Researchers propose new formation model for massive hot subdwarf stars

In a new study published in the journal Science The Astrophysical JournalDr. Zhenwei Li of the Yunnan Observatory of the Chinese Academy of Sciences and his collaborators and Dr. Yangyang Zhang of Zhoukou Normal University proposed a new model of massive hot subdwarf star formation that provides an explanation for the subset of helium observed to be abundant in hot subdwarfs in the universe.

Hot subdwarfs are extreme horizontal branch stars. Some of these stars found in tight binaries are considered potential sources of gravitational waves for future spaceborne gravitational wave detectors. Unique chemical properties, particularly their surface helium (He) abundance, can serve as valuable tools for understanding the formation and evolution of these objects.

Hot subdwarfs are typically formed through binary interactions with He core ignition when a donor initiates mass transfer near the tip of the red giant branch (RGB). This situation is called RGB channel. However, the recently discovered star SMSS J1920 (the third known binary star consisting of a hot subdwarf and an accreting white dwarf) contradicts the RGB channel.

The strong Ca H and K lines with blue shifts indicate that the binary may have originated from a recent common envelope ejection (ejection age ~10,000 years). In comparison, for hot subdwarfs produced by RGB channels, the time from common envelope ejection to the current state spans tens of millions of years.

To explain the formation of SMSS J1920, researchers propose a new route to a hot subdwarf. This involves hot subdwarfs produced by the common envelope ejection process of Asymptotic Giant Branch (AGB) stars, known as AGB channels. Unlike the thermal subdwarfs of RGB stars, the thermal subdwarfs of AGB stars contain a massive carbon and oxygen (CO) core, helium-burning shells, and hydrogen envelopes.

The researchers used state-of-the-art stellar evolution codes to construct an evolution model of hot subdwarfs from the AGB channel. The simulation results can explain most of the important observational parameters of SMSS J1920, such as evolutionary age, thermal subdwarf mass, effective temperature and surface gravity.

The AGB channel can explain not only SMSS J1920, but also some observed special hot subdwarfs. The AGB channel can produce hot subdwarfs with masses exceeding 0.48 times the mass of the Sun. However, in the RGB channel, most hot subdwarfs have masses less than 0.48 times the solar mass. This indicates that some massive hot subdwarfs may originate from the AGB channel.

Additionally, the researchers found that hot subdwarfs from AGB channels typically exhibit higher helium (He) abundances, which can be attributed to partial hydrogen burning in the envelope. Therefore, hot subdwarfs from the AGB channel can naturally explain part of the He-rich hot subdwarfs observed in the universe.