Recently, Ph.D. candidate ZANG Lei and Prof. QIAN Shengbang, from Yunnan Observatories of Chinese Academy of Sciences (CAS), as well as their cooperator, discovered that the orbital period of the persistent supersoft X-ray source QR And is continuously increasing. It reveals that the orbital evolution of QR And is mainly caused by the mass transfer from the low-mass companion to the white dwarf on its Kelvin–Helmholtz timescale. This work was published in Monthly Notices of the Royal Astronomical Society.

QR And is the optically brightest galactic supersoft X-ray source, where mass is transferred from a donor secondary to a massive white dwarf primary via an accretion disk.

To further study their mass exchange and the orbital evolution, the researchers analyzed the orbital period variation of QR And in detail, by using the continuous TESS (the Transiting Exoplanet Survey Satellite) light curves and the photometric observations collected by AAVSO (American Association of Variable Star Observers) database. The result implies that the orbital period is continuously increasing.

Moreover, based on the results of previous research, the researchers estimated that the mass of the donor secondary is about 0.5 solar mass by detailed analysis, when the white dwarf mass is about 1.2 solar mass.

Therefore, the continuous increase in the orbital period is mainly caused by the mass transfer from the low-mass companion to the white dwarf on its Kelvin–Helmholtz time-scale, while wind mass loss from the accretion disc may also contribute to the period variation.

The mass and angular momentum carried off by wind from the accretion disc delay expansion of the orbit for QR And. This allows that the companion star with a low mass can always be filling its critical Roche lobe and transferring material to the white dwarf.

The range of mass transfer rate for QR And was estimated in this scenario. And it can be roughly estimated that QR And may reach to the Chandrasekhar limit in at least 1.5 million year, then might explode as a type Ia supernova.

Contact:
ZANG Lei
Yunnan Observatories, CAS
Email: zanglei@ynao.ac.cn