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

V. Aarum Ulvås, O. Engvold
2003 Astronomy and Astrophysics  
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
more » ... cular areas and the portion of the stellar surface occupied by them.
doi:10.1051/0004-6361:20030085 fatcat:f5lm6opcpnesdgnl6zka7ockv4