Observations of Chromospheric and Coronal Emission Lines in F Stars

The Princeton telescope and ultraviolet spectrograph on the Copernicus satellite has been used to detect chromospheric emission lines of Lyman α, Si III and O VI in the F5 IV star α CMi (Procyon). The interstellar absorption component of Lyman α leads to a H I column density of $$\sim 1 \cdot 6 \times 10^{17} \,\text{cm}^{-2}$$ and a value for the neutral hydrogen density in the solar neighbourhood of $$n_{H} = 0 \cdot 015 \,\text{cm}^{-3}.$$ Mg II h and k emission was detected in Procyon and in the Fo Ib star α Car (Canopus) and the observed widths have been used to evaluate the Wilson–Bappu correlation of line width with luminosity. The analysis of the emission line intensities in Procyon leads to two models of its atmosphere in the range $$10^{4} \,\text{to} \,3 \times 10^{5} \,\text{K},$$ for the limiting cases of high and low electron pressure. In the former case a transition zone (conduction dominated regime) sets in at 10⁵ K (cf. 3 × 10⁴ K in the Sun) indicating the existence of a corona whose temperature is deduced to lie in the range 3 × 10⁵ to 6 × 10⁶ K. In the latter case, the model represents a chromosphere over the temperature range observed.