The onset of chromospheric activity among the A- and F- type stars

Abstract

We have measured the W chromospheric emission lines of H I, C II, and C IV in a large number of IUE archival spectra of late A and early F dwarf stars. The measurements suggest there is a peak in main sequence chromospheric activity near B - V = 0.28., which corresponds approximately to spectral type FO. We confirm the presence of stellar Lya emission in the spectrum of the A7 V star αAg1, but find no emission in the spectra of 10 other late A-type stars.

The location along the main sequence where chromospheric and coronal activity begins provides a vital observational constraint upon models of activity, structure, and convection in stars. Existing observations, primarily IUE spectra of the C II 1335 A and C IV 1550 A emission lines and ground-based spectra of the He I 5876 A absorption line, place the onset of stellar chromospheric activity near a color B − V = 0.28, which corresponds approximately to spectral type FO and an effective temperature of 7300 K (Wolff, Boesgaard, and Simon 1986; Walter and Linsky 1986). X-ray observations of A and F stars on the other hand, suggest that coronal activity may occur significantly blueward of the current “chromospheric boundary line” before vanishing among the early and mid A-type stars (Schmitt et al. 1985). In an effort to more clearly define the location of the high temperature boundary for chromospheric emission, we have recently examined UV spectra from the IUE archives of 38 dwarf stars with B - V colors between 0.16 and 0.42, from which we have measured the strengths of the C IV, C II, and Lymana emissions. Here we discuss some preliminary results of that study.