Chinese astronomers from the National Astronomical Observatories of Chinese Academy of Sciences (NAOC) have used spectroscopic data from LAMOST and P200/DBSP and multi-band photometric data from Catalina and Zwicky Transient Facility to discover a binary system containing an extremely low-mass (ELM) white dwarf (WD) progenitor and a compact invisible companion. The system challenges the current formation theory of ELM WDs since the visible star, a pre-ELM WD, has a mass of about 0.09 solar masses, which is lower than the theoretical limit of its species.
Most stars in the universe will end their lives as white dwarfs, with carbon-oxygen white dwarfs (CO WD) being the most common. When the mass of a star is greater than 1.4 solar masses, the white dwarf will continue to collapse into a neutron star. White dwarfs between 0.33 and 0.5 solar masses can have cores dominated by either CO or Helium (He). White dwarfs with even lower masses, known as ELM WDs, are composed of degenerate He.
ELMs are formed within interacting binary systems. The formation of such single low mass He WDs via the single star evolution channel requires a correspondingly very low initial mass and an extremely long evolution time.
The binary system's invisible compact component is more likely to be a WD, but a neutron star cannot be ruled out. The ELM binary systems with compact companions may be continuous gravitational wave sources, making them among the most revealing objects in the gravitational wave detection project.
According to Dr. Yuan Hailong, lead author of the study, the pre-ELM-WD appears to be a typical F-type dwarf star that orbits an invisible companion every 5.27 hours. He believes it have just finished its mass-transfer phase and is slowly making its way to the white dwarf cooling path.
Despite its constant luminosity, the energy of the pre-ELM-WD is provided by a small hydrogen shell that burns outside the degenerate He core. However, its dynamical mass is a mere 0.09 solar masses, which is below the theoretical lower limit, making it a unique discovery.
The system's mass was estimated using a combination of multi-band time-domain photometric and spectroscopic data and Gaia parallax. Even after accounting for all possible errors, the estimated mass is still notably low. The team tested several theoretical models, but none could accurately explain the results. This discovery raises questions about the current ELM formation mechanism that remains unanswered.
The discovery is an important milestone for the LAMOST compact object search project, proving its ability to study ELMs. With more time-domain plates expected to come in during LAMOST's second regular five-year survey, more interesting compact binary systems are expected to be discovered.