Are exoplanets orbiting m dwarfs extreme?

I gave an invited talk at Extreme Solar Systems IV in REYKJAVIK, ICELAND. However, please note that all talks were officially "invited" according to the organizers.M dwarf stars have long spin-down timescales, long activity lifetimes and persistent magnetic activity, all of which have implications for the potential habitability of orbiting planets. I will present results from several research programs investigating M dwarf rotation, activity and evolution. I will discuss a new technique to measure chemical-kinematic ages of main-sequence M dwarf stars. We applied that technique to a variety of nearby M dwarfs, both planet hosts and non-planet hosts, and rapid (young) and slow (old) rotators. We find that relatively slow rotators (P 100 days) do not appear to be α enriched, indicating that they are not over 10 Gyrs old. Second, for the rapid rotators, we see clear evidence of Zeeman enhancement of Y-band Ti I lines as a function of Rossby number. While other activity indicators, such as H-α and X-ray emission, appear to saturate with low Rossby number, Zeeman enhancement does not, indicating that the saturation mechanism is confined to the chromosphere and corona. Finally, I will present new results on the M dwarf radius problem. Using spectral synthesis methods, we find that large magnetic star spot fractions are primarily responsible for observed discrepancies between model and measured stellar radii in fully convective M dwarf stars. As most M dwarfs appear discrepant, our results suggest the vast majority of M dwarfs have large spot fractions and correspondingly high localization of magnetic fields.Published versio

Design considerations for a ground-based search for transiting planets around L and T dwarfs

We present design considerations for a ground-based survey for transiting exoplanets around L and T dwarfs, spectral classes that have yet to be thoroughly probed for planets. We simulate photometry for L and T targets with a variety of red-optical and near-infrared (NIR) detectors, and compare the scatter in the photometry to anticipated transit depths. Based on these results, we recommend the use of a low-dark-current detector with H-band NIR photometric capabilities. We then investigate the potential for performing a survey for Earth-sized planets for a variety of telescope sizes. We simulate planetary systems around a set of spectroscopically confirmed L and T dwarfs using measured M dwarf planet occurrence rates from Kepler (e.g. Dressing & Charbonneau 2015), and simulate their observation in surveys ranging in duration from 120 to 600 nights, randomly discarding 30% of nights to simulate weather losses. We find that an efficient survey design uses a 2-meter class telescope with a NIR instrument and 360─480 observing nights, observing multiple L and T targets each night with a dithering strategy. Surveys conducted in such a manner have over an 80% chance of detecting at least one planet, and detect around 2 planets, on average. The number of expected detections depends on the true planet occurrence rate, however, which may in fact be higher for L and T dwarfs than for M dwarfs. Poster at a 2-day meeting "BROWN DWARF TO EXOPLANET CONNECTION III" or BDEXOCON IIIOthe

An updated cool dwarf catalog for the transiting exoplanet survey satellite using Gaia DR2

An undergrad mentee of mine presented results at the meeting in a poster. I am a co-author of the presentation. The abstract is available on ADS: https://ui.adsabs.harvard.edu/abs/2019AAS...23314001C/abstractWe present an updated catalog of cool dwarf stars for the recently launched Transiting Exoplanet Survey Satellite (TESS) for the purpose of selecting targets for two-minute cadence observations. At launch, TESS was provided a similar catalog of cool dwarfs whose stellar parameters were approximated using proper motions and — where they existed — archival parallax measurements. Most targets therefore have significant uncertainties. Now, with the highly anticipated Gaia DR2 parallax measurements released in April 2018, we updated stellar parameters where we are able to determine a cross-match with confidence, as well as determined new targets from a cross-match between Gaia and 2MASS. We anticipate delivery to TESS by the end of 2018, at which point it will have completed about one fourth of its mission. With the updated catalog, we hope that TESS will discover Earth-sized and sub-Earth-sized exoplanets around late K and M-dwarf type stars with a higher planet yield. This project was supported in part by the NSF REU grant AST-1757321 and by the Nantucket Maria Mitchell Association.Published versio

No timing variations observed in third transit of snow-line exoplanet Kepler-421b

We observed Kepler-421 during the anticipated third transit of the snow-line exoplanet Kepler-421b in order to constrain the existence and extent of transit timing variations (TTVs). Previously, the Kepler Spacecraft only observed two transits of Kepler-421b leaving the planet's transit ephemeris unconstrained. Our visible light, time-series observations from the 4.3-meter Discovery Channel Telescope were designed to capture pre-transit baseline and the partial transit of Kepler-421b barring significant TTVs. We use the light curves to assess the probabilities of various transit models using both the posterior odds ratio and the Bayesian Information Criterion (BIC) and find that a transit model with no TTVs is favored to 3.6-sigma confidence. These observations suggest that Kepler-421b is either alone in its system or is only experiencing minor dynamic interactions with an unseen companion. With the Kepler-421b ephemeris constrained, we calculate future transit times and discuss the opportunity to characterize the atmosphere of this cold, long-period exoplanet via transmission spectroscopy. Our investigation emphasizes the difficulties associated with observing long-period exoplanet transits and the consequences that arise from failing to refine transit ephemerides

Constraining planet occurrence around ultracool dwarfs observed by K2

An undergrad mentee presented her research at the meeting. I am a co-author of the presentation. The abstract is available on ADS: https://ui.adsabs.harvard.edu/abs/2019AAS...23314019S/abstractThough we expect many planets around ultracool dwarfs, few have been detected. The K2 mission presents a unique opportunity to search for transiting planets around a large sample of ultracool dwarfs and place constraints on planet occurrence at the bottom of the main sequence. Planet detection using the transit method is dependent not only on geometric transit probability but also the effectiveness of transit-searching methods. In this work, we use K2 observations to measure transit detection efficiency in ultracool dwarfs and use our transit detection efficiency to calculate an upper limit on the planet occurrence rate. We measure our ability to recover various types of transits around dwarfs at the M/L transition. We inject synthetic planetary transits of radii from 0.1 to 3.5 Earth radii and of periods from 0.3 to 26 days into 382 K2 light curves of late-type M and L dwarfs. We attempt to recover them using Box-Least Squares and Levenberg-Marquardt optimization methods. We then calculate a detection efficiency, or fraction recovered, and a threshold of detectability relating to orbital period and radius for each dwarf. We present an upper limit on the planet occurrence rate, as well as constraints on the probability of seeing no planets around a given number of ultracool dwarfs.Published versio

Magnetic inflation and stellar mass. IV. four low-mass kepler eclipsing binaries consistent with non-magnetic stellar evolutionary models

Low-mass eclipsing binaries (EBs) show systematically larger radii than model predictions for their mass, metallicity, and age. Prominent explanations for the inflation involve enhanced magnetic fields generated by rapid rotation of the star that inhibit convection and/or suppress flux from the star via starspots. However, derived masses and radii for individual EB systems often disagree in the literature. In this paper, we continue to investigate low-mass EBs observed by NASA’s Kepler spacecraft, deriving stellar masses and radii using high-quality spacebased light curves and radial velocities from high-resolution infrared spectroscopy. We report masses and radii for three Kepler EBs, two of which agree with previously published masses and radii (KIC 11922782 and KIC 9821078). For the third EB (KIC 7605600), we report new masses and show the secondary component is likely fully convective (M2 = 0.17 ± 0.01M☉ and = - ☉ + R2 0.199 0.002R 0.001 ). Combined with KIC 10935310 from Han et al., we find that the masses and radii for four low-mass Kepler EBs are consistent with modern stellar evolutionary models for M dwarf stars and do not require inhibited convection by magnetic fields to account for the stellar radii.Published versio

Kepler planet occurrence rates for mid-type M dwarfs as a function of spectral type

Previous studies of planet occurrence rates largely relied on photometric stellar characterizations. In this paper, we present planet occurrence rates for mid-type M dwarfs using spectroscopy, parallaxes, and photometry to determine stellar characteristics. Our spectroscopic observations have allowed us to constrain spectral type, temperatures, and, in some cases, metallicities for 337 out of 561 probable mid-type M dwarfs in the primary Kepler field. We use a random forest classifier to assign a spectral type to the remaining 224 stars. Combining our data with Gaia parallaxes, we compute precise (~3%) stellar radii and masses, which we use to update planet parameters and occurrence rates for Kepler mid-type M dwarfs. Within the Kepler field, there are seven M3 V to M5 V stars that host 13 confirmed planets between 0.5 and 2.5 Earth radii and at orbital periods between 0.5 and 10 days. For this population, we compute a planet occurrence rate of ${1.19}_{-0.49}^{+0.70}$ planets per star. For M3 V, M4 V, and M5 V, we compute planet occurrence rates of ${0.86}_{-0.68}^{+1.32}$, ${1.36}_{-1.02}^{+2.30}$, and ${3.07}_{-2.49}^{+5.49}$ planets per star, respectively.Published versio

NEWS: the near-infrared Echelle for wideband spectroscopy

We present an updated optical and mechanical design of NEWS: the Near-infrared Echelle for Wide-band Spectroscopy (formerly called HiJaK: the High-resolution J, H and K spectrometer), a compact, high-resolution, near-infrared spectrometer for 5-meter class telescopes. NEWS provides a spectral resolution of 60,000 and covers the full 0.8-2.5 micron range in 5 modes. We adopt a compact, lightweight, monolithic design and developed NEWS to be mounted to the instrument cube at the Cassegrain focus of the the new 4.3-meter Discovery Channel Telescope.Comment: Proc. SPIE 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 99086M (August 9, 2016

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

Chemo-kinematic ages of eccentric-planet-hosting M dwarf stars

M dwarf stars are exciting targets for exoplanet investigations; however, their fundamental stellar properties are difficult to measure. Perhaps the most challenging property to measure is stellar age. Once on the main sequence, M dwarfs change imperceptibly in their temperature and luminosity, necessitating novel statistical techniques for estimating their ages. In this paper, we infer ages for known eccentric-planet-hosting M dwarfs using a combination of kinematics and $\alpha$-element-enrichment, both shown to correlate with age for Sun-like FGK stars. We calibrate our method on FGK stars in a Bayesian context. To measure $\alpha$-enrichment, we use publicly-available spectra from the CARMENES exoplanet survey and a recently developed [Ti/Fe] calibration utilizing individual Ti I and Fe I absorption lines in $Y$ band. Tidal effects are expected to circularize the orbits of short-period planets on short timescales; however, we find a number of mildly eccentric, close-in planets orbiting old ($\sim$8 Gyr) stars. For these systems, we use our ages to constrain the tidal dissipation parameter of the planets, $Q_\mathrm{p}$. For two mini-Neptune planets, GJ 176b and GJ 536b, we find they have $Q_\mathrm{p}$ values more similar to the ice giants than the terrestrial planets in our Solar System. For GJ 436b, we estimate an age of $8.9^{+2.3}_{-2.1}$ Gyr and constrain the $Q_\mathrm{p}$ to be $>10^5$, in good agreement with constraints from its inferred tidal heating. We find that GJ 876d has likely undergone significant orbital evolution over its $8.4^{+2.2}_{-2.0}$ Gyr lifetime, potentially influenced by its three outer companions which orbit in a Laplace resonance.Comment: accepted for publication in Ap