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

TL;DR

This study presents the first detailed chemical abundance analysis of exoplanet-hosting M-dwarfs Kepler-138 and Kepler-186 using high-resolution APOGEE spectra, revealing their metallicities, elemental compositions, and potential implications for planet formation.

Contribution

It demonstrates that APOGEE spectra can be effectively used to determine detailed chemical compositions of M-dwarfs, including multiple elements, with improved accuracy over previous methods.

Findings

Both stars have modest sub-solar metallicities around -0.09 and -0.08 dex.

Measured metallicities are higher by 0.1-0.2 dex than previous estimates.

C/O ratios are near-solar, with Kepler-186 showing a marginally enhanced [Si/Fe].

Abstract

We report the first detailed chemical abundance analysis of the exoplanet-hosting M-dwarf stars Kepler-138 and Kepler-186 from the analysis of high-resolution ($R$ $\sim$ 22,500) $H$-band spectra from the SDSS IV - APOGEE survey. Chemical abundances of thirteen elements - C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe - are extracted from the APOGEE spectra of these early M-dwarfs via spectrum syntheses computed with an improved line list that takes into account H$_{2}$O and FeH lines. This paper demonstrates that APOGEE spectra can be analyzed to determine detailed chemical compositions of M-dwarfs. Both exoplanet-hosting M-dwarfs display modest sub-solar metallicities: [Fe/H]$_{Kepler-138}$ = -0.09 $\pm$ 0.09 dex and [Fe/H]$_{Kepler-186}$ = -0.08 $\pm$ 0.10 dex. The measured metallicities resulting from this high-resolution analysis are found to be higher by $\sim$0.1-0.2 dex than previous estimates from lower-resolution spectra. The C/O ratios obtained for the two planet-hosting stars are near-solar, with values of 0.55 $\pm$ 0.10 for Kepler-138 and 0.52 $\pm$ 0.12 for Kepler-186. Kepler-186 exhibits a marginally enhanced [Si/Fe] ratio.