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

Abstract

Spectropolarimetric data allow for simultaneous monitoring of stellar chromospheric log R'HK activity and the surface-averaged longitudinal magnetic field, Bl, 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 Bl 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 |Bl|, but for G stars the relationship may undergo a change between log R'HK ~ -4.4 and -4.8. The mean log R'HK shows 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 |Bl|, or the Bl 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 asthe change in the relationships between chromospheric activity, activity variability, and mean field strength.