{
  "abstract": "We measure how young star clusters disperse over time in the two nearest star-forming complexes. Our sample comprises 44 \\texttt{SigMA}-\\textit{Gaia}-defined clusters in Orion and 31 in Sco--Cen, with isochronal ages of 2--25\\,Myr. We extend each cluster's census with \\textit{eROSITA}-selected young stars that share its parallax and motion and lie within the distance they could have traveled ballistically since birth. By 10--30\\,Myr, about one in three former cluster stars lies outside the cluster's cataloged boundary. Across both complexes and the full age range, we find that (1) the half-member radius increases linearly with age, $\\rfifty \\simeq \\sigbirth t$, (2) the velocity dispersion shows no measurable evolution, and (3) all studied clusters are unbound. These results support free expansion. Young clusters in Orion and Sco-Cen emerge unbound from their birth clouds and coast at their birth velocity dispersion without significant relaxation or a characteristic dispersal timescale. We further show that memberships defined by density contrast preferentially omit the fastest stars, making the same clusters appear to expand more slowly ($\\propto t^{0.6}$), cool with age, and approach equilibrium.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.05179",
  "author_total": 3,
  "authors": [
    {
      "name": "João Alves"
    },
    {
      "name": "Stefan Meingast"
    },
    {
      "name": "Alena Rottensteiner"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/cq23e66zh1eyz.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.05179",
  "fulltext_url": null,
  "id": "cq23e66zh1eyz",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.05179",
  "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"
    ],
    [
      "openalex",
      "3103|Astrophysics and Star Formation Studies"
    ]
  ],
  "openalex": "W7211880469",
  "pdf_url": "https://arxiv.org/pdf/2609.05179",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-04",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/cq23e66zh1eyz.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:19:14Z",
      "native_id": "2609.05179",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.05179"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen",
  "title_text": "The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen",
  "type": "preprint",
  "updated_at": "2026-10-30T12:19:14Z",
  "url": "https://research.draconitic.com/paper/cq23e66zh1eyz",
  "venue": "arXiv",
  "versions": []
}