Search for: All records

M dwarfs are ubiquitous in our Galaxy, and the rate at which they host stellar companions, and the properties of these companions, provide a window into the formation and evolution of the star(s), and of any planets that they may host. The Pervasive Overview of “Kompanions” of Every M dwarf in Our Neighborhood (POKEMON) speckle survey of nearby M dwarfs is volume limited from M0V through M9V out to 15 pc, with additional targets at larger distances. In total, 1125 stars were observed, and 455 of these are within the volume-limited, 15 pc sample of M-dwarf primaries. When we combine the speckle observations with known companions from the literature, we find that the stellar multiplicity rate of M dwarfs within 15 pc is 23.5% ± 2.0%, and that the companion rate is 28.8% ± 2.1%. We also find that the projected separation distribution for multiples that are known to host planets peaks at 198 au, while the distribution for multiples that are not yet known to host planets peaks at 5.57 au. This result suggests that the presence of close-in stellar companions inhibits the formation of M-dwarf planetary systems, similar to what has been found for FGK stars.

Based on photometric observations by TESS, we present the discovery of a potential Venus analog transiting LHS 475, an M3 dwarf located 12.5 pc from the Sun. The mass of the star is 0.274 ± 0.015M_☉. The planet, originally reported as TOI 910.01, has an orbital period of 2.0291010 ± 0.0000017 days and an estimated radius of 0.975 ± 0.058R_⊕. We confirm the validity and source of the transit signal with MEarth and Las Cumbres Observatory Global Telescope ground-based follow-up photometry. We present radial velocity data from CHIRON that rule out massive companions. In accordance with the observed mass–radius distribution of exoplanets as well as planet formation theory, we expect this planetary companion to be terrestrial, with an estimated radial velocity semiamplitude of 1.1 m s⁻¹. LHS 475 b is likely too hot to be habitable but is a suitable candidate for emission and transmission spectroscopy.

Stellar multiplicity is correlated with many stellar properties, yet multiplicity measurements have proven difficult for the M dwarfs—the most common type of star in our galaxy—due to their faintness and the fact that a reasonably complete inventory of later M dwarfs did not exist until recently. We have therefore carried out the Pervasive Overview of “Kompanions” of Every M dwarf in Our Neighborhood (POKEMON) survey, which made use of the Differential Speckle Survey Instrument on the 4.3 m Lowell Discovery Telescope, along with the NN-EXPLORE Exoplanet Stellar Speckle Imager on the 3.5 m WIYN telescope. The POKEMON sample is volume limited from M0V through M9V out to 15 pc, with additional brighter targets at larger distances. In total, 1125 targets were observed. New discoveries were presented in the first paper in the series. In this second paper in the series, we present all detected companions, gauge our astrometric and photometric precision, and compare our filtered and filterless speckle observations. We find that the majority (58.9%) of the companions we detect in our speckle images are not resolved in Gaia, demonstrating the need for high-resolution imaging in addition to long-term astrometric monitoring. Additionally, we find that the majority (73.2%) of simulated stellar companions would be detectable by our speckle observations. Specifically within 100 au, we find that 70.3% of simulated companions are recovered. Finally, we discuss future directions of the POKEMON survey.

We present a study of the relationship between Galactic kinematics, flare rates, chromospheric activity, and rotation periods for a volume-complete, nearly all-sky sample of 219 single stars within 15 pc and with masses between 0.1 and 0.3M_⊙observed during the primary mission of TESS. We find all stars consistent with a common value ofα= 1.984 ± 0.019 for the exponent of the flare frequency distribution. Using our measured stellar radial velocities and Gaia astrometry, we determine GalacticUVWspace motions. We find 78% of stars are members of the Galactic thin disk, 7% belong to the thick disk, and for the remaining 15% we cannot confidently assign membership to either component. If we assume star formation has been constant in the thin disk for the past 8 Gyr, then based on the fraction that we observe to be active, we estimate the average age at which these stars transition from the saturated to the unsaturated flaring regime to be 2.4 ± 0.3 Gyr. This is consistent with the ages that we assign from Galactic kinematics: we find that stars with rotation periodP_rot< 10 days have an age of 2.0 ± 1.2 Gyr, stars with 10 days <P_rot≤ 90 days have an age of 5.6 ± 2.7 Gyr, and stars withP_rot> 90 days have an age of 12.9 ± 3.5 Gyr. We find that the average age of stars withP_rot< 10 days increases with decreasing stellar mass from 0.6 ± 0.3 Gyr (0.2–0.3M_⊙) to 2.3 ± 1.3 Gyr (0.1–0.2M_⊙).

We present a study of the variation timescales of the chromospheric activity indicator Hαon a sample of 13 fully convective, active mid-to-late M stars with masses between 0.1 and 0.3 solar masses. Our goal was to determine the dominant variability timescale and, by inference, a possible mechanism responsible for the variation. We gathered 10 or more high-resolution spectra each of 10 stars using the TRES spectrograph at times chosen to span all phases of stellar rotation, as determined from photometric data from the MEarth Observatories. All stars varied in their Hαemission. For nine of these stars, we found no correlation between Hαand rotational phase, indicating that constant emission from fixed magnetic structures, such as star spots and plage, are unlikely to be the dominant source of Hαemission variability. In contrast, one star, G 7–34, shows a clear relationship between Hαand stellar rotational phase. Intriguingly, we found that this star is a member of the AB Doradus moving group and hence has the young age of 149 Myr. High-cadence spectroscopic observations of three additional stars revealed that they are variable on timescales ranging from 20 to 45 minutes, which we posit may be due to flaring behavior. For one star, GJ 1111, simultaneous TESS photometry and spectroscopic monitoring show an increase in Hαemission with increased photometric brightness. We conclude that low-energy flares are able to produce variation in Hαon the timescales we observe and thus may be the dominant source of Hαvariability on active fully convective M dwarfs.

M dwarfs are favorable targets for exoplanet detection with current instrumentation, but stellar companions can induce false positives and inhibit planet characterization. Knowledge of stellar companions is also critical to our understanding of how binary stars form and evolve. We have therefore conducted a survey of stellar companions around nearby M dwarfs, and here we present our new discoveries. Using the Differential Speckle Survey Instrument at the 4.3 m Lowell Discovery Telescope, and the similar NN-EXPLORE Exoplanet Stellar Speckle Imager at the 3.5 m WIYN telescope, we carried out a volume-limited survey of M-dwarf multiplicity to 15 parsecs, with a special emphasis on including the later M dwarfs that were overlooked in previous surveys. Additional brighter targets at larger distances were included for a total sample size of 1070 M dwarfs. Observations of these 1070 targets revealed 26 new companions; 22 of these systems were previously thought to be single. If all new discoveries are confirmed, then the number of known multiples in the sample will increase by 7.6%. Using our observed properties, as well as the parallaxes and 2MASSKmagnitudes for these objects, we calculate the projected separation, and estimate the mass ratio and component spectral types, for these systems. We report the discovery of a new M-dwarf companion to the white dwarf Wolf 672 A, which hosts a known M-dwarf companion as well, making the system trinary. We also examine the possibility that the new companion to 2MASS J13092185-2330350 is a brown dwarf. Finally, we discuss initial insights from the POKEMON survey.

We present the first results of a multiyear program to map the orbits of M-dwarf multiples within 25 pc. The observations were conducted primarily during 2019–2020 using speckle interferometry at the Southern Astrophysical Research Telescope in Chile, using the High-Resolution Camera mounted on the adaptive optics module (HRCam+SAM). The sample of nearby M dwarfs is drawn from three sources: multiples from the RECONS long-term astrometric monitoring program at the SMARTS 0.9 m; known multiples, for which these new observations will enable or improve orbit fits; and candidate multiples flagged by their astrometric fits in Gaia Data Release 2 (DR2). We surveyed 333 of our 338 M dwarfs via 830 speckle observations, detecting companions for 63% of the stars. Most notably, this includes new companions for 76% of the subset selected from Gaia DR2. In all, we report the first direct detections of 97 new stellar companions to the observed M dwarfs. Here we present the properties of those detections, the limits of each nondetection, and five orbits with periods 0.67–29 yr already observed as part of this program. Companions detected have projected separations of 0.″024–2.″0 (0.25–66 au) from their primaries and have ΔI≲ 5.0 mag. This multiyear campaign will ultimately map complete orbits for nearby M dwarfs with periods up to 3 yr, and provide key epochs to stretch orbital determinations for binaries to 30 yr.

TESS has proven to be a powerful resource for finding planets, including those that orbit the most prevalent stars in our galaxy: M dwarfs. Identification of stellar companions (both bound and unbound) has become a standard component of the transiting planet confirmation process in order to assess the level of light-curve dilution and the possibility of the target being a false positive. Studies of stellar companions have also enabled investigations into stellar multiplicity in planet-hosting systems, which has wide-ranging implications for both exoplanet detection and characterization, as well as for the formation and evolution of planetary systems. Speckle and AO imaging are some of the most efficient and effective tools for revealing close-in stellar companions; we therefore present observations of 58 M-dwarf TOIs obtained using a suite of speckle imagers at the 3.5 m WIYN telescope, the 4.3 m Lowell Discovery Telescope, and the 8.1 m Gemini North and South telescopes. These observations, as well as near-infrared adaptive optics images obtained for a subset (14) of these TOIs, revealed only two close-in stellar companions. Upon surveying the literature, and cross-matching our sample with Gaia, SUPERWIDE, and the catalog from El-Badry et al., we reveal an additional 15 widely separated common proper motion companions. We also evaluate the potential for undetected close-in companions. Taking into consideration the sensitivity of the observations, our findings suggest that the orbital period distribution of stellar companions to planet-hosting M dwarfs is shifted to longer periods compared to the expected distribution for field M dwarfs.

In order to assess the multiplicity statistics of stars across spectral types and populations in a volume-limited sample, we censused nearby stars for companions with Robo-AO. We report on observations of 1157 stars of all spectral types within 25 pc with decl. >−13° searching for tight companions. We detected 154 companion candidates with separations ranging from ∼0.″15 to 4.″0 and magnitude differences up to Δm${}_{i\prime }\le 7$using the robotic adaptive optics instrument Robo-AO. We confirmed physical association from Gaia EDR3 astrometry for 53 of the companion candidates, 99 remain to be confirmed, and two were ruled out as background objects. We complemented the high-resolution imaging companion search with a search for comoving objects with separations out to 10,000 au in Gaia EDR3, which resulted in an additional 147 companions registered. Of the 301 total companions reported in this study, 49 of them are new discoveries. Out of the 191 stars with significant acceleration measurements in the Hipparcos–Gaia catalog of accelerations, we detect companions around 115 of them, with the significance of the acceleration increasing as the companion separation decreases. From this survey, we report the following multiplicity fractions (compared to literature values): 40.9% ± 3.0% (44%) for FGK stars and 28.2% ± 2.3% (27%) for M stars, as well as higher-order fractions of 5.5% ± 1.1% (11%) and 3.9% ± 0.9% (5%) for FGK stars and M-type stars, respectively.