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Low-mass (<1.2 Msun) 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, LHα/Lbol and LX/Lbol, and to calculate Rossby numbers Ro. 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 Ro–LHα/Lbol plane, unsaturated single stars follow a power law with index β = −5.9 ± 0.8 for Ro > 0.3. In the Ro–LX/Lbol plane, we see evidence for supersaturation for single stars with R < 0.01, following a power law with index βsup = 0.5(+0.2,-0.1) supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical Ro value at which activity saturates is smaller for LX/Lbol than for LHα/Lbol. Finally, we observed an almost 1:1 relation between LHα/Lbol and LX/Lbol, suggesting that both the corona and the chromosphere experience similar magnetic heating. 

Abstract The intermediate period gap, discovered by Kepler, is an observed dearth of stellar rotation periods in the temperature–period diagram at ∼20 days for G dwarfs and up to ∼30 days for early-M dwarfs. However, because Kepler mainly targeted solar-like stars, there is a lack of measured periods for M dwarfs, especially those at the fully convective limit. Therefore it is unclear if the intermediate period gap exists for mid- to late-M dwarfs. Here, we present a period catalog containing 40,553 rotation periods (9535 periods >10 days), measured using the Zwicky Transient Facility (ZTF). To measure these periods, we developed a simple pipeline that improves directly on the ZTF archival light curves and reduces the photometric scatter by 26%, on average. This new catalog spans a range of stellar temperatures that connect samples from Kepler with MEarth, a ground-based time-domain survey of bright M dwarfs, and reveals that the intermediate period gap closes at the theoretically predicted location of the fully convective boundary ( G BP − G RP ∼ 2.45 mag). This result supports the hypothesis that the gap is caused by core–envelope interactions. Using gyro-kinematic ages, we also find a potential rapid spin-down of stars across this period gap. 

Abstract The Tucana–Horologium association (Tuc-Hor) is a 40 Myr old moving group in the southern sky. In this work, we measure the rotation periods of 313 Tuc-Hor objects with TESS light curves derived from TESS full-frame images and membership lists driven by Gaia EDR3 kinematics and known youth indicators. We recover a period for 81.4% of the sample and report 255 rotation periods for Tuc-Hor objects. From these objects we identify 11 candidate binaries based on multiple periodic signals or outlier Gaia DR2 and EDR3 renormalized unit weight error values. We also identify three new complex rotators (rapidly rotating M dwarf objects with intricate light-curve morphology) within our sample. Along with the six previously known complex rotators that belong to Tuc-Hor, we compare their light-curve morphology between TESS Cycle 1 and Cycle 3 and find that they change substantially. Furthermore, we provide context for the entire Tuc-Hor rotation sample by describing the rotation period distributions alongside other youth indicators such as Hα and Li equivalent width, as well as near-ultraviolet and X-ray flux. We find that measuring rotation periods with TESS is a fast and effective means to confirm members in young moving groups. 

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. 

Abstract We use three campaigns of K2 observations to complete the census of rotation in low-mass members of the benchmark, ≈670 Myr old open cluster Praesepe. We measure new rotation periods (P_rot) for 220 ≲1.3 M_⊙ Praesepe members and recovery periods for 97% (793/812) of the stars with aP_rot in the literature. Of the 19 stars for which we do not recover a P_rot, 17 were not observed by K2. As K2’s three Praesepe campaigns took place over the course of 3 yr, we test the stability of our measured P_rot for stars observed in more than one campaign. We measure P_rot consistent to within 10% for >95% of the 331 likely single stars with ≥2 high-quality observations; the median difference in P_rot is 0.3%, with a standard deviation of 2%. Nearly all of the exceptions are stars with discrepant P_rot measurements in Campaign 18, K2’s last, which was significantly shorter than the earlier two (≈50 days rather than ≈75 days). This suggests that, despite the evident morphological evolution we observe in the light curves of 38% of the stars, P_rot measurements for low-mass stars in Praesepe are stable on timescales of several years. A P_rot can therefore be taken to be representative even if measured only once.