Scientists discover pre-low-mass white dwarf binary using LAMOST

A binary system containing a particularly low-mass (ELM) white dwarf (WD) progenitor and a compact invisible companion, was found by Chinese language astronomers from the Nationwide Astronomical Observatories of Chinese language Academy of Sciences (NAOC), utilizing spectroscopic information from LAMOST and P200/DBSP, and multi-band photometric information from Catalina and Zwicky Transient Facility.

The mass of the seen star, a pre-ELM WD, is about 0.09 solar masses, decrease than the theoretical restrict of its species, and should problem the present formation concept of the ELM WD.

The research has been revealed in The Astronomical Journal.

Most stars within the universe will finish their lives as white dwarfs after their nuclear gasoline had been burned out. A lot of the white dwarfs are carbon-oxygen white dwarfs (CO WD), which suggests they’re principally composed of Carbon and Oxygen. They’ve lots of 0.5-1.4 solar lots.

When the mass is larger than 1.4 solar lots, the electron degeneracy strain on the core can’t resist the gravity and the white dwarf will proceed to break down right into a neutron star. White dwarfs between 0.33 and 0.5 solar lots can have cores dominated by both CO or Helium (He). White dwarfs with even decrease lots, often called ELM WDs, are composed of degenerate He.

It’s believed that these ELM WDs can’t type by way of the one star evolution channel, because the formation of such single low mass He WDs requires a progenitor star with a correspondingly very low preliminary mass and a particularly lengthy evolution time, a time even longer than the present age of our universe. Subsequently, it’s usually accepted that ELMs are fashioned inside interacting binary methods.

Specifically, it’s proposed that the even decrease mass fraction of ELMs (beneath 0.18 solar lots) loses most of its preliminary mass by way of a steady Roche Lobe overflow channel.

To efficiently type an ELM, the onset of the mass transfer must be on the proper time. If the mass switch begins too early, the donor will evolve right into a low-mass fundamental sequence star, just like the secondary within the Cataclysmic Variable.

However, if it begins too late, the core of the donor will probably be huge sufficient to evolve to the following stage by helium flash. Thus, there’s a theoretical decrease mass restrict of the ELM-WD in such a restricted mass switch course of, which is about 0.14-0.16 solar lots.

“The particular pre-ELM-WD appears to be like like a standard F-type dwarf star orbiting an invisible part each 5.27 hours. It could have simply accomplished its mass-transfer phase and is transferring slowly in the direction of the white dwarf cooling path. Its fixed luminosity implies that its power is provided by a tiny burning hydrogen shell outdoors the degenerate He core. Nevertheless, its dynamical mass is simply about 0.09 solar lots, beneath the decrease restrict of theoretical predictions, which is certainly puzzling,” stated Dr. Yuan Hailong, first creator of the research.

The mass of the system is estimated based mostly on multi-band time-domain photometric and spectroscopic information and Gaia parallax. After considering all error budgets, the estimated mass remains to be considerably low. The workforce has examined a number of theoretical fashions, none of which might match the outcomes correctly. This discovery raises a query in regards to the present ELM formation mechanism that’s nonetheless awaiting a solution.

The invisible compact part might have a mass of ~1.0M☉, and is extra more likely to be a WD, however a neutron star can’t be dominated out at current.

The ELM binary methods with compact companions could also be steady gravitational wave sources and are among the many most revealing objects within the gravitational wave detection mission.

As an vital progress of the LAMOST compact object search mission, the invention proves the power of LAMOST to review the ELMs. With extra time-domain plates coming in throughout LAMOST’s second common five-year survey, extra attention-grabbing compact binary methods are anticipated to be found.