J0526+5934: a peculiar ultra-short period double white dwarf
arxiv(2024)
摘要
Ultra-short period compact binaries are important sources of gravitational
waves, which include e.g. the progenitors of type Ia supernovae or the
progenitors of merger episodes that may lead to massive and magnetic single
white dwarfs. J0526+5934 is an unresolved compact binary star with an orbital
period of 20.5 minutes that belongs to this category. The visible component of
J0526+5934 has been recently claimed to be a hot sub-dwarf star with a CO white
dwarf companion. Our aim is to provide strong observational plus theoretical
evidence that the primary star is rather an extremely-low mass white dwarf,
although the hot subdwarf nature cannot be completely ruled out. We analyse
optical spectra together with time-series photometry of the visible component
of J0526+5934 to constrain its orbital and stellar parameters. We also employ
evolutionary sequences for low-mass white dwarfs to derive independent values
of the primary mass. From the analysis of our observational data, we find a
stellar mass for the primary star in J0526+5934 of 0.26+-0.05 Msun, which
perfectly matches the 0.237+-0.035 Msun independent measurement we derived from
the theoretical evolutionary models. This value is considerably lower than the
theoretically expected and generally observed mass range of hot subdwarf stars,
but falls well within the mass limit values of extremely low-mass white dwarfs.
We conclude J0526+5934 is the fifth ultra-short period detached double white
dwarf currently known.
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