Chromospheric Variability in Early F-type Stars

TL;DR

This study investigates chromospheric activity variability in early F-type stars using helium D3 line measurements, revealing short-term fluctuations likely caused by global processes or numerous small active regions, unlike the solar-like activity in late F-types.

Contribution

It provides the first detailed analysis of chromospheric variability in early F-type stars, highlighting differences from late F-types and suggesting alternative mechanisms for activity changes.

Findings

Early F-type stars show short-term variability not caused by rotational modulation.

Late F-type stars exhibit solar-like activity patterns.

Variability does not account for the wide activity range in early F-type stars.

Abstract

Using precise measurements of the helium D3 line, we have searched for statistically significant variations in the strength of chromospheric activity in 13 early F-type stars and two late F-type stars. In two early F-type stars, we find short-term (hours to days) variability based on ~25 observations over the course of a week. In an additional two cases we find significant differences between observations taken years apart, but we can most likely explain this apparent long-term variation as an artifact of probable short-term variations. The evidence suggests that pure rotational modulation of discrete active regions is not responsible for the short-term variations in the early F-type stars and that either a more global process is at work, or we are seeing large number of small active regions spread across the star. In contrast, the two late F-type stars in the sample show strength and/or wavelength variations that are consistent with "solar-type" activity typified by the rotational modulation of active regions. Our results suggest that variability does not cause the wide range in activity levels observed within the early F-type stars.