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Sofia Feltzing

Sofia Feltzing

Professor of astronomy

Sofia Feltzing

Determination of robust metallicities for metal-rich red giant branch stars : An application to the globular cluster NGC 6528

Author

  • C. Liu
  • G. Ruchti
  • S. Feltzing
  • F Primas

Summary, in English

Context. The study of the Milky Way relies on our ability to interpret the light from stars correctly. With the advent of the astrometric ESA mission Gaia we will enter a new era where the study of the Milky Way can be undertaken on much larger scales than currently possible. In particular we will be able to obtain full 3D space motions of red giant stars at large distances. This calls for a reinvestigation of how reliably we can determine, for example, iron abundances in such stars and how well they reproduce those of dwarf stars. Aims. Here we explore robust ways of determining the iron content of metal-rich giant stars. We aim to understand what biases and shortcomings the widely applied methods suffer from. Methods. In this study we were mainly concerned with standard methods of analysing stellar spectra. These include the analysis of individual lines to determine stellar parameters, and analysis of the broad wings of certain lines (e.g. Hα and calcium lines) to determine effective temperature and surface gravity for the stars. Results. For NGC 6528 we find that [Fe/H] = + 0.04 dex with a scatter of σ = 0.07 dex, which gives an error in the derived mean abundance of 0.02 dex. Conclusions. Our work has two important conclusions for analysis of metal-rich red giant branch stars. Firstly, for spectra with S/N of below about 35 per reduced pixel, [Fe/H] becomes too high. Secondly, determination of Teff using the wings of the Hα line results in [Fe/H] values about 0.1 dex higher than if excitational equilibrium is used. The last conclusion is perhaps unsurprising, as we expect the NLTE effect to become more prominent in cooler stars and we can not use the wings of the Hα line to determine Teff for the cool stars in our sample. We therefore recommend that in studies of metal-rich red giant stars care should be taken to obtain sufficient calibration data to enable use of the cooler stars.

Department/s

  • Lund Observatory - Has been reorganised

Publishing year

2017-05-01

Language

English

Publication/Series

Astronomy and Astrophysics

Volume

601

Document type

Journal article

Publisher

EDP Sciences

Topic

  • Astronomy, Astrophysics and Cosmology

Keywords

  • Galaxy: bulge
  • Globular clusters: individual: NGC 6528
  • Stars: Atmospheres
  • Stars: fundamental parameters

Status

Published

ISBN/ISSN/Other

  • ISSN: 0004-6361