Spectral energy distribution of M-subdwarfs: A study of their atmospheric properties^⋆

¹ Astronomy & Astrophysics DivisionPhysical Research Laboratory, 380009 Ahmedabad, India
e-mail: arvindr@prl.res.in
² Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ Institut UTINAM CNRS 6213, Observatoire des Sciences de l’Univers THETA Franche-Comté Bourgogne, Univ. Bourgogne Franche-Comté, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
⁴ Univ Lyon, ENS de Lyon, Univ Lyon1, CNRS, Centre de Recherche Astrophysique de Lyon UMR 5574, 69007 Lyon, France
⁵ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, 5030 Casilla, Valparaíso, Chile
⁷ Clausthal University of Technology, Institute for Theoretical Physics, Leibnizstr. 10, 38678 Clausthal-Zellerfeld, Germany

Received: 18 June 2015
Accepted: 22 September 2016

Abstract

Context. M-type subdwarfs are metal-poor low-mass stars and are probes for the old populations in our Galaxy. Accurate knowledge of their atmospheric parameters and especially their composition is essential for understanding the chemical history of our Galaxy.

Aims. The purpose of this work is to perform a detailed study of M-subdwarf spectra covering the full wavelength range from the optical to the near-infrared. It allows us to perform a more detailed analysis of the atmospheric composition in order to determine the stellar parameters, and to constrain the atmospheric models. The study will allow us to further understand physical and chemical processes such as increasing condensation of gas into dust, to point out the missing continuum opacities, and to see how the main band features are reproduced by the models. The spectral resolution and the large wavelength coverage used is a unique combination that can constrain the processes that occur in a cool atmosphere.

Methods. We obtained medium-resolution spectra (R = 5000−7000) over the wavelength range 0.3−2.5 μm of ten M-type subdwarfs with X-shooter at VLT. These data constitute a unique atlas of M-subdwarfs from optical to near-infrared. We performed a spectral synthesis analysis using a full grid of synthetic spectra computed from BT-Settl models and obtained consistent stellar parameters such as effective temperature, surface gravity, and metallicity.

Results. We show that state-of the-art atmospheric models correctly represent the overall shape of their spectral energy distribution, as well as atomic and molecular line profiles both in the optical and near-infrared. We find that the actual fitted gravities of almost all our sample are consistent with old objects, except for LHS 73 where it is found to be surprisingly low.