{
  "abstract": "The scarcity of optically visible O stars near the theoretical zero-age main sequence (ZAMS) has puzzled astronomers for over four decades. We argue that it reflects prolonged accretion. If massive stars assemble over Myr timescales, as the inertial-inflow model predicts, core-hydrogen burning begins before accretion ends, and the stars become optically visible at the end of accretion on a birthline displaced from the standard ZAMS. We test this interpretation by comparing the hot-side envelope of Galactic O stars in the Kiel diagram with accreting stellar-evolution models, adopting a growth law $t_{\\mathrm{form}}=τ_0(m_{\\mathrm{f}}/60 M_\\odot)^α$ with formation time $t_{\\mathrm{form}}$ and final mass $m_{\\mathrm{f}}$. The resulting birthlines reproduce both the displacement and the high-mass turnover of the observed envelope, with best-fitting parameters $(α,τ_0)=(0.52,4.3\\,\\mathrm{Myr})$, consistent with independent constraints from the luminosity functions of compact H II regions and OB stars. The two constraints have orthogonal degeneracy directions, so combining them narrows the allowed parameter space, yielding projected 90% confidence intervals $0.43<α<0.60$ and $3.9<τ_0/\\mathrm{Myr}<5.0$. The inferred growth law agrees with cloud-scale simulations and the inertial-inflow model, and implies that mass assembly and main-sequence evolution overlap substantially in massive stars, with broader implications for stellar evolution and feedback.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.31482",
  "author_total": 4,
  "authors": [
    {
      "name": "Paolo Padoan"
    },
    {
      "name": "Samuel Shipkoff"
    },
    {
      "name": "Mark Gieles"
    },
    {
      "name": "Fabrice Martins"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/a2hz1m7hnd5yp.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.31482",
  "fulltext_url": null,
  "id": "a2hz1m7hnd5yp",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.31482",
  "language": "en",
  "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"
    ],
    [
      "hal",
      "0.phys"
    ],
    [
      "hal",
      "1.phys.astr"
    ],
    [
      "openalex",
      "3103|Astrophysics and Star Formation Studies"
    ]
  ],
  "openalex": "W7223353838",
  "pdf_url": "https://arxiv.org/pdf/2610.31482",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-22",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/a2hz1m7hnd5yp.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:34:34Z",
      "native_id": "2610.31482",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.31482"
    },
    {
      "first_seen": "2026-11-04T12:20:53Z",
      "native_id": "hal-05851340",
      "source": "hal",
      "url": "https://hal.science/hal-05851340v1"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    },
    {
      "field": "physics",
      "origin": "hal:0.phys",
      "subfield": "general"
    }
  ],
  "title": "Slow Massive-Star Formation and the O-Star ZAMS Deficit",
  "title_text": "Slow Massive-Star Formation and the O-Star ZAMS Deficit",
  "type": "preprint",
  "updated_at": "2026-11-06T12:20:44Z",
  "url": "https://research.draconitic.com/paper/a2hz1m7hnd5yp",
  "venue": "arXiv",
  "versions": []
}