A STUDY OF ACTIVITY IN F-TYPE MAIN-SEQUENCE STARS USING THE D3 LINE OF HEI

We have observed the He I D3 lambda 5876 angstrom line as a chromosphe ric activity indicator in 36 stars (34 F-type and 2 G-type) belonging to the Hyades open cluster, the Ursa Major group and the field. Comple menting our data with measurements taken from the literature, we have correlated the equivalent width and absorbed energy flux of the He I l ine with other chromospheric and coronal indicators across the F-K ran ge of spectral classes. We have confirmed that D3 is a particularly su itable chromospheric activity indicator for the early F range, and hav e used it to set limits to the onset of chromospheres (B - V approxima tely 0.3) and the onset of magnetic activity (B - V approximately 0.42 ), as well as to assess the kinds of mechanisms which heat chromospher es in that range, finding acoustic waves to be the most probable domin ant energy source.