For the first time, researchers have found long-term sunspots and other phenomena in δ variable stars in the constellation Shield

δ shield variable stars are typically moderate-mass stars with an effective temperature between 6900 and 8900 K and a spectral type A-F. Most of these variables have rich pulsation patterns, which can be used to explore the structure of stars in astrological seismology, and are of great significance for revealing the mysteries of stellar evolution.

Recently, Ma Shuguo, a doctoral student in the Optical Astronomy and Technology Application Research Laboratory of the Xinjiang Astronomical Observatory, under the guidance of his supervisor, Researcher Eli Ishamuddin, analyzed the light changes of a low-temperature shield constellation δ variable star KIC 5768203 with an effective temperature of about 6934 (134) K, and found for the first time that there was a long-term sunspot phenomenon on its surface, and it only pulsated in a radial mode with low amplitude. The results of this research have been officially published in the Journal of Astronomy (2022, AJ, 264, 22).

Based on the metering data from KIC 5768203’s space telescopes (Kepler and TESS), the researchers analyzed the periodic modulation of the star’s starlight changes and found that there were sunspots on its surface for a long time, which means that the star has a stable surface magnetic field. A necessary condition for the existence of a magnetic field is convective events in the outer cladding, but current models of stellar evolution show that the stellar cladding at a temperature of around 7000 K is already in complete radiation equilibrium and should not have convective events. Recent studies using space telescope data have shown that sunspots may also exist on hotter A-type stars, adding another observational evidence.

Figure 1: Periodic photometric evolution of KIC 5768203 removing the influence of pulsations, and the yellow band region is the region of the star’s surface brightness affected by sunspots.

In addition, through the analysis of astrological seismology, it was found that the star only pulsates in a radial mode with very low amplitude, and the pulsation mode is similar to that of the large-amplitude shield constellation δ variable star (pulsating amplitude greater than 150 mmag), but its pulsation amplitude is only 1.4 mmag (millisterine magnitude). Only two pure radially pulsating small-amplitude shield δ variable stars have been found so far, making them very important and rare research samples in stellar evolution. (Source: Xinjiang Astronomical Observatory, Chinese Academy of Sciences)

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