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1. Activity and Rotation of Nearby Field M Dwarfs in the TESS Southern Continuous Viewing Zone

   https://doi.org/10.3847/1538-3881/ac6110  

   Anthony, Francys ; Núñez, Alejandro ; Agüeros, Marcel A ; Curtis, Jason L ; do_Nascimento_Jr, J-D ; Machado, João M ; Mann, Andrew W ; Newton, Elisabeth R ; Rampalli, Rayna ; Thao, Pa Chia ; et al ( May 2022 , The Astronomical Journal)

   The evolution of magnetism in late-type dwarfs remains murky, as we can only weakly predict levels of activity for M dwarfs of a given mass and age. We report results from our spectroscopic survey of M dwarfs in the Southern Continuous Viewing Zone (CVZ) of the Transiting Exoplanet Survey Satellite (TESS). As the TESS CVZs overlap with those of the James Webb Space Telescope, our targets constitute a legacy sample for studies of nearby M dwarfs. For 122 stars, we obtained at least one R≈ 2000 optical spectrum with which we measure chromospheric Hαemission, a proxy for magnetic field strength. The fraction of active stars is consistent with what is expected for field M dwarfs; as in previous studies, we find that late-type M dwarfs remain active for longer than their early-type counterparts. While the TESS light curves for ≈20% of our targets show modulations consistent with rotation, TESS systematics are not well enough understood for confident measurements of rotation periods (P_rot) longer than half the length of an observing sector. We report periods for 12 stars for which we measure P_rot ≲ 15 days or find confirmation for the TESS-derived P_rot in the literature. Our sample of 21 P_rot, which includes periods from the literature, is consistent with our targets being spun-down field stars. Finally, we examine the Hα-to-bolometric luminosity distribution for our sample. Two stars are rotating fast enough to be magnetically saturated, but are not, hinting at the possibility that fast rotators may appear inactive in Hα.

    

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2. Galactic Kinematics and Observed Flare Rates of a Volume-complete Sample of Mid-to-late M Dwarfs: Constraints on the History of the Stellar Radiation Environment of Planets Orbiting Low-mass Stars

   https://doi.org/10.3847/1538-4357/ac77f9  

   Medina, Amber A. ; Winters, Jennifer G. ; Irwin, Jonathan M. ; Charbonneau, David ( August 2022 , The Astrophysical Journal)

   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_⊙).

    

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3. The Factory and the Beehive. IV. A Comprehensive Study of the Rotation X-Ray Activity Relation in Praesepe and the Hyades

   https://doi.org/10.3847/1538-4357/ac6517  

   Núñez, Alejandro ; Agüeros, Marcel A. ; Covey, Kevin R. ; Douglas, Stephanie T. ; Drake, Jeremy J. ; Rampalli, Rayna ; Bowsher, Emily C. ; Cargile, Phillip A. ; Kraus, Adam L. ; Law, Nicholas M. ( May 2022 , The Astrophysical Journal)

   X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses ≲1M_⊙. We present an updated study of the rotation–X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods—an increase of 6× relative to what was previously available. We find that at ≈700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosityL_X/L_bolin unsaturated single members, suggesting a power-law index between −3.2 and −3.9. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars’ atmospheres, thereby informing the development of robust age-rotation-activity relations.

    

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4. Using Stellar Rotation to Identify Tidally Stripped Members of the Praesepe Open Cluster

   https://doi.org/10.3847/2515-5172/ac7793  

   McDivitt, Jessica ; Douglas, Stephanie T. ; Curtis, Jason Lee ; Popinchalk, Mark ; Núñez, Alejandro ( June 2022 , Research Notes of the AAS)

   As an open cluster orbits the Milky Way, gravitational fields distort it, stripping stars from the core and forming tidal tails. Recent work has identified tidal tails of the Praesepe cluster; we explore rotation periods as a way to confirm these candidate members. In open clusters, the rotation period distribution evolves over time due to magnetic braking. Since tidally stripped stars originally formed within the cluster, they should follow the same period distribution as in the cluster core. We analyze 96 candidate members observed by NASA’s Transiting Exoplanet Survey Satellite mission. We measure reliable rotation periods for 32 stars, while 64 light curves are noise-dominated. The 32 newly identified rotators are consistent with the period distribution in the core, and with past membership in Praesepe. We therefore suggest that for nearby open clusters, stellar rotation offers a quick and inexpensive method for confirming past members dispersed into tidal tails.

    

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5. Another Superdense Sub-Neptune in K2-182 b and Refined Mass Measurements for K2-199 b and c*

   https://doi.org/10.3847/1538-3881/ac2830  

   Akana Murphy, Joseph M. ; Kosiarek, Molly R. ; Batalha, Natalie M. ; Gonzales, Erica J. ; Isaacson, Howard ; Petigura, Erik A ; Weiss, Lauren M. ; Grunblatt, Samuel K. ; Ciardi, David R. ; Fulton, Benjamin ; et al ( December 2021 , The Astronomical Journal)

   Abstract We combine multiple campaigns of K2 photometry with precision radial velocity measurements from Keck-HIRES to measure the masses of three sub-Neptune-sized planets. We confirm the planetary nature of the massive sub-Neptune K2-182 b ( P b = 4.7 days, R b = 2.69 R ⊕ ) and derive refined parameters for K2-199 b and c ( P b = 3.2 days, R b = 1.73 R ⊕ and P c = 7.4 days, R c = 2.85 R ⊕ ). These planets provide valuable data points in the mass–radius plane, especially as TESS continues to reveal an increasingly diverse sample of sub-Neptunes. The moderately bright ( V = 12.0 mag) early K dwarf K2-182 (EPIC 211359660) was observed during K2 campaigns 5 and 18. We find that K2-182 b is potentially one of the densest sub-Neptunes known to date (20 ± 5 M ⊕ and 5.6 ± 1.4 g cm −3 ). The K5V dwarf K2-199 (EPIC 212779596; V = 12.3 mag), observed in K2 campaigns 6 and 17, hosts two recently confirmed planets. We refine the orbital and planetary parameters for K2-199 b and c by modeling both campaigns of K2 photometry and adding 12 Keck-HIRES measurements to the existing radial velocity data set ( N = 33). We find that K2-199 b is likely rocky, at 6.9 ± 1.8 M ⊕ and 7.2 − 2.0 + 2.1 g cm −3 , and that K2-199 c has an intermediate density at 12.4 ± 2.3 M ⊕ and 2.9 − 0.6 + 0.7 g cm −3 . We contextualize these planets on the mass–radius plane, discuss a small but intriguing population of “superdense” sub-Neptunes ( R p < 3 R ⊕ , M p >20 M ⊕ ), and consider our prospects for the planets’ atmospheric characterization. 

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