Metallicity determination of M dwarfs

Abstract

Context. Several new techniques to determine the metallicity of M dwarfs with better precision have been developed over the last decades. However, most of these studies were based on empirical methods. In order to enable detailed abundance analysis, standard methods established for warmer solar-like stars, i.e. model-dependent methods using fitting of synthetic spectra, still need to be used. Aims. In this work we continue the reliability confirmation and development of metallicity determinations of M dwarfs using highresolution infrared spectra. The reliability was confirmed through analysis of M dwarfs in four binary systems with FGK dwarf companions and by comparison with previous optical studies of the FGK dwarfs. Methods. The metallicity determination was based on spectra taken in the J band (1.1-1.4 µm) with the CRIRES spectrograph. In this part of the infrared, the density of stellar molecular lines is limited, reducing the amount of blends with atomic lines enabling an accurate continuum placement. Lines of several atomic species were used to determine the stellar metallicity. Results. All binaries show excellent agreement between the derived metallicity of the M dwarf and its binary companion. Our results are also in good agreement with values found in the literature. Furthermore, we propose an alternative way to determine the effective temperature of M dwarfs of spectral types later than M2 through synthetic spectral fitting of the FeH lines in our observed spectra. Conclusions. We have confirmed that a reliable metallicity determination of M dwarfs can be achieved using high-resolution infrared spectroscopy. We also note that metallicites obtained with photometric metallicity calibrations available for M dwarfs only partly agree with the results we obtain from high-resolution spectroscopy.

Chemical Abundances of M-Dwarfs from the Apogee Survey. I. The Exoplanet Hosting Stars Kepler-138 and Kepler-186

“…As previously mentioned, the APOGEE line list does not include any transitions of iron hydride, an important contributor to the spectra of M-dwarfs (Önehag et al 2012;Lindgren et al 2016). It was clear from our test syntheses that the DR13 line list could not match well several features in the observed spectra of the target stars.…”

“…To produce detailed elemental abundance results for M-dwarfs that rival those for F-G-K dwarfs, it is useful to shift the analysis to the near-infrared (near-IR) part of the spectrum (λ∼1.1-2.5 μm), where both the strength and density of molecular absorption features drops relative to optical wavelengths, especially for the warmer M-dwarfs. Recent highresolution analyses in the J-and K-bands have derived metallicities (Önehag et al 2012;Lindgren et al 2016) and C/O ratios (Tsuji & Nakajima 2014, 2016Tsuji et al 2015) in M-dwarfs.…”

A Physically Motivated and Empirically Calibrated Method to Measure the Effective Temperature, Metallicity, and Ti Abundance of M Dwarfs

“…Tsuji & Nakajima (2014) and Tsuji et al (2015) measured C and O abundances of M dwarfs by comparing the equivalent widths of blended CO and H 2 O lines in high-resolution K-band spectra of M dwarfs to their Unified Cloudy Models. Önehag et al (2012), Lindgren et al (2016), and Lindgren & Heiter (2017) utilized MARCS models (Gustafsson et al 2008) and the Spectroscopy Made Easy (SME; Valenti & Piskunov 1996;Piskunov & Valenti 2017) spectral synthesis code to infer M dwarf effective temperatures and metallicities to a precision of 100 K and 0.05 dex, respectively. Souto et al (2017) used MARCS models and the turbospectrum code (Alvarez & Plez 1998;Plez 2012) to synthesize SDSS APOGEE spectra of two planet-hosting, early-M dwarfs (Kepler-138 and Kepler-186) and measured chemical abundances for 13 elements with a precision of the order of 0.1 dex.…”

“…The overall metallicities derived by Lindgren et al (2016) and Lindgren & Heiter (2017) dex difference) between metallicities measured independently from either component. Önehag et al (2012), Lindgren et al (2016), and Lindgren & Heiter (2017) did not fit for individual elemental abundances, so the accuracy of their methods for detailed chemical analysis is unknown.…”

“…Önehag et al (2012), Lindgren et al (2016), and Lindgren & Heiter (2017) did not fit for individual elemental abundances, so the accuracy of their methods for detailed chemical analysis is unknown. Furthermore, they found discrepancies between temperatures derived through their spectral synthesis and those derived from empirical calibrations.…”