Trigonometric Parallaxes for 1,507 Nearby Mid-to-Late M-dwarfs

The MEarth survey is a search for small rocky planets around the smallest, nearest stars to the Sun as identified by high proper motion with red colors. We augmented our planetary search time series with lower cadence astrometric imaging and obtained two million images of approximately 1800 stars suspected to be mid-to-late M dwarfs. We fit an astrometric model to MEarth's images for 1507 stars and obtained trigonometric distance measurements to each star with an average precision of 5 milliarcseconds. Our measurements, combined with the 2MASS photometry, allowed us to obtain an absolute K_s magnitude for each star. In turn, this allows us to better estimate the stellar parameters than those obtained with photometric estimates alone and to better prioritize the targets chosen to monitor at high cadence for planetary transits. The MEarth sample is mostly complete out to a distance of 25 parsecs for stars of type M5.5V and earlier, and mostly complete for later type stars out to 20 parsecs. We find eight stars that are within ten parsecs of the Sun for which there did not exist a published trigonometric parallax distance estimate. We release with this work a catalog of the trigonometric parallax measurements for 1,507 mid-to-late M-dwarfs, as well as new estimates of their masses and radii.Comment: ApJ, accepted. 36 pages, 8 figures, 2 tables. Please find our data table here: http://www.cfa.harvard.edu/MEarth/DataDR2.htm

A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100 Hour Spitzer Campaign

We present the results of a search for additional bodies in the GJ 1132 system through two methods: photometric transits and transit timing variations of the known planet. We collected 21 transit observations of GJ 1132b with the MEarth-South array since 2015. We obtained 100 near-continuous hours of observations with the $Spitzer$ Space Telescope, including two transits of GJ 1132b and spanning 60\% of the orbital phase of the maximum period at which bodies coplanar with GJ 1132b would pass in front of the star. We exclude transits of additional Mars-sized bodies, such as a second planet or a moon, with a confidence of 99.7\%. When we combine the mass estimate of the star (obtained from its parallax and apparent $K_s$ band magnitude) with the stellar density inferred from our high-cadence $Spitzer$ light curve (assuming zero eccentricity), we measure the stellar radius of GJ 1132 to be $0.2105^{+0.0102}_{-0.0085} R_\odot$, and we refine the radius measurement of GJ 1132b to $1.130 \pm 0.056 R_\oplus$. Combined with HARPS RV measurements, we determine the density of GJ 1132b to be $6.2 \pm 2.0$\ g cm$^{-3}$, with the mass determination dominating this uncertainty. We refine the ephemeris of the system and find no evidence for transit timing variations, which would be expected if there was a second planet near an orbital resonance with GJ 1132b.Comment: 29 pages, 4 Tables, 8 Figures, Submitted to ApJ. Comments welcom

The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood

Rotation is a directly-observable stellar property, and drives magnetic field generation and activity through a magnetic dynamo. Main sequence stars with masses below approximately 0.35Msun (mid-to-late M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insights into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth transit survey, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no period-amplitude relation for stars below 0.25Msun and an anti-correlation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly-rotating stars without H{\alpha} emission that nevertheless have strong photometric variability. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the Solar Neighborhood. We use the W space velocities and established age-velocity relations to estimate that stars with P<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs. The period distribution is mass dependent: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods. [Abridged]Comment: Accepted to ApJ. Machine readable tables and additional figures are available in the published article or on reques

Trigonometric Parallaxes for 1,507 Nearby Mid-to-late M-dwarfs

The MEarth survey is a search for small rocky planets around the smallest, nearest stars to the Sun as identified by high proper motion with red colors. We augmented our planetary search time series with lower cadence astrometric imaging and obtained two million images of approximately 1800 stars suspected to be mid-to-late M dwarfs. We fit an astrometric model to MEarth’s images for 1507 stars and obtained trigonometric distance measurements to each star with an average precision of 5 mas. Our measurements, combined with the Two Micron All Sky Survey photometry, allowed us to obtain an absolute Ks magnitude for each star. In turn, this allows us to better estimate the stellar parameters than those obtained with photometric estimates alone and to better prioritize the targets chosen to monitor at high cadence for planetary transits. The MEarth sample is mostly complete out to a distance of 25 pc for stars of type M5.5V and earlier, and mostly complete for later type stars out to 20 pc. We find eight stars that are within 10 pc of the Sun for which there did not exist a published trigonometric parallax distance estimate. We release with this work a catalog of the trigonometric parallax measurements for 1507 mid-to-late M dwarfs, as well as new estimates of their masses and radii.Astronom

The Impact of Stellar Rotation on the Detectability of Habitable Planets Around M Dwarfs

Stellar activity and rotation frustrate the detection of exoplanets through the radial velocity technique. This effect is particularly of concern for M dwarfs, which can remain magnetically active for billions of years. We compile rotation periods for late-type stars and for the M dwarf planet-host sample in order to investigate the rotation periods of older field stars across the main sequence. We show that for stars with masses between 0.25 and 0.5 solar masses (M4V to M1V), the stellar rotation period typical of field stars coincides with the orbital periods of planets in the habitable zone. This will pose a fundamental challenge to the discovery and characterization of potentially habitable planets around early M dwarfs. Due to the longer rotation periods reached by mid M dwarfs and the shorter orbital period at which the planetary habitable zone is found, stars with masses between 0.1 and 0.25 solar masses (M6V to M4V) offer better opportunities for the detection of habitable planets via radial velocities.Astronom

Robo-AO M-dwarf Multiplicity Survey: Catalog

We analyze observations from Robo-AO's field M dwarf survey taken on the 2.1 m Kitt Peak telescope and perform a multiplicity comparison with Gaia DR2. Through its laser-guided, automated system, the Robo-AO instrument has yielded the largest adaptive optics M dwarf multiplicity survey to date. After developing an interface to visually identify and locate stellar companions, we selected 11 low-significance Robo-AO detections for follow-up on the Keck II telescope using NIRC2. In the Robo-AO survey we find 553 candidate companions within 4'' around 534 stars out of 5566 unique targets, most of which are new discoveries. Using a position cross-match with DR2 on all targets, we assess the binary recoverability of Gaia DR2 and compare the properties of multiples resolved by both Robo-AO and Gaia. The catalog of nearby M dwarf systems and their basic properties presented here can assist other surveys which observe these stars, such as the NASA TESS mission

An Activity-Rotation Relationship and Kinematic Analysis of Nearby Mid-to-Late-Type M Dwarfs

Using spectroscopic observations and photometric light curves of 238 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by Hα emission), rotation period, and stellar age. Previous attempts to investigate the relationship between magnetic activity and rotation in these stars were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that v sin i measurements probe only rapid rotation). However, the photometric data from MEarth allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from shorter than than one to longer than 100 days). Over all M spectral types that we probe, we find that the presence of magnetic activity is tied to rotation, including for late-type, fully convective M dwarfs. We also find evidence that the fraction of latetype M dwarfs that are active may be higher at longer rotation periods compared to their early-type counterparts, with several active, late-type, slowly rotating stars present in our sample. Additionally, we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active. This potential mismatch suggests that the physical mechanisms that connect stellar rotation to chromospheric heating may be different in fully convective stars. A kinematic analysis suggests that the magnetically active, rapidly rotating stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.Astronom

Coronal X-Ray Emission from Nearby, Low-Mass, Exoplanet Host Stars Observed by the MUSCLES and Mega-MUSCLES HST Treasury Survey Projects

The high energy X-ray and ultraviolet (UV) radiation fields of exoplanet host stars play a crucial role in controlling the atmospheric conditions and the potential habitability of exoplanets. Major surveys of the X-ray/UV emissions from late-type (K and M spectral type) exoplanet hosts have been conducted by the MUSCLES and Mega-MUSCLES Hubble Space Telescope (HST) Treasury programs. These samples primarily consist of relatively old, ``inactive'', low mass stars. In this paper we present results from X-ray observations of the coronal emission from these stars obtained using the Chandra X-ray Observatory, the XMM-Newton Observatory, and the Neil Gehrels Swift Observatory. The stars effectively sample the coronal activity of low-mass stars at a wide range of masses and ages. The vast majority (21 of 23) of the stars are detected and their X-ray luminosities measured. Short-term flaring variability is detected for most of the fully-convective (M $\leq$ 0.35 M$_{\odot}$) stars but not for the more massive M dwarfs during these observations. Despite this difference, the mean X-ray luminosities for these two sets of M dwarfs are similar with more massive (0.35 M$_{\odot}$ $\leq$ M $\leq$ 0.6 M$_{\odot}$) M dwarfs at $\sim$5 $\times$ 10$^{26}$ erg s$^{-1}$ compared to $\sim$2 $\times$ 10$^{26}$ erg s$^{-1}$ for fully-convective stars older than 1 Gyr. Younger, fully-convective M dwarfs have X-ray luminosities between 3 and 6 $\times$ 10$^{27}$ erg s$^{-1}$.The coronal X-ray spectra have been characterized and provide important information that is vital for the modeling of the stellar EUV spectra.Comment: 39 pages, 15 figures. Accepted for publication in The Astronomical Journa