Chinese astronomers from the National Astronomical Observatories of Chinese Academy of Sciences (NAOC) discovered a binary system with an extremely low-mass (ELM) white dwarf (WD) progenitor and a compact invisible companion using spectroscopic data from LAMOST and P200/DBSP and multi-band photometric data from Catalina and Zwicky Transient Facility.

The observable star, a pre-ELM WD, has a mass of 0.09 solar masses, lower below the theoretical limit of its species, which may contradict the ELM WD creation theory.

Astronomical Journal published the study.

Most stars become white dwarfs after their nuclear fuel runs out. Carbon and oxygen make up most white dwarfs (CO WD). 0.5-1.4 solar masses.

When the mass exceeds 1.4 solar masses, the core electron degeneracy pressure cannot resist gravity and the white dwarf collapses into a neutron star. White dwarfs with cores of CO or He weigh 0.33–0.5 solar masses. ELM WDs are degenerate He.

These ELM WDs cannot form via the single star evolution channel because they require a progenitor star with a very low initial mass and an extremely long evolution time, even longer than the age of our universe. Thus, binary systems create ELMs.

In particular, a stable Roche Lobe overflow channel loses most of the mass of ELMs below 0.18 solar masses.

Mass transfer must start at the right time to generate an ELM. The donor will evolve into a low-mass main sequence star like the Cataclysmic Variable secondary if mass transfer begins too early.

However, if it starts late, the donor’s core will be large enough to evolve by helium flash. Thus, in a constrained mass transfer mechanism, the ELM-WD has a theoretically lower mass limit of 0.14-0.16 solar masses.

The unusual pre-ELM-WD resembles a regular F-type dwarf star orbiting an unseen component every 5.27 hours. It may have finished mass transfer and is going slowly toward white dwarf cooling. A little burning hydrogen shell outside the degenerate He core provides its continuous luminosity.

ELM binary systems with compact partners may be continuous gravitational wave generators.

“However, its dynamical mass is only about 0.09 solar masses, below the lower limit of theoretical predictions, which is indeed puzzling,” stated Dr. Yuan Hailong, first author of the study.

Multi-band time-domain photometric and spectroscopic data and Gaia parallax estimate system mass. The predicted mass is still low after error budgets. The scientists tried numerous theoretical models, but none suited the results. This discovery questions the established ELM generation method.

The unseen compact component is likely a WD with a mass of ~1.0M, however a neutron star cannot be ruled out.

The discovery demonstrates LAMOST can examine ELMs. During LAMOST’s second regular five-year survey, more time-domain plates are likely to reveal interesting compact binary structures.

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