A&A 399, L11-L14 (2003)
DOI: 10.1051/0004-6361:20030085

Letter

Why does the spotted UX Ari system get bluer as it gets darker?

V. Aarum Ulvås and O. Engvold

Institute of Theoretical Astrophysics, PO Box 1029 Blindern, 0315 Oslo, Norway
(Received 2 October 2002 / Accepted 12 January 2003)

Abstract
We present simplified calculations of UX Ari B-V colour index and V magnitude for a range of parameter values. The results of our calculations are compared to previously published measurements. We find that our calculations reproduce the well established colour-brightness relation ( UX Ari gets bluer as it gets darker) only if the dark spots on the surface of the K star are accompanied by bright facular areas. There is currently an ambiguity between the temperature of the facular areas and the portion of the stellar surface occupied by them.

Key words: stars: activity -- binaries: spectroscopic -- stars: individual: UX Ari -- stars: late-type -- starspots

Offprint request: V. Aarum Ulvås, v.a.ulvas@astro.uio.no

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