M giants with IGRINS. I. Stellar parameters and α-abundance trends of the solar neighborhood population

Abstract

Context. Cool stars, such as M giants, can only be analyzed in the near-infrared (NIR) regime due to the ubiquitous titanium oxide features in optical spectra of stars with T_eff < 4000 K. In dust-obscured regions, the inner bulge and Galactic center region, the intrinsically bright M giants observed in the NIR are an optimal option for studying stellar abundances and the chemical evolution of stellar populations. Because of the uncertainties in photometric methods, a method for determining the stellar parameters for M giants from the NIR spectra themselves is needed.
Aims: We develop a method for determining the stellar parameters for M giants from the NIR spectra. We validate the method by deriving the stellar parameters for nearby well-studied M giants with spectra from the spectral library of the Immersion GRating INfrared Spectrograph (IGRINS). We demonstrate the accuracy and precision of our method by determining the stellar parameters and α-element trends versus metallicity for solar neighborhood M giants.
Methods: We carried out new observations of 44 M giant stars with IGRINS mounted on the Gemini South telescope. We also obtained the full H and K band IGRINS spectra of six nearby well-studied M giants at a spectral resolving power of R = 45 000 from the IGRINS spectral library. We used the tool called spectroscopy made easy in combination with one-dimensional (1D) model atmospheres in a radiative and convective scheme (MARCS) stellar atmosphere models to model the synthetic spectrum that fits the observed spectrum best.
Results: The effective temperatures that we derive from our new method (tested for 3400 ≲ T_eff ≲ 4000 K here) agree excellently with those of the six nearby well-studied M giants, which indicates that the accuracy is indeed high. For the 43 solar neighborhood M giants, our T_eff, log g, [Fe/H], ξ_micro, [C/Fe], [N/Fe], and [O/Fe] agree with APOGEE with mean differences and a scatter (our method - APOGEE) of −67±33 K, −0.31±0.15 dex, 0.02±0.05 dex, 0.22±0.13 km s⁻¹, −0.05±0.06 dex, 0.06±0.06 dex, and 0.02±0.09 dex, respectively. Furthermore, the tight offset with a small dispersion compared to the APOGEE T_eff indicates a high precision in our derived temperatures and those derived from the APOGEE pipeline. The typical uncertainties in the stellar parameters are found to be ±100 K in T_eff, ±0.2 dex in log g, ±0.1 dex in [Fe/H], and ±0.1 km s⁻¹ in ξ_micro. The α-element trends versus metallicity for Mg, Si, Ca, and Ti are consistent with the APOGEE DR17 trends for the same stars and with the GILD optical trends. We also find a clear enhancement in the abundances for thick-disk stars.

Data table is only available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/675/A23