Volume 640, August 2020
|Number of page(s)||12|
|Published online||17 August 2020|
Facing problems in the determination of stellar temperatures and gravities: Galactic globular clusters★
Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna,
Via Gobetti 93/2,
2 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
3 GEPI, Observatoire de Paris, Université PSL, CNRS, 5, Place Jules Janssen 92195 Meudon, France
Accepted: 16 March 2020
We analysed red giant branch stars in 16 Galactic globular clusters, computing their atmospheric parameters both from the photometry and from excitation and ionisation balances. The spectroscopic parameters are lower than the photometric ones and this discrepancy increases with decreasing metallicity, reaching differences of ~350 K in effective temperature and ~1 dex in surface gravity at [Fe/H] ~ –2.5 dex. We demonstrate that the spectroscopic parameters are inconsistent with the position of the stars in the colour-magnitude diagram, providing overly low temperatures and gravities, and predicting that the stars are up to about 2.5 magnitudes brighter than the observed magnitudes. The parameter discrepancy is likely due to inadequacies in the adopted physics; in particular the assumption of a one-dimensional geometry could be the origin of the observed slope between iron abundances and excitation potential that leads to low temperatures. However, the current modelling of 3D/NLTE radiative transfer for giant stars seems to be unable to totally erase this slope. We conclude that the spectroscopic parameters are incorrect for metallicity lower than –1.5 dex and that photometric temperatures and gravities should be adopted for these red giant stars. We provide a simple relation to correct the spectroscopic temperatures in order to put them onto a photometric scale.
Key words: globular clusters: general / stars: abundances / stars: atmospheres / techniques: spectroscopic
© ESO 2020
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.