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Effect of chromospheric activity on the mean
colours of late­type stars
P. Amado and P.B. Byrne
A&A, accepted July 96
In this paper we show, for the first time, the effect of activity on the (U \Gamma B) o
vs. (B \Gamma V ) o
colour­colour diagram for a large sample of late type stars. we
clearly see that for single­lined spectroscopic RS CVn systems there exists an
ultraviolet excess over that recorded for quiescent stars of the same spectral
type. The same effect is seen in main­sequence stars but to a lesser extent. From
simple arguments we demonstrate that the cause is unlikely to be unresolved
flaring, the combined effect of strong chromospheric emission lines, or X­ray/UV
back heating of the photosphere. White light faculae are a resonable candidate
for the effect however. In our investigation we have not taken into account the
effect that metallicity might have on the colour indices.
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