There exists a unique class of binary systems known as Algol-type binaries. These systems consist of a hot primary star and a cooler companion. The companion, having expanded to fill its Roche lobe, transfers material via a steady stream onto the primary. The system 2MASS J06281154+164439.3 is precisely such a pair currently undergoing active mass transfer.

Leveraging 1,082 days of continuous photometric data from NASA’s Transiting Exoplanet Survey Satellite(TESS) and 21 medium-resolution spectra from China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST), Dr.YANG Daoye, a Ph.D. student at the Xinjiang Astronomical Observatory (XAO), Chinese Academy of Sciences (CAS), has constructed the first comprehensive profile of this long-period system (orbital period approximately 21.6 days),under the supervision of Prof. Esamdin Ali from XAO and Prof. SHI Jianrong from National Astronomical Observatories, CAS. Related results were published in The Astronomical Journal.

The researchers discovered that the transferred material does not directly impact the primary star. Instead, it forms a rotating structure—an accretion disk. Hydrogen atoms within this disk emit a characteristic Hα line exhibiting a stable double-peaked profile, akin to two rotating searchlights, clearly signaling the presence of the disk.

Intriguingly, the separation between the peaks remains nearly constant, indicating that the outer boundary of the disk is stabilized at approximately 26 solar radii from the primary, precisely within the star's gravitational domain. However, slight fluctuations in the intensity of these peaks suggest the presence of a "hot spot" on the disk, likely generated by the impact of the accretion stream.

To validate this model, the researchers integrated light curves and spectral data into a physical model, successfully reconstructing the gas density, temperature (approximately 6,000 Kelvin), and internal turbulence velocity (nearly 50 km/s) of the disk. They further identified a hot spot at the disk's outer edge that accounts for subtle asymmetries in the light curve, thereby eliminating the need for ad hoc assumptions of "starspots" on the stellar surface.

This work not only provides precise measurements of the masses, radii, and temperatures of the binary components but also clarifies the stable structure of the accretion disk during mass transfer. It demonstrates that even with an orbital period of three weeks, an accretion disk can persist over extended timescales, offering an exceptional sample for understanding stellar mass transport. Future high-precision spectroscopy will enable astronomers to trace the dynamic evolution of the hot spot and the disk, further unveiling the secrets of binary star evolution.

Figure：Doppler tomography of Hα using a maximum-entropy inversion. The left panel shows the observed trailed spectra as a function of orbital phase and radialvelocity. The right panel shows the reconstructed Doppler map displayed as a polar projection