Paper

{
  "raw_xml": "<entry>\n    <id>http://arxiv.org/abs/2603.23756v1</id>\n    <title>Thermally inflated accretors in post-mass transfer binaries: Abell 35 and its class revisited</title>\n    <updated>2026-03-24T22:28:44Z</updated>\n    <link href='https://arxiv.org/abs/2603.23756v1' rel='alternate' type='text/html'/>\n    <link href='https://arxiv.org/pdf/2603.23756v1' rel='related' title='pdf' type='application/pdf'/>\n    <summary>A small but growing class of binaries containing hot ($T_{\\rm eff}\\sim10^5\\rm~K$) white dwarfs (WDs) and rapidly rotating, apparently subgiant companions -- including the prototype, Abell 35 -- show companions that are too large and luminous to be ordinary main-sequence stars yet too numerous to be explained as finely tuned near-twin binaries. We argue that these stars are instead main-sequence accretors temporarily inflated out of thermal equilibrium by recent mass transfer. For the subgiant of Abell 35, a new Gaia DR3 astrometric orbit ($P_{\\rm orb} = 790$ d) combined with updated photometric and spectroscopic constraints yield $T_{\\rm eff} \\approx 4900~\\rm K$, $R \\approx 3~R_{\\odot}$, near-solar metallicity, and rapid rotation aligned with the orbit ($v_{\\rm rot} \\approx 195~\\rm km~s^{-1}$), indicating substantial recent accretion and spin-up. Dynamical mass limits disfavor a coeval twin-binary origin, supporting the inflated-accretor interpretation. We test this scenario using self-consistent MESA binary evolution calculations with a new accretion prescription in which accreted material retains a fraction of its infall energy. The accretor expands to giant-like radii when $\\dot{M}$ is high yet remains within its Roche lobe, allowing stable mass transfer even for mass ratios traditionally considered unstable. After mass transfer ceases, the star contracts on Myr timescales through a bloated, rapidly rotating phase whose temperatures, radii, and spins match those observed in Abell 35-type systems. This framework explains the population without fine tuning and unifies Abell 35-type binaries with post-AGB binaries, blue lurkers, and wide WD$+$main-sequence systems as successive stages of the same post-mass-transfer evolutionary pathway.</summary>\n    <category scheme='http://arxiv.org/schemas/atom' term='astro-ph.SR'/>\n    <category scheme='http://arxiv.org/schemas/atom' term='astro-ph.GA'/>\n    <published>2026-03-24T22:28:44Z</published>\n    <arxiv:comment>20 pages (including appendix), 2 tables, 16 figures. Submitted to PASP. Comments are welcome! All codes and MESA inlists can be found in https://github.com/Soumin1908/InflatedStars_A35_MesaModels.git</arxiv:comment>\n    <arxiv:primary_category term='astro-ph.SR'/>\n    <author>\n      <name>Soumyadeep Bhattacharjee</name>\n    </author>\n    <author>\n      <name>Kareem El-Badry</name>\n    </author>\n    <author>\n      <name>Jim Fuller</name>\n    </author>\n    <author>\n      <name>Cheyanne Shariat</name>\n    </author>\n    <author>\n      <name>Natsuko Yamaguchi</name>\n    </author>\n  </entry>"
}