Stellar parameters of early-M dwarfs from ratios of spectral features at optical wavelengths

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 pseudoequivalent 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.

Fundamental M-dwarf parameters from high-resolution spectra using PHOENIX ACES models

“…They showed that the calibration of Casagrande et al (2008) underestimates temperatures because Casagrande et al (2008) assumed M dwarfs to be blackbodies, but M dwarfs have more flux in the infrared than predicted for a blackbody. For metallicities, however, Maldonado et al (2015) reported good agreement of their results with Neves et al (2012) and Neves et al (2014).…”

“…Their results in metallicity agree excellently well with those of Rojas-Ayala et al (2012), with mean differences of 0.03 dex. A recent work by Maldonado et al (2015) used pseudo-equivalent widths of optical spectra to determine effective temperature and [Fe/H]. They showed that the calibration of Casagrande et al (2008) underestimates temperatures because Casagrande et al (2008) assumed M dwarfs to be blackbodies, but M dwarfs have more flux in the infrared than predicted for a blackbody.…”

The HADES RV Programme with HARPS-N at TNG

“…We used the stellar parameters published by Maldonado et al (2017), which were calculated applying the empirical relations by Maldonado et al (2015) on the same HARPS-N spectra from which we derived the RV time series. This technique calculates stellar temperatures from ratios of pseudo-equivalent widths of spectral features, and calibrate the metallicity on combinations and ratios of different features.…”

“…This technique calculates stellar temperatures from ratios of pseudo-equivalent widths of spectral features, and calibrate the metallicity on combinations and ratios of different features. Although such techniques are mainly used for solar-type stars, Maldonado et al (2015) proved them to be equally effective for low-mass stars. Howard et al (2014) derived a rotational period of 43.82 ± 0.56 d, both from their Keck-HIRES measurements of the S HK index and their APT photometric observations at Fairborn Observatory.…”

HADES RV Programme with HARPS-N at TNG

“…The data analyzed in this paper have been collected during 7 semesters, from September 2012 to February 2016. In Maldonado et al (2016) we have measured the stellar parameters using the methodology described in Maldonado et al (2015). The most relevant stellar parameters for the current analysis are listed in Table 1.…”