Recently, Dr. LI Linjia and Prof. QIAN Shengbang from Yunnan Observatories of the Chinese Academy of Sciences discovered a RR Lyrae star with rapidly decreasing period, AX UMa, and found that AX UMa showed some characteristics different from the normal RR Lyrae stars, indicating that it is a rare binary evolution pulsator. The study was published in The Astronomical Journal.The researchers made multi-band photometric observation on AX UMa by using the 60 cm telescope and Sino-Thai 70 cm telescope located in Yunnan Observatories.By analyzing the light curve data collected from several sky survey observation projects, they obtained the O-C diagram of AX UMa, and calculated that 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 diagram (H-R) diagram.
The researchers 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. They believed that AX UMa has a hot subdwarf star, and the system should be a binary evolution pulsator.
Besides, the researchers found that the radial velocities of AX UMa provided by Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) survey varied greatly. The differences in radial velocity indirectly indicated 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 to study the formation and evolution of horizontal branch stars.
Reference: L.-J. Li, S.-B. Qian et al., “AX UMa, an ab-type RR Lyrae Star with a Rapidly Decreasing Pulsation Period, and Its Binarity”, Astronomical Journal, 161(4), 2021. https://iopscience.iop.org/article/10.3847/1538-3881/abe4df
Provided by Chinese Academy of Sciences