{
  "id": "1911.00014",
  "version": "v1",
  "published": "2019-10-31T18:00:03.000Z",
  "updated": "2019-10-31T18:00:03.000Z",
  "title": "From Hydrogen to Helium: The Spectral Evolution of White Dwarfs as Evidence for Convective Mixing",
  "authors": [
    "Tim Cunningham",
    "Pier-Emmanuel Tremblay",
    "Nicola Pietro Gentile Fusillo",
    "Mark Hollands",
    "Elena Cukanovaite"
  ],
  "comment": "14 pages, 13 figures, submitted to MNRAS",
  "categories": [
    "astro-ph.SR"
  ],
  "abstract": "We present a study of the hypothesis that white dwarfs undergo a spectral change from hydrogen- to helium-dominated atmospheres using a volume-limited photometric sample drawn from the $Gaia$ DR2 catalogue, the Sloan Digital Sky Survey (SDSS) and the $Galaxy~Evolution~Explorer$ (GALEX). We exploit the strength of the Balmer jump in hydrogen-atmosphere DA white dwarfs to separate them from helium-dominated objects in SDSS colour space. Across the effective temperature range from 20000K to 9000K we find that 22% of white dwarfs will undergo a spectral change, with no spectral evolution being ruled out at 5$\\sigma$. The most likely explanation is that the increase in He-rich objects is caused by the convective mixing of DA stars with thin hydrogen layers, in which helium is dredged up from deeper layers by a surface hydrogen convection zone. The rate of change in the fraction of He-rich objects as a function of temperature, coupled with a recent grid of 3D radiation-hydrodynamic simulations of convective DA white dwarfs - which include the full overshoot region - lead to a discussion on the distribution of total hydrogen mass in white dwarfs. We find that 60% of white dwarfs must have a hydrogen mass larger than $M_{\\rm H}/M_{\\rm WD} = 10^{-10}$, another 25% have masses in the range $M_{\\rm H}/M_{\\rm WD} = 10^{-14}-10^{-10}$, and 15% have less hydrogen than $M_{\\rm H}/M_{\\rm WD} = 10^{-14}$. These results have implications for white dwarf asteroseismology, stellar evolution through the asymptotic giant branch (AGB) and accretion of planetesimals onto white dwarfs.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-10-31T18:00:03.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "spectral evolution",
      "convective mixing",
      "he-rich objects",
      "sloan digital sky survey",
      "hydrogen-atmosphere da white dwarfs"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 14,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}