J0526+5934: A peculiar ultra-short-period double white dwarf

¹ Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
e-mail: alberto.rebassa@upc.edu
² Institute for Space Studies of Catalonia (IEEC), c/Esteve Terradas,1, Edifici RDIT, Despatx 212, Campus del Baix Llobregat UPC – Parc Mediterrani de la Tecnologia, 08860 Castelldefels, Spain
³ Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH UK
⁴ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
⁵ Instituto de Astrofísica La Plata, UNLP-CONICET, Paseo del Bosque s/n, 1900 La Plata, Argentina
⁶ Department of Physics, University of Warwick, Coventry, CV4 7AL UK
⁷ National Astronomical Research Institute of Thailand, 260 Moo 4, T. Donkaew, A. Maerim, Chiangmai, 50180 Thailand

Received: 6 February 2024
Accepted: 8 April 2024

Abstract

Context. Ultra-short-period compact binaries are important sources of gravitational waves. The class of short-period compact binaries includes, for example, the progenitors of type Ia supernovae and the progenitors of merger episodes that may lead to massive and magnetic single white dwarfs. J0526+5934 is one such example: it is an unresolved compact binary star with an orbital period of 20.5 min.

Aims. The visible component of J0526+5934 was recently claimed to be a hot sub-dwarf star with a CO white dwarf companion. Our aim is to provide strong observational and theoretical evidence that the primary star is instead an extremely low-mass white dwarf, although the hot sub-dwarf nature cannot be completely ruled out.

Methods. We analysed optical spectra together with time-series photometry of the visible component of J0526+5934 to constrain its orbital and stellar parameters. We also employed evolutionary sequences for low-mass white dwarfs to derive independent values of the primary mass.

Results. From the analysis of our observational data, we find a stellar mass for the primary star in J0526+5934 of 0.26 ± 0.05 M_⊙, which perfectly matches the 0.237 ± 0.035 M_⊙ independent measurement we derive from the theoretical evolutionary models. This value is considerably lower than the theoretically expected and generally observed mass range for hot sub-dwarf stars, but falls well within the mass limit values of extremely low-mass white dwarfs.

Conclusions. We conclude J0526+5934 is the sixth ultra-short-period detached double white dwarf currently known.