Exploring globular clusters with the lens of asteroseismology

The global cluster M4 (Credit: ESA/Hubble & NASA)

Asteroseismology - one of the most fascinating and sophisticated methods for measuring the mass of stars, and by extension, their age - can also be successfully used to reveal the characteristics of the stars within globular clusters: very large groups of stars (in the order of hundreds of thousands) condensed in a relatively small space and all at approximately the same distance from us. This is shown by a study - published in Astronomy & Astrophysics - carried out by an international team of astrophysicists as part of the ERC Asterochronometry project, active at the University of Bologna; in close, complementary collaboration with several institutes in the INAF – the National Institute for Astrophysics - network, the University of Padua and the University of Aarhus (Denmark).

The scientists focused in particular on M4, a globular cluster of our galaxy, located within the Scorpio constellation. The data employed in the study are those taken by the NASA Kepler satellite, which observed M4 during the second part of its mission (called K2), providing about 80 days of continuous photometric monitoring: the longest, continuous observations available of this type for these stellar associations.

"Almost all globular clusters are composed of a collection of different stellar populations, which shows its complexity when these stellar associations are observed through special combinations of optical and ultraviolet filters and colors, or when they are investigated with high-resolution spectroscopy", explains Marco Tailo, a research fellow at the Department of Physics and Astronomy of the University of Bologna, and first author of the study. "But Asteroseismology can also play a crucial role in this respect, thanks to its ability to provide a direct and accurate estimate of the mass, radius, age, and some other features of the stars belonging to the individual populations in a globular cluster."

"Indeed, Asteroseismology makes it possible to determine the mass and age of stars through the study of stellar oscillations, which are measured by means of the luminosity variations observed on the surface of the stars, which are then analyzed using refined mathematical methods," explains Enrico Corsaro, a researcher at the INAF - Astrophysical Observatory of Catania and second author of the study.

The study analyzed a sample of 37 stars within M4: currently the highest number for this kind of study for the distant globular clusters. Of these, most (31) are red giants, very bright stars in the advanced stages of their evolution, having small or intermediate mass, while the other 6 are horizontal branch stars, a later evolutionary stage composed of stars that are burning helium in their cores.

"Through the analysis of the asteroseismic data, we were able to obtain the mass and radius of all these stars with good precision and accuracy", continues Tailo. "Moreover, the results obtained are in agreement with other estimates made with more traditional methods: a confirmation of the efficiency and robustness of the asteroseismic method also for the study of stars within globular clusters”.

Not only that: the study succeeded in providing an asteroseismic characterization of the two stellar populations in M4 for the first time, providing a new point of view on the matter, thus integrating what is already in the literature. In the first population, the stars have similar chemical characteristics to those outside the cluster, while stars in the second have completely different chemical characteristics: it is in fact a stellar population born from the material ejected from the stars of the first population. With respect to this issue, the scientists suggest that the average mass difference between the two populations is actually very small, which indicates the two populations were born very close together.

"This work reveals the enormous potential that asteroseismic studies can bring to this field. However, determining the mass of stars is only a first step. If we had more precise data at our disposal, we could even reconstruct in detail the structure and internal rotation of stars belonging to different populations, thus clarifying their different origins. This is actually one of the main scientific objectives of HAYDN, a proposal for a future ESA space mission that we are pursuing together with a large European consortium”, explains Andrea Miglio, professor at the University of Bologna and principal investigator of the Asterochronometry project.

Other important results emerged from the sample of horizontal branch stars. Indeed, the relationship between their mass and temperature was identified in the sample, thus obtaining the first direct confirmation of the behavior already predicted for these stars by theoretical stellar evolution.

"Since horizontal branch stars are the direct descendants of red giants, studying their mass has allowed us to have important information on what happens during the earlier phase, in particular, we also got information on the phenomenon of mass loss, occurring during the ascent of the red giant branch," explains Tailo. "In this context, the amount of mass loss that stars in globular clusters undergo is much higher than that of their counterparts in less dense systems or for stars not belonging to stellar associations, a result that has involved many astronomers and astrophysicists in various interdisciplinary studies."

The study was published in the journal Astronomy & Astrophysics under the title “Asterosesimology of the multiple stellar populations in the Globular Cluster M4”. Participating for the University of Bologna were Marco Tailo, Andrea Miglio, Josefina Montalbán, Amalie Stokholm and Giada Casali, all from the Department of Physics and Astronomy “Augusto Righi”.