Austrian Early Career Conference 2024

Contribution:
Poster

Authors:
Suntinger T. [1], Beck P. G. [1,2], Camalich Jorge Martin [2], Hanslmeier A. [1]

Affiliations:
1: Institut für Physik, Institutsbereich Geophysik, Astrophysik und Meteorologie, NAWI Graz, University of Graz, Universitätsplatz 5/II, 8010 Graz, Austria; 2: Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain

Title:
Determination of the Luminosity of Stars at the Tip of the Red-Giant Branch from Gaia Observations as Proxy for the Effectiveness of Axion Cooling

Abstract:
The Tip of the Red Giant Branch (TRGB) of a Globular Cluster (GC) marks the point of helium ignition in a star. Neutrinos, produced by plasma oscillations in the degenerate core of a red-giant branch (RGB) star , are cooling its core. Consequently, the star requires longer to reach helium's ignition temperature, allowing it to continue its evolution on the RGB. Consequently, the cooling of the core leads to an increased luminosity at the TRGB. This opens a possibility to test for the existence of an additional exotic cooling channel based on axions - hypothetical particles and dark matter candidates.
The challenge in determining such a precise luminosity at the TRGB evaluation lies in the sparse population of the upper RGB and its contamination by AGB stars. Moreover, the statistical probability of finding a star at the TRGB is infinitesimally small. Approximations based on statistical RGB distributions and the brightest found RGBs in the GCs are used as a solution. Variable RGBs pose yet another challenge to these approximations. Additionally, systematic uncertainties as reasons for the slight TRGB brightness increase need to be excluded.
To date, the Gaia satellite mission provides the largest coherent star catalogs (including high-precision astrometry, photometry, and spectroscopy).
In this poster, we present methods for deriving the bolometric absolute GC TRGB brightness as a proxy for the effectiveness of axion cooling on GC M5 (NGC5904) using recent Gaia DR3 catalogs. With Gaia DR3 catalogs, we derived the bolometric TRGB brightness of GC M5 (NGC5904). Using epoch photometry from Gaia DR3 for the four brightest RGs on the branch led to identifying one member as a long-period variable with a brightness range spanning over 0.9 mag, and a  maximum brightness range of 0.2 mag for the remaining three members. Thanks to the epoch photometry, long-period brightness variability was also considered in the uncertainty estimation, leading to the bolometric GC TRGB brightness (-3.50±0.31) mag for GC M5.
Methodologies based on axion-electron interactions via bremsstrahlung and provided by Straniero et al. (2020) were applied to the bolometric TRGB brightness, yielding an even lower stringent upper bound on the axion-electron coupling constant of 2.22, compared to the result of 2.30 from Straniero et al. (2020). A re-investigation of interstellar extinction, bolometric correction, and the use of future Gaia data releases 4 and 5 will lead to a significant quality increase of the TRGB brightness and most likely to an even tighter stringent upper axion-electron coupling bound.