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

• Context. We present the discovery of J22564-5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in thesystem and it has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence pointstowards 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 TESSlight 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 ourinterpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based onthe triple-peaked H lines, the magnetic field strength was estimated and, by using the shellspec code, qualitative models of gasconfigurations 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 centrifugalmagnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disc or torus. Both scenarios are not mutuallyexclusive 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 thefirst post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. If themagnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-whitedwarf object. Thus, it could represent the long sought-after immediate ancestor of strongly magnetic white dwarfs.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

Stars: binaries

Stars: circumstellar matter

Stars: evolution

Stars: magnetic field

Stars: subdwarfs

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