Anchoring the Chromospheric Activity$-$Rotation Relation: A Study of the Pleiades and Group X
Year: 2026
Type: preprint
Abstract: We present a study of the rotation and the chromospheric H$α$ activity of low-mass stars in the $\approx$120 Myr-old Pleiades and the $\approx$400 Myr-old Group X open clusters. We combine published and new rotation periods ($P_\mathrm{rot}$) from TESS and ZTF, and use LAMOST, SDSS, and MDM spectra to measure H$α$ equivalent widths (EWs). While overall the Group X distributions of $P_\mathrm{rot}$ and EWs are as expected, detailed comparisons to the Pleiades and to the 700-Myr-old benchmark Praesepe produce some surprises. For F through mid K stars, the Group X color$-$period sequence lies between those of the Pleiades and of Praesepe, as expected. But the $P_\mathrm{rot}$ for its late K and early M dwarfs and the Praesepe slow-rotator sequence overlap, indicating that the 700 Myr-old stars have not spun down significantly despite being nearly twice as old as their Group X counterparts. In the Rossby number$-L_\mathrm{Hα}/L_\mathrm{bol}$ plane, rapid rotators in Group X are saturated, as is seen in older clusters, whereas the rapid rotators in the Pleiades show a weak but measurable dependence of $L_\mathrm{Hα}/L_\mathrm{bol}$ on rotation. This implies that the chromospheric activity$-$rotation relation attains its mature form between 120 and 400 Myr. Dividing the sample by color reveals that K, M0, and M1 dwarfs are no longer saturated by $\approx$120 Myr, M2 and M3 dwarfs become unsaturated between $\approx$400 and $\approx$700 Myr, and later M dwarfs remain largely saturated through field ages even as their saturated $L_\mathrm{Hα}/L_\mathrm{bol}$ levels decline, indicating that chromospheric evolution is not determined by rotation period alone.
Source: arXiv (Cornell University)
Institutions: Columbia University, Universidad de Antioquia
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Yale University, Carnegie Institution of Washington, Ohio State University, California Institute of Technology, Northwestern University, Harvard University, University of Washington, Johns Hopkins University, Dartmouth College, New Mexico State University, Space Telescope Science Institute, European Space Agency, Flatiron Health, University of Warwick, Trinity College Dublin, École Polytechnique Fédérale de Lausanne, Chinese Academy of Sciences, University of Toronto, Universidad Nacional Autónoma de México, Yunnan University, Nanjing University, Stockholms Universitet, China National Textile and Apparel Council, National Development and Reform Commission, Leibniz-Institut für Astrophysik Potsdam, Max-Planck-Institut für Astronomie, University of Illinois at Urbana-Champaign, University of Colorado Boulder, Smithsonian Astrophysical Observatory, Science Mission Directorate, Deutsches Elektronen-Synchrotron, Leibniz-Gemeinschaft, Institut National de Physique Nucléaire et de Physique des Particules, National Astronomical Observatories, Chinese Academy of Sciences
OpenAlex ID: W7161705017