tabl
An Observational Diagnostic for Distinguishing Between Clouds and Haze in Hot Exoplanet Atmospheres
The nature of aerosols in hot exoplanet atmospheres is one of the primary
vexing questions facing the exoplanet field. The complex chemistry, multiple
formation pathways, and lack of easily identifiable spectral features
associated with aerosols make it especially challenging to constrain their key
properties. We propose a transmission spectroscopy technique to identify the
primary aerosol formation mechanism for the most highly irradiated hot Jupiters
(HIHJs). The technique is based on the expectation that the two key types of
aerosols -- photochemically generated hazes and equilibrium condensate clouds
-- are expected to form and persist in different regions of a highly irradiated
planet's atmosphere. Haze can only be produced on the permanent daysides of
tidally-locked hot Jupiters, and will be carried downwind by atmospheric
dynamics to the evening terminator (seen as the trailing limb during transit).
Clouds can only form in cooler regions on the night side and morning terminator
of HIHJs (seen as the leading limb during transit). Because opposite limbs are
expected to be impacted by different types of aerosols, ingress and egress
spectra, which primarily probe opposing sides of the planet, will reveal the
dominant aerosol formation mechanism. We show that the benchmark HIHJ,
WASP-121b, has a transmission spectrum consistent with partial aerosol coverage
and that ingress-egress spectroscopy would constrain the location and formation
mechanism of those aerosols. In general, using this diagnostic we find that
observations with JWST and potentially with HST should be able to distinguish
between clouds and haze for currently known HIHJs.Comment: 10 pages, 4 figures, accepted to ApJ Letter
A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ~300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds
Kepler Transit Depths Contaminated by a Phantom Star
We present ground-based observations from the Discovery Channel Telescope
(DCT) of three transits of Kepler-445c---a supposed super-Earth exoplanet with
properties resembling GJ 1214b---and demonstrate that the transit depth is
approximately 50 percent shallower than the depth previously inferred from
Kepler Spacecraft data. The resulting decrease in planetary radius
significantly alters the interpretation of the exoplanet's bulk composition.
Despite the faintness of the M4 dwarf host star, our ground-based photometry
clearly recovers each transit and achieves repeatable 1-sigma precision of
approximately 0.2 percent (2 millimags). The transit parameters estimated from
the DCT data are discrepant with those inferred from the Kepler data to at
least 17-sigma confidence. This inconsistency is due to a subtle miscalculation
of the stellar crowding metric during the Kepler pre-search data conditioning
(PDC). The crowding metric, or CROWDSAP, is contaminated by a non-existent
"phantom star" originating in the USNO-B1 catalog and inherited by the Kepler
Input Catalog (KIC). Phantom stars in the KIC are likely rare, but they have
the potential to affect statistical studies of Kepler targets that use the PDC
transit depths for a large number of exoplanets where individual follow-up
observation of each is not possible. The miscalculation of Kepler-445c's
transit depth emphasizes the importance of stellar crowding in the Kepler data,
and provides a cautionary tale for the analysis of data from the Transiting
Exoplanet Survey Satellite (TESS), which will have even larger pixels than
Kepler.Comment: 11 pages, 10 figures, 5 tables. Accepted for publication in AJ.
Transit light curves will be available from AJ as Db
A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ~300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds
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