# Estimates of Active Region Area Coverage through Simultaneous Measurements of He I $\lambda\lambda$ 5876 and 10830 Lines

## TL;DR

This study measures the active region coverage in solar-type stars using helium triplet lines, revealing that most stars have less than full coverage and that activity differences are mainly due to the area coverage of active regions.

## Contribution

It introduces a method to estimate active region filling factors in stars using simultaneous helium line measurements and chromospheric models, highlighting the role of area coverage in stellar activity.

## Key findings

- Most stars have filling factors less than one.
- Discrepancies in K-type and F-type stars are due to model limitations and measurement uncertainties.
- Active region heating rates are similar across different stars.

## Abstract

Simultaneous, high-quality measurements of the neutral helium triplet features at 5876~\AA\ and 10830~\AA, respectively, in a sample of solar-type stars are presented. The observations were made with ESO telescopes at the La Silla Paranal Observatory under program ID 088.D-0028(A) and MPG Utility Run for FEROS 088.A-9029(A). The equivalent widths of these features combined with chromospheric models are utilized to infer the fractional area coverage, or filling factor, of magnetic regions outside of spots. We find that the majority of the sample is characterized by filling factors less than unity. However, discrepancies occur among the coolest K-type and warmest and most rapidly rotating F-type dwarf stars. We discuss these apparently anomalous results and find that in the case of K-type stars they are an artifact of the application of chromospheric models best suited to the Sun than to stars with significantly lower $T_\mathrm{eff}$. The case of the F-type rapid rotators can be explained with the measurement uncertainties of the equivalent widths, but they may also be due to a non-magnetic heating component in their atmospheres. With the exceptions noted above, preliminary results suggest that the average heating rates in the active regions are the same from one star to the other, differing in the spatially integrated, observed level of activity due to the area coverage. Hence, differences in activity in this sample are mainly due to the filling factor of active regions.