Anchoring the Chromospheric Activity−Rotation Relation: A Study of the Pleiades and Group X
Abstract
We present a study of the rotation and the chromospheric Hα activity of low-mass stars in the ≈120 Myr-old Pleiades and the ≈400 Myr-old Group X open clusters. We combine published and new rotation periods (Prot) from TESS and ZTF, and use LAMOST, SDSS, and MDM spectra to measure Hα equivalent widths (EWs). While overall the Group X distributions of Prot 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 Prot 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−LHα/Lbol 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 LHα/Lbol 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 ≈120 Myr, M2 and M3 dwarfs become unsaturated between ≈400 and ≈700 Myr, and later M dwarfs remain largely saturated through field ages even as their saturated LHα/Lbol levels decline, indicating that chromospheric evolution is not determined by rotation period alone.
Related
- TESS flares and chromospheric variability in the Pleiades, the Hyades, and Praesepe
- TESS flares and chromospheric variability in the Pleiades, the Hyades, and Praesepe
- Cross-layer coupling in active-region upflows: Statistical connections between photospheric magnetic field, chromospheric activity, and coronal blueshifts
- Relation of rotation and pulsation properties in γ Doradus stars
- Relation of Rotation and Pulsation Properties in γ Doradus Stars
BibTeX
@misc{nunez2026anchoring,
title = {{Anchoring the Chromospheric Activity$-$Rotation Relation: A Study of the Pleiades and Group X}},
author = {A. Núñez and M. A. Agüeros and J. L. Curtis and C. A. Ospinal Achicanoy},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18644},
eprint = {2610.18644},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18644},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Anchoring the Chromospheric Activity−Rotation Relation: A Study of the Pleiades and Group X
AU - A. Núñez
AU - M. A. Agüeros
AU - J. L. Curtis
AU - C. A. Ospinal Achicanoy
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18644
UR - https://arxiv.org/abs/2610.18644
AB - We present a study of the rotation and the chromospheric Hα activity of low-mass stars in the ≈120 Myr-old Pleiades and the ≈400 Myr-old Group X open clusters. We combine published and new rotation periods (P_rot) from TESS and ZTF, and use LAMOST, SDSS, and MDM spectra to measure Hα equivalent widths (EWs). While overall the Group X distributions of P_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_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_Hα/L_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_Hα/L_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 ≈120 Myr, M2 and M3 dwarfs become unsaturated between ≈400 and ≈700 Myr, and later M dwarfs remain largely saturated through field ages even as their saturated L_Hα/L_bol levels decline, indicating that chromospheric evolution is not determined by rotation period alone.
ER -