An INAF study provides new assumptions on oscillation in star layers
Stars contain sound waves and their investigation is important to understand their structure, keeping into account that the frequency and wavelength of individual oscillations are related to the physical status of the different layers through which wave passes. Astroseismology is the study of the internal structure of pulsating stars through the interpretation of their pulsations and their spectrum.
A team of researchers led by Enrico Corsaro, fellow of the AstroFIt2 programme at the National Institute for Astrophysics (INAF) in Catania, has recently published a study in Nature Astronomy conducted using astroseismology, which shows the relationship between the orientation of the stellar-spin axis and the environment where they formed.
The international team of researchers has shown the rotational movement of a group of giant red stars belonging to two open stellar clusters, with a mass similar to our Sun but more mature. The analysis of their oscillations has shown that the rotating axes of each cluster is mainly oriented towards a specific direction in the sky: this status contains data on the formation of stellar clusters.
The gravitational collapse of a molecular cloud, characterized by turbulent motions that occur in its interior, originates star clusters, each of which contains thousands of stars; however, it is not easy to observe the process that starts new stars nor rebuild it theoretically. But analyzing the star clusters already formed, it is possible to obtain information about their past.
The study focuses on NGC 6791 and NGC 6819, two open stellar clusters belonging to our galaxy, of which 48 red giant stars were analyzed, showing periodic expansions and contractions of their structures, observed by the NASA Kepler space mission.
“Thanks to their fluctuations, we have measured the angle of inclination of the rotation axis of each star,” says Corsaro, first author of the article published in Nature Astronomy which describes the study results. “The pattern observed is that almost all the stars in the sample have strongly aligned rotation axes, since they point in the same direction in the sky. The evidence for this result does not leave room for any doubt and makes us infer that this phenomenon must necessarily have originated in the process of formation of the same star clusters, which occurred billions of years ago.”
“What we have observed, that is, the strong alignment of the rotation axes of a large number of stars, can be reproduced by assuming that at least half of the total kinetic energy of the molecular cloud that originated each of the two clusters, has a rotational nature” adds Corsaro. "This shows that the global angular momentum of the cloud has been transferred efficiently to the individual stars that are formed in its interior. However, this does not occur if the stars have smaller masses than the Sun, because the cloud does not transfer enough mass to oppose to the turbulent motions, which then redistribute the angular momentum in all directions, losing all traces of a possible alignment of the rotation axes.”
The result achieved by the researchers shows for the first time that the process of formation of a star can be understood and studied in detail by astroseismology even in stars having an age similar to that of the Universe, going back to the energy components, the structure and geometry of the first steps that led to the formation of star clusters.