Near-infrared narrow-band photometry of M-giant and Mira stars: models meet observations

TL;DR

This study combines near-infrared photometry and advanced atmospheric models to analyze M-giant and Mira stars, revealing phase lags and color variations that enhance understanding of stellar atmospheres.

Contribution

It provides the first detailed comparison of observed and modeled near-infrared colors of M-giants and Miras, confirming phase lag phenomena and improving spectral modeling techniques.

Findings

Normal M-giants colors match models for spectral types earlier than M7.

Mira stars show phase lag in molecular band-strength indices.

Propagation of atmospheric perturbations explains phase lag phenomena.

Abstract

From near-infrared, narrow-band photometry of 256 oxygen-rich Mira variables we obtain evidence about the loops that these stars follow in colour-colour diagrams. We also find a phase lag between indices related to molecular band-strength of titanium oxide and vanadium oxide. We compute colours for normal M-giants and Miras using hydrostatic and hydrodynamic model atmospheres and very extensive up-to-date line lists. Normal M-giants colours are well reproduced, reaching a high quantitative agreement with observations for spectral types earlier than M7. The out-of-phase variations of the various spectral features of Miras are also acceptably reproduced, despite limitations in the modelling. This enables us to confirm that the phase lag phenomenon results from the propagation of perturbations in the extended atmosphere. It opens new perspectives in the spectral modelling of Miras.