Colloquium on Dec. 19, 2023

RGB mass loss from the study of the Horizontal Branch morphology

Speaker: Marco Tailo (University of Bologna)

Venue: Video Conference

Time: 15:00 PM, Tuesday, Dec. 19, 2023

Abstract:

The amount of mass lost by stars during the red-giant branch (RGB) phase is one of the main parameters needed to fully understand later stages of stellar evolution and, from a wider point of view, to gain insights on many problems in astrophysics (such as the survival rate of planetary systems). In spite of its importance, a fully-comprehensive physical understanding of this phenomenon is still missing, and we need to rely on empirical and semi-empirical formulations. In time, these have been obtained by studying stellar associations and by comparing the mass of the RGB stars with the mass of the core Helium Burning ones. This can be successfully done because core Helium Burning is the evolutionary phase immediately subsequent the RGB, therefore studying the mass defect between these two phases provides the sought after total mass loss and mass loss rate. Galactic Globular Clusters (GC) are ideal targets to derive formulations of mass loss laws thanks to the special features of their core Helium Burning stars: these are hosted in a locus in the CMD called Horizontal Branch (HB) and have special features that make their study useful to solve this problem. Indeed, the colour and, with lesser extent, the luminosity of a HB star are tied to its mass which, in turn, is tied to the many factors contributing to alter the mass of a star evolving though the RGB and the helium flash, such as differences in surface chemistry and mass loss. The sensitivity of these stars to any changes in these parameters produces the large variety of HB morphologies we observe and was one of the culprits that led to the discovery of the multiple populations phenomenon. However, until recently, the very presence of multiple populations has been a major challenge in properly constraining RGB mass loss in GCs during the modern era of precise, multi-wavelenght observations. Indeed, the same special features of the core HB stars pose an additional challenge, because the effects of increased helium abundance, typical of the multiple populations in GCs, is degenerate with mass loss and their effects difficult to disentangle. As a consequence, precise evaluation of mass loss for the stellar populations hosted along the HB was possible only under few circumstances.  A leap forward has been made when recent works, based on a large collection of HST data, measured the helium abundance in a large collection of GCs, breaking this decades long degeneracy and allowing the characterization of these old stellar populations in unprecedented ways and unlocking the path to fully study each HB. In this talk, I will discuss how these recent advancements in the field of multi-band photometry can provide solutions to some new and decades-old problems and give us a new formulation of mass loss, useful to describe old, dense stellar environment. I will also describe new observations and phenomena specific for GCs which could shed lights on the origin of multiple populations phenomenon itself. Finally I will briefly show how asteroseismology can help us extend these observations to the field stars and to younger, metal-richer and less dense stellar association paving the way to a possible wider understanding of the phenomenon of stellar mass loss.