Abstract
Chromospheric activity in late A- and early F-type field stars of luminosity classes III through V has been investigated using the helium D3 absorption feature. This feature shows a detection boundary near b - y = 0.19 (B - V = 0.29). This color index corresponds to a dividing line in activity levels as determined from the C II λ1335 chromospheric emission line. On the red side of this boundary, stars exhibit strong or moderately strong C II emission and strong or moderately strong D3 absorption. However, on the blue side, D3 absorption does not conclusively appear, while several stars show moderately strong C II emission. The data suggest that D3 is sensitive to the boundary at B - V = 0.29, but they also suggest limitations in the use of D3 as an activity indicator in the late A-type stars. To within observational errors, the D3 boundary appears at the same color index for the full range of luminosity classes explored, in contradiction with some acoustic energy calculations. In addition, the strength of D3 absorption shows no significant trend with luminosity class or the Stromgren δc1 index, with a wide range of activity levels at a given luminosity or surface gravity.
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