Mass evaluation for red giant stars based on the spectroscopically determined atmospheric parameters

Abstract

The mass (M) of a star can be evaluated from its spectroscopically determined effective temperature (Teff) and metallicity ([Fe/H]) along with the luminosity (L; derived from parallax), while comparing them with grids of theoretical evolutionary tracks. It has been argued, however, that such a track-based mass (Mtrk) may tend to be overestimated for the case of red giants. Meanwhile, there is an alternative approach of evaluating mass (MgLT) directly from surface gravity (g), L, and Teff. The practical reliability of MgLT was examined for ∼100 benchmark giants in the Kepler field, for which atmospheric parameters are already determined and the reliable mass (Mseis) along with the evolutionary status are known from asteroseismology. In addition, similar check was also made for the accuracy of Mtrk for comparison. It turned out that, while a reasonable correlation is seen between MgLT and Mseis almost irrespective of the stellar property, its precision is rather insufficient because log(MgLT/Mseis) distributes rather widely within ∼±0.2–0.3 dex. In contrast, the reliability of Mtrk was found to depend on the evolutionary status. Although Mtrk and Mseis are satisfactorily consistent with each other (typical dispersion of log(Mtrk/Mseis) is within ∼± 0.1 dex) for H-burning red giants as well as He-burning 2nd clump giants of higher mass, Mtrk tends to be considerably overestimated as compared to Mseis by up to ≲ 0.4 dex for He-burning 1st clump giants of lower mass. Accordingly, MgLT and Mtrk are complementary with each other in terms of their characteristic merit and demerit.