High-resolution spectroscopic atlas of M subdwarfs. Effective temperature and metallicity^⋆,⋆⋆

¹ Institut UTINAM, CNRS UMR 6213, Observatoire des Sciences de l’Univers THETA Franche-Comté-Bourgogne, Université de Franche Comté, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
e-mail: [email protected]
² Centre de Recherche Astrophysique de Lyon, CNRS UMR 5574, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Université de Nice Sophia-Antipolis, Observatoire de Côte d’Azur, Lagrange, CNRS UMR 7293, 06304 Nice Cedex 4, France
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago, Chile
⁶ Max Planck Institut fur Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany

Received: 21 October 2013
Accepted: 13 January 2014

Abstract

Context. M subdwarfs are metal-poor and cool stars. They are important probes of the old galactic populations. However, they remain elusive because of their low luminosity. Observational and modelling efforts are required to fully understand their physics and to investigate the effects of metallicity in their cool atmospheres.

Aims. We performed a detailed study of a sample of subdwarfs to determine their stellar parameters and constrain the state-of-the art atmospheric models.

Methods. We present UVES/VLT high-resolution spectra of three late-K subdwarfs and 18 M subdwarfs. Our atlas covers the optical region from 6400 Å to the near-infrared at 8900 Å. We show spectral details of cool atmospheres at very high-resolution (R ~ 40 000) and compare them with synthetic spectra computed from the recent BT-Settl atmosphere models.

Results. Our comparison shows that molecular features (TiO, VO, CaH), and atomic features (Fe I, Ti I, Na I, K I) are well-fitted by current models. We produce a relation of effective temperature versus spectral type over the entire subdwarf spectral sequence. The high-resolution of our spectra unabled us to perform a detailed comparison of the line profiles of individual elements such as Fe I, Ca II, and Ti I, and we determined accurate metallicities of these stars. These determinations in turn enabled us to calibrate the relation between metallicity and molecular-band strength indices from low-resolution spectra.

Conclusions. The new generation of models is able to reproduce various spectral features of M subdwarfs. These high-resolution spectra allowed us to separate the atmospheric parameters (effective temperature, gravity, metallicity), which is not possible when using low-resolution spectroscopy or photometry.