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.
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