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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)

   Abstract 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. The Factory and the Beehive. V. Chromospheric and Coronal Activity and Its Dependence on Rotation in Praesepe and the Hyades

   https://doi.org/10.3847/1538-4357/ad117e  

   Núñez, Alejandro; Agüeros, Marcel A; Curtis, Jason L; Covey, Kevin R; Douglas, Stephanie T; Chu, Sabine R; DeLaurentiis, Stanislav; Wang, Minzhi Luna; Drake, Jeremy J (February 2024, The Astrophysical Journal)

   Abstract Low-mass (≲1.2M_⊙) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional Hα and X-ray luminosities, L_Hα/L_bolandL_X/L_bol, and to calculate Rossby numbers R_o. We found that at ≈700 Myr almost all M dwarfs exhibit Hα emission, with binaries having the same overall color–Hα equivalent width distribution as single stars. In the R_o–L_Hα/L_bol plane, unsaturated single stars follow a power law with index β = −5.9 ± 0.8 for R_o > 0.3. In the R_o–L_X/L_bol plane, we see evidence for supersaturation for single stars with R_o ≲ 0.01, following a power law with index ${\beta }_{\sup }={0.5}_{-0.1}^{+0.2}$, supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical R_o value at which activity saturates is smaller for L_X/L_bol than for L_Hα/L_bol. Finally, we observed an almost 1:1 relation between L_Hα/L_bol and L_X/L_bol, suggesting that both the corona and the chromosphere experience similar magnetic heating. 

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3. The CARMENES search for exoplanets around M dwarfs: Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star

   https://doi.org/10.1051/0004-6361/202245322  

   Kossakowski, D.; Kürster, M.; Trifonov, T.; Henning, Th.; Kemmer, J.; Caballero, J. A.; Burn, R.; Sabotta, S.; Crouse, J. S.; Fauchez, T. J.; et al (February 2023, Astronomy & Astrophysics)

   We present the discovery of an Earth-mass planet (M_bsini= 1.26 ± 0.21M_⊕) on a 15.6 d orbit of a relatively nearby (d ~9.6 pc) and low-mass (0.167 ± 0.011M_⊙) M5.0 V star, Wolf 1069. Sitting at a separation of 0.0672 ± 0.0014 au away from the host star puts Wolf 1069 b in the habitable zone (HZ), receiving an incident flux ofS= 0.652 ± 0.029S_⊕. The planetary signal was detected using telluric-corrected radial-velocity (RV) data from the CARMENES spectrograph, amounting to a total of 262 spectroscopic observations covering almost four years. There are additional long-period signals in the RVs, one of which we attribute to the stellar rotation period. This is possible thanks to our photometric analysis including new, well-sampled monitoring campaigns undergone with the OSN and TJO facilities that supplement archival photometry (i.e., from MEarth and SuperWASP), and this yielded an updated rotational period range ofP_rot= 150–170 d, with a likely value at 169.3_−3.6^+3.7. The stellar activity indicators provided by the CARMENES spectra likewise demonstrate evidence for the slow rotation period, though not as accurately due to possible factors such as signal aliasing or spot evolution. Our detectability limits indicate that additional planets more massive than one Earth mass with orbital periods of less than 10 days can be ruled out, suggesting that perhaps Wolf 1069 b had a violent formation history. This planet is also the sixth closest Earth-mass planet situated in the conservative HZ, after Proxima Centauri b, GJ 1061 d, Teegarden’s Star c, and GJ 1002 b and c. Despite not transiting, Wolf 1069 b is nonetheless a very promising target for future three-dimensional climate models to investigate various habitability cases as well as for sub-m s⁻¹RV campaigns to search for potential inner sub-Earth-mass planets in order to test planet formation theories. 

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4. 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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5. A 4 Gyr M-dwarf Gyrochrone from CFHT/MegaPrime Monitoring of the Open Cluster M67

   https://doi.org/10.3847/1538-4357/ac90be  

   Dungee, Ryan; van Saders, Jennifer; Gaidos, Eric; Chun, Mark; García, Rafael A.; Magnier, Eugene A.; Mathur, Savita; Santos, Ângela R. (October 2022, The Astrophysical Journal)

   Abstract We present stellar rotation periods for late K- and early M-dwarf members of the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and Pan-STARRS (PS1) photometry for binary identification, we build this set of rotation periods from a campaign of monitoring M67 with the Canada–France–Hawaii Telescope’s MegaPrime wide-field imager. We identify 1807 members of M67, of which 294 are candidate single members with significant rotation period detections. Moreover, we fit a polynomial to the period versus color-derived effective temperature sequence observed in our data. We find that the rotation of very cool dwarfs can be explained by simple solid-body spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the predictions of gyrochronological models and find that the best match is Skumanich-like spin-down,P_rot∝t^0.62, applied to the sequence of Ruprecht 147. This suggests that, for spectral types K7–M0 with near-solar metallicity, once a star resumes spinning down, a simple Skumanich-like relation is sufficient to describe their rotation evolution, at least through the age of M67. Additionally, for stars in the range M1–M3, our data show that spin-down must have resumed prior to the age of M67, in conflict with the predictions of the latest spin-down models. 

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