On High-Contrast Characterization of Nearby, Short-Period Exoplanets with Giant Segmented-Mirror Telescopes

Measurements of the frequency with which short-period planets occur around main sequence stars allows a direct prediction of the number and types of such planets that will be amenable to characterization by high-contrast instruments on future giant segmented- mirror telescopes (GSMTs). Adopting conservative assumptions, I predict of order 10 planets with radii R_P=1-8 R_Earth and equilibrium temperatures <400 K should be accessible around stars within 8 pc of the Sun. These numbers are roughly the same for both near-infrared observations of scattered starlight and mid-infrared observations of planetary thermal emission, with the latter observations demonstrating greater relative sensitivity to smaller and cooler planets. Adopting the conservative assumption that planets with R_P=1-2 R_E and 2-4 R_E occur with equal frequency, I predict a 40% chance that a planet with R_P=1-2 R_E and equilibrium temperature 200-250 K will accessible to high-contrast thermal infrared characterization; this would be a compelling object for further study. To validate these predictions, more detailed analyses are needed of the occurrence frequencies of low-mass planets around M dwarfs, both in the Kepler field and in the solar neighborhood. Several planets already discovered by radial velocity surveys will be accessible to near-infrared high-contrast GSMT observations, including the low-mass planets alpha Cen Bb and (depending on their albedos) GJ 139c and d, GJ 876b and c, and tau Cet b, c, and d; tau Cet f would be amenable to thermal infrared characterization. Further efforts to model the near-infrared reflectance and mid-infrared emission of these and other short-period planets are clearly warranted, and will pave the way for the interpretation of future high-contrast characterization of a variety of planets around the nearest stars.Comment: A&A Accepted. 10 pages, 5 figures, 1 tabl

Doppler Imaging of Exoplanets and Brown Dwarfs

Doppler Imaging produces 2D global maps of rotating objects using high-dispersion spectroscopy. When applied to brown dwarfs and extrasolar planets, this technique can constrain global atmospheric dynamics and/or magnetic effects on these objects in un- precedented detail. I present the first quantitative assessment of the prospects for Doppler Imaging of substellar objects with current facilities and with future giant ground-based telescopes. Observations will have the greatest sensitivity in K band, but the H and L bands will also be useful for these purposes. To assess the number and availability of targets, I also present a compilation of all measurements of photometric variability, rotation period (P), and projected rotational velocity (v sin i) for brown dwarfs and exoplanets. Several bright objects are already accessible to Doppler Imaging with currently available instruments. With the development of giant ground-based telescopes, Doppler Imaging will become feasible for many dozens of brown dwarfs and for the few brightest directly imaged extrasolar planets (such as beta Pic b). The present set of measurements of P, v sin i, and variability are incomplete for many objects, and the sample is strongly biased toward early-type objects (< L5). Thus, surveys to measure these quantities for later-type objects will be especially helpful in expanding the sample of candidates for global weather monitoring via Doppler Imaging.Comment: 11 pages, 4 figures, 1 electronic table. Recommended for publication in A&A. Includes referee correction

Photometry as a proxy for stellar activity in radial velocity analyses

Stellar activity remains a limiting factor in measuring precise planet parameters from radial velocity spectroscopy, not least in the search for Earth mass planets orbiting in the habitable zones of Sun-like stars. One approach to mitigate stellar activity is to use combined analyses of both radial velocity and time-series photometry. We present an analysis of simultaneous disk-integrated photometry and radial velocity data of the Sun in order to determine the useful limits of a combined analysis. We find that simple periodogram or autocorrelation analysis of solar photometry give the correct rotation period <50% of the time. We therefore use a Gaussian process to investigate the time variability of solar photometry and to directly compare simultaneous photometry with radial velocity data. We find that the hyperparameter posteriors are relatively stable over 70 years of solar photometry and the amplitude tracks the solar cycle. We observe good agreement between the hyperparameter posteriors for the simultaneous photometry and radial velocity data. Our primary conclusion is a recommendation to include an additional prior in Gaussian process fits to constrain the evolutionary timescale to be greater than the recurrence timescale (ie., the rotation period) to recover more physically plausible and useful results. Our results indicate that such simultaneous monitoring may be a useful tool in enhancing the precision of radial velocity surveys.Comment: 10 pages, accepted in A

Light Curve Modulation of Low Mass Stars in K2. I. Identification of 508 Fast Rotators in the Solar Neighborhood

The K2 mission is targeting large numbers of nearby (d<100 pc) GKM dwarfs selected from the SUPERBLINK proper motion survey (mu>40 mas yr^-1, V<20). Additionally, the mission is targeting low-mass, high proper motion stars associated with the local (d<500 pc) Galactic halo population also selected from SUPERBLINK. K2 campaigns 0 through 8 monitored a total of 27,382 of these cool main-sequence stars. We used the auto-correlation function to search for fast rotators by identifying short-period photometric modulations in the K2 light curves. We identified 508 candidate fast rotators with rotation periods <4 days that show light curve modulations consistent with star spots. Their kinematics show low average transverse velocities, suggesting they are part of the young disk population. A subset (13) of the fast rotators are found among those targets with colors and kinematics consistent with the local Galactic halo population and may represent stars spun up by tidal interactions in close binary systems. We further demonstrate the M dwarf fast rotators selected from the K2 light curves are significantly more likely to have UV excess, and discuss the potential of the K2 mission to identify new nearby young GKM dwarfs on the basis of their fast rotation rates. Finally, we discuss the possible use of local halo stars as fiducial, non-variable sources in the Kepler fields.Comment: 14 pages, 14 figures, to appear in the Astronomical Journal (AJ

A Super-Earth and Sub-Neptune Transiting the Late-type M Dwarf LP 791-18

This work is licensed under a Creative Commons Attribution 3.0 Unported License.Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R ⊙, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1R ⊕ planet on a 0.95 day orbit and a 2.3R ⊕ planet on a 5 day orbit. Because the host star is small the decrease in light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets