Astronomers have discovered pulsating subdwarf stars in the cluster NGC 6791 an unusual type of pulsating space objects. The results of the study are published in the arXiv preprint repository.
Hot subdwarf stars B (sdB) are objects consisting of a helium core and a very thin hydrogen envelope. Their mass is about half that of the Sun, the radius is 0.1-0.3 of the Sun's radius, and the effective temperature reaches 20-40 thousand Kelvin. However, the most interesting for scientists are the so-called pulsating subdwarf stars B (sdBV), which change their brightness due to short-period pressure changes (p-modes) and long-period gravitational modes (g-modes).
Researchers have searched for sdBV in open cluster NGC 6791, eight billion years old. It is located about 13,300 light years from Earth in the constellation Lyra, and its mass is comparable to the mass of four thousand Suns.
Three sdBVs have been identified in NGC 6791: KIC 2569576 (B3), KIC 2438324 (B4) and KIC 2437937 (B5). The measured effective temperatures B3, B4 and B5 were 24 250, 24 786 and 23 844 kelvin, respectively. The scientists also discovered that B4 is a binary system containing the star sdBV and its main sequence companion, with an orbital period of about 9.5 hours. B3 and B5 could also be binaries, as astronomers have found signs of changes in their radial velocities.