{
  "id": "2201.05614",
  "version": "v2",
  "published": "2022-01-14T19:00:00.000Z",
  "updated": "2022-08-24T11:58:54.000Z",
  "title": "Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer",
  "authors": [
    "Kareem El-Badry",
    "Charlie Conroy",
    "Eliot Quataert",
    "Hans-Walter Rix",
    "Jonathan Labadie-Bartz",
    "Tharindu Jayasinghe",
    "Todd Thompson",
    "Phillip Cargile",
    "Keivan G. Stassun",
    "Ilya Ilyin"
  ],
  "comment": "25 pages, 19 figures, accepted to MNRAS",
  "categories": [
    "astro-ph.SR"
  ],
  "abstract": "Motivated by recent suggestions that many Be stars form through binary mass transfer, we searched the APOGEE survey for Be stars with bloated, stripped companions. From a well-defined parent sample of 297 Be stars, we identified one mass-transfer binary, HD 15124. The object consists of a main-sequence Be star ($M_{\\rm Be}=5.3\\pm 0.6 \\,M_{\\odot}$) with a low-mass ($M_{\\rm donor}=0.92\\pm 0.22\\,M_{\\odot}$), subgiant companion on a 5.47-day orbit. The emission lines originate in an accretion disk caused by ongoing mass transfer, not from a decretion disk as in classical Be stars. Both stars have surface abundances bearing imprint of CNO processing in the donor's core: the surface helium fraction is $Y_{\\rm He}\\approx 0.6$, and the nitrogen-to-carbon ratio is 1000 times the solar value. The system's properties are well-matched by binary evolution models in which mass transfer begins while a $3-5\\,M_{\\odot}$ donor leaves the main sequence, with the secondary becoming the Be star. These models predict that the system will soon become a detached Be + stripped star binary like HR 6819 and LB-1, with the stripped donor eventually contracting to become a core helium-burning sdOB star. Discovery of one object in this short-lived ($\\sim$1 Myr) evolutionary phase implies the existence of many more that have already passed through it and are now Be + sdOB binaries. We infer that $(28_{-16}^{+27})\\,\\%$ of Be stars have stripped companions, most of which are faint. Together with the dearth of main-sequence companions to Be stars and recent discovery of numerous Be + sdOB binaries in the UV, our results imply that binarity plays an important role in the formation of Be stars.",
  "revisions": [
    {
      "version": "v2",
      "updated": "2022-08-24T11:58:54.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "binary mass transfer",
      "stars form",
      "apogee search",
      "sdob binaries",
      "surface abundances bearing imprint"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 25,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}