Looking into the cradle of the grave: J22564–5910, a potential young post-merger hot subdwarf

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
e-mail: joris.vos@uv.cl
² Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranska Lomnica, Slovak Republic
⁵ Lund University, Department of Astronomy and Theoretical Physics, Box 43 221-00 Lund, Sweden
⁶ Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
⁷ Astroserver.org, 8533 Malomsok, Hungary
⁸ Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA
⁹ University of North Carolina at Chapel Hill, Department of Physics and Astronomy, Chapel Hill, NC 27599, USA
¹⁰ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
¹¹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile

Received: 21 January 2021
Accepted: 5 June 2021

Abstract

Context. We present the discovery of J22564–5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and it has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564–5910 being observed very shortly after the merger phase that formed it.

Aims. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features.

Methods. The photometry, spectra, and light curves were all analysed, and their results were combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength was estimated and, by using the SHELLSPEC code, qualitative models of gas configurations were fitted to the observations.

Results. All observations can either be explained by a magnetic field of ∼650 kG, which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disc or torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger.

Conclusions. J22564–5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long sought-after immediate ancestor of strongly magnetic white dwarfs.