Recently, a binary system containing an extremely low-mass (ELM) white dwarf (WD) progenitor and a compact invisible companion, was discovered by Chinese astronomers from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), by using spectroscopic data from LAMOST and P200/DBSP, and multi-band photometric data from Catalina and ZTF. The mass of the visible star, a pre-ELM WD, is about 0.09 solar masses, lower than the theoretical limit of their species, and may challenge the current formation theory of the ELM WD.

Figure 1. ELM WD schematic diagram (Image Credit: Caltech / IPAC)

Most stars in the universe will end their life as white dwarfs after their nuclear fuel were burned out. Most of the white dwarfs are Carbon-Oxygen white dwarfs (CO WD), which means they are mostly composed of Carbon and Oxygen. They have masses between 0.5-1.4 solar masses.

When the mass is more massive than 1.4 solar masses, the electron degeneracy pressure at the core cannot resist the gravity and the white dwarf will collapse further into a neutron star. White dwarfs between 0.33 and 0.5 solar masses may have cores dominated by either CO or Helium (He). White Dwarfs with even lower mass, known as extremely low-mass white dwarf (ELM WD), are composed of degenerate Helium.

It is believed that these ELM WDs cannot be formed via single star evolution channel, due to the fact that the formation of such single low mass He WDs requires the progenitor star with correspondingly very low initial mass and an extremely long evolution time, a time even longer than the current age of our universe. Therefore, it is generally accepted that ELMs are formed within interacting binary systems. Especially, the even lower mass part of ELMs (below 0.18 solar masses) are proposed to lost most of their initial masses via stable Roche Lobe overflow channel.

To successfully form an ELM, the onset of the mass transfer should be at the right moment. If the mass transfer starts too early, the donor will evolve to a low-mass main sequences star, similar to the secondary in Cataclysmic Variable. On the other hand, if it starts too late, the core of the donor will be massive enough to evolve into the next stage via helium flash. So there is a theoretical lower mass limit of the ELM-WD in such a restricted mass transfer process, which is about 0.14-0.16 solar masses.

“The special pre-ELM-WD looks like a normal F type dwarf star orbiting an invisible components every 5.27 hours. It may have just finished its mass transferring stage, and is moving slowly towards the white dwarf cooling track. The constant luminosity implies that its energy is supplied by a tiny burning Hydrogen shell outside the degenerate He core. However, its dynamical mass is only about 0.09 solar masses, below the lower limit of theoretical predictions, which is indeed confusing,” said Dr. YUAN Hailong, the first author of the research paper.

The mass of the system is estimated based on multi-band time domain photometric and spectroscopic data, and the parallax from Gaia. After counting all the error budget, the estimated mass is still significantly low. The team has tested different theoretical models, none could fit the results properly. This discovery put a question on the current ELM formation mechanism and is still waiting for an answer.

The invisible compact component may have a mass of ~1.0M_☉, and it is more likely a WD, but a neutron star cannot be ruled out at present.

The ELM binary systems with compact companions may be continuous gravitational wave sources and are among the most telling objects in the gravitational wave detection project.

As an important progress of the LAMOST compact object hunting project, the discovery proves the ability LAMOST on the ELMs study. As more time-domain plates are coming in the second 5-year regular survey of LAMOST, more interesting compact binary systems are expected to be discovered.

Figure 2. The time domain resolution spectra, RV curve and multi-band light curves of J2240. (Credit: YUAN Hailong)

The research paper has been published in Astronomical Journal on February 21, 2023, and can be accessed at https://iopscience.iop.org/article/10.3847/1538-3881/acaf07

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