M Subdwarf Research III. Spectroscopic Diagnostics for Breaking Parameter Degeneracy

TL;DR

This paper develops spectroscopic diagnostics using spectral features in the 0.6-2.5 μm range to resolve parameter degeneracies in low-resolution M subdwarf spectra, enabling more accurate stellar parameter determination.

Contribution

It introduces a four-step process utilizing spectral indices to independently determine stellar parameters, extending previous methods and applicable at low spectral resolutions.

Findings

Spectral features vary distinctly with atmospheric parameters.

The diagnostic method is effective down to a spectral resolution of R~200.

The approach successfully breaks degeneracies in parameter estimation.

Abstract

To understand the parameter degeneracy of M subdwarf spectra at low resolution, we assemble a large number of spectral features in the wavelength range of 0.6-2.5 {\mu}m with band strength quantified by narrowband indices. Based on the index trends of BT-Settl model sequences, we illustrate how the main atmospheric parameters (Teff, log g, [M/H], and [alpha/Fe]) affect each spectral feature differently. Furthermore, we propose a four-step process to determine the four parameters sequentially, which extends the basic idea proposed by Jao et al. Each step contains several spectral features that break the degeneracy effect when determining a specific stellar parameter. Finally, the feasibility of each spectroscopic diagnostic with different spectral qualities is investigated. The result is resolution-independent down to R~200.