Ton S 415: a close binary containing an intermediate helium subdwarf discovered with SALT and TESS

Edward Snowdon*, C. Simon Jeffery, Saskia Schlagenhauf, Matti Dorsch, Itumaleng M. Monageng

*Corresponding author for this work

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Hot subdwarfs are early-type low-mass stars lying between the main sequence and the white dwarf sequence. The majority have helium-burning cores and helium-poor surfaces. A minority have helium-rich surfaces. A few, including Ton S 415, have surfaces with between 10 per cent and 90 per cent helium (by number). Their properties are diverse and their origins mostly unknown. Ton S 415 was identified as a suspected binary in a Southern African Large Telescope (SALT) survey of hot subdwarfs from its large radial velocity. Using SALT spectroscopy and Transiting Exoplanet Survey Satellite light curves, we confirm that Ton S 415 is a close binary with an orbital period and velocity semi-amplitude. Analysis of the SALT spectrum and broad-band spectral energy distribution shows the visible star to have an effective temperature T_eff = 43300 +/- 1000 K, surface gravity log g/cms^(-2) = 5.89 +/- 0.10⁠, surface helium-to-hydrogen ratio log y = −0.62 +/- 0.10, radius R = 0.1074 +/- 0.0025 R_sun, and mass M = 0.33 +/- 0.09 M_sun. By modelling the light curve, we estimate the binary mass ratio to be q = 0.7 ± 0.3 and infer an unseen white dwarf companion with a mass of M = 0.47 +/- 0.24 M_sun. The results are consistent with a post-common-envelope binary that lost its hydrogen envelope before reaching the peak of the red giant branch, becoming a hot subdwarf with a non-canonical mass. We predict that the system will evolve into a double white dwarf binary before eventually either forming a stably accreting AM CVn system, or merging to form an R CrB star.
Original languageEnglish
Pages (from-to)183-189
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date28 Jul 2023
Publication statusPublished - Oct 2023


  • stars: binaries: close
  • stars: chemically-peculiar
  • stars: horizontal branch
  • stars: subdwarfs


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