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Summary: Mon. Not. R. Astron. Soc. 000, 000{000 (0000) Printed 14 July 1999 (MN L A T E X style le v1.4)
Photometric modelling of starspots I. A Barnes-Evans-like
surface brightness{colour relation using (I c
{K)
P.J. Amado 1 ? , C.J. Butler 1 and P.B. Byrne y
1 Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
14 July 1999
ABSTRACT
In the rst part of this work, the empirical correlation of stellar surface brightness
FV as a function of the (I c {K) broadband colour has been investigated by using a
sample of stars cooler than the Sun. A bilinear correlation is found to well represent the
brightness of G, K and M giant stars. The change in slope occurs at an (I c {K) of 2:1
or at about the transition from K to M spectral types. The same relationship was also
investigated for dwarf stars and found to be distinctly di erent from that of the giants.
Their correlation di ers by an average of 0:4 in (I c {K) or a maximum di erence in
FV 0:1, positioning it below that of the giants', with both trends tending to
convergence for the hotter stars in our sample. The
ux distribution derived from the
FV {(I c {K) relationship for the giant stars, together with that derived from a FV {
(V {K) relationship and the blackbody
ux distribution, is then utilized to compute
synthetic light V and colour (V {R) c , (V {I) c and (V {K) curves of cool spotted stars.
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