Search for: All records

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 Hαand X-ray luminosities,L_Hα/L_bolandL_X/L_bol, and to calculate Rossby numbersR_o. We found that at ≈700 Myr almost all M dwarfs exhibit Hαemission, with binaries having the same overall color–Hαequivalent width distribution as single stars. In theR_o–L_Hα/L_bolplane, unsaturated single stars follow a power law with indexβ= −5.9 ± 0.8 forR_o> 0.3. In theR_o–L_X/L_bolplane, we see evidence for supersaturation for single stars withR_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 criticalR_ovalue at which activity saturates is smaller forL_X/L_bolthan forL_Hα/L_bol. Finally, we observed an almost 1:1 relation betweenL_Hα/L_bolandL_X/L_bol, suggesting that both the corona and the chromosphere experience similar magnetic heating. 

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 Stellar positions and velocities from Gaia are yielding a new view of open cluster dispersal. Here we present an analysis of a group of stars spanning Cepheus (l= 100°) to Hercules (l= 40°), hereafter the Cep-Her complex. The group includes four Kepler objects of interest: Kepler-1643 b (R_p= 2.32 ± 0.13R_⊕,P= 5.3 days), KOI-7368 b (R_p= 2.22 ± 0.12R_⊕,P= 6.8 days), KOI-7913 Ab (R_p= 2.34 ± 0.18R_⊕,P= 24.2 days), and Kepler-1627 Ab (R_p= 3.85 ± 0.11R_⊕,P= 7.2 days). The latter Neptune-sized planet is in part of the Cep-Her complex called theδLyr cluster. Here we focus on the former three systems, which are in other regions of the association. Based on kinematic evidence from Gaia, stellar rotation periods from TESS, and spectroscopy, these three objects are also ≈40 million years (Myr) old. More specifically, we find that Kepler-1643 is ${46}_{-7}^{+9}$Myr old, based on its membership in a dense subcluster of the complex called RSG-5. KOI-7368 and KOI-7913 are ${36}_{-8}^{+10}$Myr old, and are in a diffuse region that we call CH-2. Based on the transit shapes and high-resolution imaging, all three objects are most likely planets, with false-positive probabilities of 6 × 10⁻⁹, 4 × 10⁻³, and 1 × 10⁻⁴for Kepler-1643, KOI-7368, and KOI-7913, respectively. These planets demonstrate that mini-Neptunes with sizes of ≈2 Earth radii exist at ages of 40 Myr. 

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.