Discovery of three magnetic helium-rich hot subdwarfs with SALT

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
² Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK
³ School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
⁴ Institute of Astronomy, The Observatories, Madingley Road, Cambridge CB3 OHA, UK
⁵ South African Astronomical Observatory, PO Box 9, Observatory Rd., Observatory 7935, Cape Town, South Africa
⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁷ Australian Astronomical Optics, Faculty of Science and Engineering, Macquarie University, North Ryde NSW 2113, Australia
⁸ Department of Physics, University of Nebraska at Omaha, 6001 Dodge St, Omaha, NE 68182-0266, USA

^★ Corresponding author; dorsch@uni-potsdam.de

Received: 28 June 2024
Accepted: 30 September 2024

Abstract

Magnetic fields with strengths ranging from 300 to 500 kG have recently been discovered in a group of four extremely similar helium-rich hot subdwarf (He-sdO) stars. In addition to their strong magnetic fields, these He-sdO stars are characterised by common atmospheric parameters, clustering around T_eff = 46 500 K, a log ɡ/cm s⁻¹ close to 6, and intermediate helium abundances. Here we present the discovery of three additional magnetic hot subdwarfs, J123359.44–674929.11, J125611.42-575333.45, and J144405.79–674400.93. These stars are again almost identical in terms of atmospheric parameters, but, at B ≈ 200 kG, their magnetic fields are somewhat weaker than those previously known. The close similarity of all known He-sdOs implies a finely tuned formation channel. We propose the merging of a He white dwarf with a H+He white dwarf. A differential rotation at the merger interface may initiate a toroidal magnetic field that evolves via a magnetic dynamo to produce a poloidal field. This field is either directly visible at the surface or might diffuse towards the surface if initially buried. We further discuss a broad absorption line centred at about 4630 Å that is common to all magnetic He-sdOs. This feature may not be related to the magnetic field but instead to the intermediate helium abundances in these He-sdO stars, allowing the strong He II 4686 Å line to be perturbed by collisions with hydrogen atoms.