Linking chromospheric activity and magnetic field properties for late-type dwarf stars

Abstract

Spectropolarimetric data allow for simultaneous monitoring of stellar chromospheric log 𝑅HK activity and the surface-averaged longitudinal magnetic field, 𝐵 𝑙 , giving the opportunity to probe the relationship between large-scale stellar magnetic fields and chromospheric manifestations of magnetism. We present log 𝑅 HK and/or 𝐵 𝑙 measurements for 954 mid-F to mid-M stars derived from spectropolarimetric observations contained within the PolarBase database. Our magnetically active sample complements previous stellar activity surveys that focus on inactive planet-search targets. We find a positive correlation between mean log 𝑅 HK and mean log |𝐵 𝑙 |, but for G stars the relationship may undergo a change between log 𝑅 HK ∼ −4.4 and −4.8. The mean log 𝑅 HKshows a similar change with respect to the log 𝑅 HK variability amplitude for intermediately-active G stars. We also combine our results with archival chromospheric activity data and published observations of large-scale magnetic field geometries derived using Zeeman Doppler Imaging. The chromospheric activity data indicate a slight under-density of late-F to early-K stars with −4.75 ≤ log 𝑅 HK ≤ −4.5. This is not as prominent as the original Vaughan-Preston gap, and we do not detect similar underpopulated regions in the distributions of the mean |𝐵 𝑙 |, or the 𝐵 𝑙 and log 𝑅 HK variability amplitudes. Chromospheric activity, activity variability and toroidal field strength decrease on the main sequence as rotation slows. For G stars, the disappearance of dominant toroidal fields occurs at a similar chromospheric activity level as the change in the relationships between chromospheric activity, activity variability and mean field strength.