Mg and TiO spectral features at the near-IR: Spectrophotometric index definitions and empirical calibrations

TL;DR

This study defines new near-infrared spectroscopic indices for Mg I and TiO features, analyzes their dependence on stellar parameters, and provides empirical calibrations to estimate stellar properties and chemical abundances.

Contribution

It introduces novel spectroscopic indices for Mg I and TiO in the near-IR, characterizes their sensitivities, and offers empirical calibration functions for stellar parameters.

Findings

MgI index is a good indicator of Mg abundance.

sTiO index effectively discriminates dwarf and giant stars.

Empirical calibration functions are provided for stellar parameters.

Abstract

Using the near-infrared spectral stellar library of Cenarro et al. (2001a,b), the behaviours of the Mg I line at 8807 angstrom and nearby TiO bands are analyzed in terms of the effective temperature, surface gravity, and metallicity of the library stars. New spectroscopic indices for both spectral features -namely MgI and sTiO- are defined, and their sensitivities to different signal-to-noise ratios, spectral resolutions, flux calibrations, and sky emission line residuals are characterized. The new two indices exhibit interesting properties. In particular, MgI reveals as a good indicator of the Mg abundance, whereas sTiO is a powerful dwarf-to-giant discriminator for cold spectral types. Empirical fitting polynomials that reproduce the strength of the new indices as a function of the stellar atmospheric parameters are computed, and a FORTRAN routine with the fitting functions predictions is made available. A thorough study of several error sources, non-solar [Mg/Fe] ratios, and their influence on the fitting function residuals is also presented. From this analysis, a [Mg/Fe] underabundance of ~ -0.04 is derived for the Galactic open cluster M67.