Class B sub dwarfs in the open cluster of stars NGC 6791

(ORDO NEWS) — Using NASA’s Kepler Space Telescope and the MMT Telescope in Arizona, USA, an international team of astronomers studied the population of class B sub dwarfs in the open cluster of stars known as NGC 6791.

In general, hot B class sub dwarfs are extreme objects of the horizontal branch, consisting of nuclei, in which the combustion of helium occurs, and very thin hydrogen shells. They are compact objects, the masses of which are usually about half the mass of the Sun, the radii are from 0.1 to 0.3 of the radius of our star, and the effective temperatures are in the range from 20,000 to 40,000 Kelvin.

Astronomers are especially interested in pulsating B-type sub dwarfs, which exhibit two kinds of variations in light intensity. The first type is associated with short-period pressure modes (p-modes) and is characterized by pulsation periods of the order of minutes and pulsation amplitudes of the order of tens of a thousandth of a magnitude unit. The second type of variations in the intensity of the light flux is associated with long-period gravitational oscillations (g-modes) and differs in pulsation periods of the order of hours and pulsation amplitudes of less than 10 thousandths of a unit of magnitude.

Recently, a team of researchers led by Sachi Sanjaya of the Pedagogical University of Krakow, Poland, searched for class B sub dwarfs in the open star cluster NGC 6791, which is about 8 billion years old and is one of the oldest and most metal-rich star clusters in the Milky paths. This cluster is located about 13,300 light-years from us in the direction of the Lyra cluster, and it is unusually massive (its mass is about 4,000 solar masses) and densely populated with stars.

Sanjaya’s team discovered three B-class sub dwarfs in the NGC 6791 star cluster: KIC 2569576 (B3), KIC 2438324 (B4), and KIC 2437937 (B5). Although previous studies have already shown that these stars are pulsating, astronomers were able this time to obtain more detailed and covering a longer period of time data on the periods of their pulsations. This made it possible to confirm the previously found out frequencies of pulsations in the g-modes, as well as to find additional new frequencies of such pulsations.

According to the study, the effective temperatures of the stars B3, B4 and B5 were, respectively, 24 250, 24 786 and 23 844 Kelvin. The B4 system is binary and has an orbital period of about 9.5 hours, scientists have found. The authors also do not exclude the possibility that the sources B3 and B5 can also be binary systems.

The research appeared on the arxiv.org preprint server.

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