Abstract
Context. Different relationships between the Hα and Ca II chromospheric emissions have been reported in solar-type stars. In particular, the time-series of emissions in these two lines are clearly anti-correlated for a few percent of the stars, contrary to what is observed on the Sun. Aims. Our objective is to characterise these relationships in more detail using complementary criteria, and to constrain the properties of filaments and plages that are necessary to explain the observations. Methods. We analysed the average level and variability of the Hα and Ca II emission for 441 F-G-K stars, paying particular attention to their (anti-)correlations on both short and long timescales. We also computed synthetic Hα and Ca II time-series for different assumptions of plage and filament properties and compared them with the observations. Results. We were not able to find plage properties that, alone, are sufficient to reproduce the observations at all timescales simultaneously, even when allowing different Hα and Ca II emission relationships for different stars. We also specified the complex and surprising relationship between the average activity levels of both indexes, in particular for low-activity stars. Conclusions. We conclude that plages alone are unlikely to explain the observed variety of relationships between Ca II and Hα emission, and that the presence of other phenomena like filaments may help to reconcile the models with observations.
Highlights
Chromospheric emission is a widely used indicator of stellar activity, especially based on the emission in the core of Ca II H and K lines (e.g. Baliunas et al 1995; Radick et al 1998, 2018; Lockwood et al 2007; Hall et al 2009)
We identified the first signalto-noise ratio (S/N) bin for which the standard deviation was lower than σ30 and the first S/N bin for which the average was smaller than m30 + 3σ30, and took the highest of these S/N values
There are more stars that show significant Ca II variability than stars showing significant variability in Hα, which may be due to a larger S/N for Ca II: there are 14 stars (9%) showing significant variability in Ca II (
Summary
Chromospheric emission is a widely used indicator of stellar activity, especially based on the emission in the core of Ca II H and K lines (e.g. Baliunas et al 1995; Radick et al 1998, 2018; Lockwood et al 2007; Hall et al 2009). Marchenko et al (2021) found that the other Balmer lines, such as Hβ, Hγ, and Hδ, do not trace the usual chromospheric activity indicators as well as expected on rotational timescales, possibly being more affected by photospheric variability due to spots This was confirmed by Cincunegui et al (2007) who used simultaneous observations and a better temporal sampling, and found a correlation between averaged (overall values for each star) indexes in a large sample of F-G-K stars.
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