Stellar activity with LAMOST – II. Chromospheric activity in open clusters

Abstract

We use the LAMOST spectra of member stars in Pleiades, M34, Praesepe, and Hyades to study how chromospheric activity vary as a function of mass and rotation at different age. We measured excess equivalent widths of Hα, Hβ , and Ca ii K based on estimated chromospheric contributions from old and inactive field dwarfs, and excess luminosities are obtained by normalizing bolometric luminosity, for more than 700 late-type stars in these open clusters. Results indicate two activity sequences in cool spot coverage and Hα excess emission among GK dwarfs in Pleiades and M dwarfs in Praesepe and Hyades, paralleling with well known rotation sequences. A weak dependence of chromospheric emission on rotation exists among ultra fast rotators in saturated regime with Rossby number Ro 0.1. In the unsaturated regime, chromospheric and coronal emission show similar dependence on Ro, but with a shift toward larger Ro, indicating chromospheric emission gets easily saturated than coronal emission, and/or convective turnover time-scales based on X-ray data do not work well with chromospheric emission. More interestingly, our analysis show fully convective slow rotators obey the rotation-chromospheric activity relation similar to hotter stars, confirming the previous finding. We found correlations among Hα, Hβ , and Ca ii K emissions, in which Hα losses are more important than Ca ii K for cooler and more active stars. In addition, a weak correlation is seen between chromospheric emission and photospheric activity that shows dependency on stellar spectral type and activity level, which provides some clues on how spot configuration vary as a function of mass and activity level.

Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

“…This Hα emission is relatively weak for an object with an M3.5 spectral type (Newton et al 2017;Jeffers et al 2018;Kiman et al 2021). This indicates that Ross 19B has an age greater than ≈600-800 Myr, as all mid-M type members of the Hyades and Praesepe clusters show strong Hα emission (Douglas et al 2014;Fang et al 2018). We also note here that there is no notable lithium absorption at λ6707 Å in the spectrum of Ross 19A.…”

Rapid contraction of giant planets orbiting the 20-million-year-old star V1298 Tau

“…V1298 Tau exhibits an X-ray emission of 4.58 +1.71 −1.44 × 10 30 erg s −1 (ref. 69 ), compatible with an activity stronger than that of the stars in the Pleiades 70,71 , and has an ultraviolet excess (8.15 ± 0.05 magnitude difference between the magnitude in the near-ultraviolet and the magnitude in the J band) characteristic of stars younger than ~100 Myr (ref. 72 ).…”

TOI-811b and TOI-852b: New Transiting Brown Dwarfs with Similar Masses and Very Different Radii and Ages from the TESS Mission

“…We assume that each line has equal width and the two Li doublet lines have equal heights. We compare the Li EW to empirical relationships of EW and T eff of stars in clusters and associations (e.g., Somers & Pinsonneault 2015;Fang et al 2018) in Figure 10, and from this we find that TOI-811 has an age not inconsistent with the stars in Pleiades (125 Myr) or IC 2602 and IC 2391 (30-50 Myr). Though we do not explicitly use the Li λ6708 EW measurement to constrain the age of TOI-811 in our analysis, we may qualitatively use it to confirm that TOI-811 is likely younger than 200 Myr.…”