Stellar parameters of early-M dwarfs from ratios of spectral features at optical wavelengths^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF−Osservatorio Astronomico di Palermo, Piazza Parlamento 1, 90134 Palermo, Italy
e-mail: jmaldonado@astropa.inaf.it
² INAF−Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ INAF−Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ INAF−Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
⁵ INAF−Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ INAF−Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
⁷ INAF−Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Naples, Italy
⁸ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
⁹ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹¹ INAF−IASF Milano, via Bassini 15, 20133 Milano, Italy
¹² Institut de Ciéncies de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ciéncies, Torre C5 parell, 2a planta, 08193 Bellaterra, Spain
¹³ Dip. di Fisica e Astronomia Galileo Galilei − Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹⁴ Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA
¹⁵ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK

Received: 3 February 2015
Accepted: 10 March 2015

Abstract

Context. Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. As a consequence, Doppler search programmes using high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. However, determining the stellar parameters of M dwarfs using optical spectra has proven to be challenging.

Aims. We aim to calibrate empirical relationships to determine accurate stellar parameters for early-M dwarfs (spectral types M0-M4.5) using the same spectra as those that are used for radial velocity determinations, without the necessity of acquiring IR spectra or relying on atmospheric models and/or photometric calibrations.

Methods. Our methodology consists of using ratios of pseudo-equivalent widths of spectral features as a temperature diagnostic, a technique frequently used in solar-type stars. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are currently being observed in the framework of the HARPS GTO search for extrasolar planets. The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters.

Results. A long list of spectral features in the optical spectra of early-M dwarfs was identified. This list shows that the pseudo-equivalent width of roughly 43% of the features is strongly anticorrelated with the effective temperature. The correlation with the stellar metallicity is weaker. A total of 112 temperature sensitive ratios were identified and calibrated over the range 3100−3950 K, providing effective temperatures with typical uncertainties of about 70 K. Eighty-two ratios of pseudo-equivalent widths of features were calibrated to derive spectral types within 0.5 subtypes for stars with spectral types between K7V and M4.5V. We calibrated 696 combinations of the pseudo-equivalent widths of individual features and temperature-sensitive ratios for the stellar metallicity over a metallicity range from −0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07−0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity. These parameters depend on the stellar metallicity. For a given effective temperature, lower metallicities predict lower masses and radii as well as higher gravities.