Tracers of Chromospheric Structure I: Observations of Ca II K and H alpha in M Dwarfs

TL;DR

This study investigates the relationship between Ca II K and Balmer lines in M dwarf stars to understand chromospheric temperature structures, revealing correlations and activity-dependent behaviors through simultaneous optical spectra and multi-wavelength data.

Contribution

It provides the first simultaneous spectral observations of Ca II and Balmer lines in M dwarfs, clarifying their correlation patterns and implications for chromospheric modeling.

Findings

Strong correlation between Ca II K and Balmer lines in active stars.

H alpha absorption varies with Ca II K emission, showing initial increase then filling in.

Positive correlation between chromospheric and coronal emissions across activity levels.

Abstract

We report on our observing program to capture simultaneous spectra of Ca II and Balmer lines in a sample of nearby M3 dwarfs. Our goal is to investigate the chromospheric temperature structure required to produce these lines at the observed levels. We find a strong positive correlation between instantaneous measurements of Ca II K and the Balmer lines in active stars, although these lines may not be positively correlated in time-resolved measurements. The relationship between H alpha and Ca II K remains ambiguous for weak and intermediate activity stars, with H alpha absorption corresponding to a range of Ca II K emission. A similar relationship is also observed between Ca II K and the higher order Balmer lines. As our sample consists of a single spectral type, correlations between these important chromospheric tracers cannot be ascribed to continuum effects, as suggested by other authors. These data confirm prior non-simultaneous observations of the H alpha line behavior with increasing activity, showing an initial increase in the H alpha absorption with increasing Ca II K emission, prior to H alpha filling in and eventually becoming a pure emission line in the most active stars. We also compare our optical measurements with archival UV and X-ray measurements, finding a positive correlation between the chromospheric and coronal emission for both high and intermediate activity stars. We compare our results with previous determinations of the active fraction of low mass stars, and discuss them in the context of surface inhomogeneity. Lastly, we discuss the application of these data as empirical constraints on new static models of quiescent M dwarf atmospheres.