Using spectroscopic data from LAMOST and P200, Chinese astronomers from the National Astronomical Observatory of the Chinese Academy of Sciences (NAOC) have discovered a binary system containing an extremely low-mass (ELM) white dwarf (WD) and a compact invisible companion. /DBSP and multi-channel photometric data from Catalina and Zwicky Transient Facility. The visible star’s mass before ELM WD is about 0.09 solar masses; this is below the theoretical limit of its kind and may challenge existing theory of the formation of ELM WD.

Most stars in the universe will end their lives as white dwarfs after their nuclear fuel runs out. Most white dwarfs are carbon-oxygen white dwarfs (CO WD), meaning they are mostly made up of carbon and oxygen. They have a mass of 0.5-1.4 times the mass of the sun.

Once the mass exceeds 1.4 solar masses, the pressure of electron degeneracy in the core cannot withstand gravity and the white dwarf continues to collapse into a neutron star. White dwarfs with masses between 0.33 and 0.5 solar masses can have cores dominated by CO or helium (He). Even lower-mass white dwarfs, known as ELM WDs, are composed of degenerate He.

Since the formation of such single low-mass He WDs requires a progenitor star with a corresponding very low initial mass and an extremely long evolution time, it is believed that these ELM WDs could not have formed through a single-star evolutionary channel. It is longer than the current age of our universe. Therefore, it is generally accepted that ELMs occur within interacting binary systems. Notably, even a lower mass fraction (less than 0.18 solar masses) of the ELM is predicted to lose most of its initial mass via the stable Roche Lobe overflow channel.

To successfully build an ELM, the start of mass transfer must be at the right time. If mass transfer begins too early, the donor will transform into a low-mass main sequence star, similar to a Disaster Variable secondary star. On the other hand, if it starts too late, the donor core will be large enough to evolve to the next stage thanks to the helium explosion. Therefore, there is a theoretical lower limit of ELM-WD mass in such a limited mass transfer process of about 0.14-0.16 solar masses.

“The pre-ELM special WD looks like an ordinary F-type dwarf star orbiting an invisible component every 5.27 hours. It may have just completed the mass transfer phase and may be slowly moving towards the cooling path of the white dwarf. Its constant brightness means that its energy is provided by a small shell of hydrogen that burns outside the degenerate helium core. “However, its dynamic mass is only about 0.09 solar masses, which is below the lower limit of theoretical estimates, which is really surprising.”

The mass of the system is estimated from multi-channel photometric and spectroscopic data in the time domain and Gaia parallax. After accounting for all error budgets, the estimated mass is still significantly low. The team tested several theoretical models, none of which fit the results adequately. This discovery raises questions about the current mechanism of ELM formation that still await an answer.

The invisible compact component may have a mass of ~1.0M☉ and is most likely a WD, but a neutron star cannot be ruled out at this time. ELM binaries with compact companions can be continuous sources of gravitational waves and are among the most revealing objects in a gravitational wave detection project. This discovery is a significant advance for the LAMOST compact object search project, demonstrating LAMOST’s ability to study ELMs. As more time domain plates are obtained during LAMOST’s second regular search every five years, more interesting compact binaries are expected to be discovered.