{
  "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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.18644",
  "author_total": 4,
  "authors": [
    {
      "name": "A. Núñez"
    },
    {
      "name": "M. A. Agüeros"
    },
    {
      "name": "J. L. Curtis"
    },
    {
      "name": "C. A. Ospinal Achicanoy"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/0jkg6az572fpj.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.18644",
  "fulltext_url": null,
  "id": "0jkg6az572fpj",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.18644",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ],
    [
      "openalex",
      "3103|Stellar, planetary, and galactic studies"
    ]
  ],
  "openalex": "W7221810203",
  "pdf_url": "https://arxiv.org/pdf/2610.18644",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-13",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/0jkg6az572fpj.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:29:11Z",
      "native_id": "2610.18644",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.18644"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    }
  ],
  "title": "Anchoring the Chromospheric Activity$-$Rotation Relation: A Study of the Pleiades and Group X",
  "title_text": "Anchoring the Chromospheric Activity−Rotation Relation: A Study of the Pleiades and Group X",
  "type": "preprint",
  "updated_at": "2026-10-30T12:29:11Z",
  "url": "https://research.draconitic.com/paper/0jkg6az572fpj",
  "venue": "arXiv",
  "versions": []
}