Metallicity of M dwarfs

II. A comparative study of photometric metallicity scales^⋆

¹ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
² UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
³ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁴ Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France
⁵ Centro de Astronomia e Astrofísica da Universidade de Lisboa, Campo Grande, Ed. C8 1749-016 Lisboa, Portugal
⁶ Leiden Observatory, Leiden University, The Netherlands
⁷ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland

Received: 16 September 2011
Accepted: 12 October 2011

Abstract

Stellar parameters are not easily derived from M dwarf spectra, which are dominated by complex bands of diatomic and triatomic molecules and do not agree well with the individual line predictions of atmospheric models. M dwarf metallicities are therefore most commonly derived through less direct techniques. Several recent publications propose calibrations that provide the metallicity of an M dwarf from its K_s band absolute magnitude and its V − K_s color, but disagree at the  ±0.1 dex level. We compared these calibrations using a sample of 23 M dwarfs, which we selected as wide (>5 arcsec) companions of F-, G-, or K-dwarfs with metallicities measured on a homogeneous scale and which we require to have V band photometry measured to better than  ~0.03 mag. We find that the Schlaufman & Laughlin (2010, A&A, 519, A105) calibration has the lowest offsets and residuals against our sample, and used our improved statistics to marginally refine that calibration. With more strictly selected photometry than in previous studies, the dispersion around the calibration is well in excess of the [Fe/H] and photometric uncertainties. This suggests that the origin of the remaining dispersion is astrophysical rather than observational.