Recently, Dr. LI Linjia and Prof. QIAN Shengbang from Yunnan Observatories of the Chinese Academy of Sciences discovered and identified a RR Lyrae star with rapidly decreasing period, AX UMa. They found that AX UMa also showed some characteristics different from the normal RR Lyrae stars, which indicate that it is a rare binary evolution pulsator. The results of this work are published in The Astronomical Journal.

The research team made multi-band photometric observation on AX UMa by using the 60 cm telescope and Sino-Thai 70 cm telescope located in Yunnan Observatories, and collected the light curve data from several sky survey observation projects. They obtained the O-C diagram of AX UMa, and calculate the period change rate is -7.75 days per million years. This value is one or two orders of magnitude higher than the absolute value of the rate of normal RR Lyrae stars.

So far, AX UMa is the field RR Lyrae star which has the fastest decrease in pulsation period, which means that it is rapidly moving to the left in the Hertzsprung-Russell (H-R) diagram.

The research team found that the color index of AX UMa at minimum light, B-V = 0.295 mag, is smaller than that of normal ab type RR Lyrae (RRab) stars, which indicates that the effective temperature is higher, reaching 7300 K. Combining with other evidence, the research team believed that AX UMa has a hot subdwarf star, and the system should be a binary evolution pulsator.

The research team found that the radial velocities of AX UMa provided by Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) survey vary greatly, and the maximum difference is more than 100 km/s. The great difference in radial velocity also indirectly indicates the existence of the companion star.

The rapid decrease of period of AX UMa indicates that it is in the process of rapid evolution towards the left of H-R diagram. This makes it a living sample for studying the formation and evolution of horizontal branch stars.

The binary nature also brings great enlightenment to the related research. One of the purposes of this work is to arouse people’s attention to this rare object and hope that more researchers will make more observations and studies on it.

Contact:

LI Linjia, Yunnan Observatories, CAS

lipk@ynao.ac.cn