The effective temperature scale of M dwarfs^⋆

¹ Institut UTINAM CNRS 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: arvind@obs-besancon.fr
² Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 7, France
³ Université de Nice Sophia-Antipolis, CNRS, Observatoire de Côte d’Azur, Laboratoire Cassiopée, 06304 Nice Cedex 4, France
⁴ Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia

Received: 22 February 2013
Accepted: 12 April 2013

Abstract

Context. Despite their large number in the Galaxy, M dwarfs remain elusive objects and the modeling of their photosphere has long remained a challenge (molecular opacities, dust cloud formation).

Aims. Our objectives are to validate the BT-Settl model atmospheres, update the M dwarf T_eff-spectral type relation, and find the atmospheric parameters of the stars in our sample.

Methods. We compare two samples of optical spectra covering the whole M dwarf sequence with the most recent BT-Settl synthetic spectra and use a χ² minimization technique to determine T_eff. The first sample consists of 97 low-resolution spectra obtained with New Technology Telescope (NTT) at La Silla Observatory. The second sample contains 55 medium-resolution spectra obtained at the Siding Spring Observatory (SSO). The spectral typing is realized by comparison with already classified M dwarfs.

Results. We show that the BT-Settl synthetic spectra reproduce the slope of the spectral energy distribution and most of its features. Only the CaOH band at 5570 Å and AlH and NaH hydrides in the blue part of the spectra are still missing in the models. The T_eff scale obtained with the higher resolved SSO 2.3 m spectra is consistent with that obtained with the NTT spectra. We compare our T_eff scale with those of other authors and with published isochrones using the BT-Settl colors. We also present relations between effective temperature, spectral type, and colors of the M dwarfs.