The Ultraviolet Light Curves of W Ursae Majoris

The present study examines the controversy that has surrounded explanations of W-type W Ursae Majoris light curves. It is suggested on the basis of the association of faculae with sunspots that a similar association may exist for starspots. The enhanced UV contrast betweeen solar faculae and the surrounding photosphere found by Lean (1989, 1991) implies a possible physical basis to modify theoretical UV light curves of W Ursae Majoris. A simple simulation is presented to test whether the modification might explain the failure of a hot secondary Roche model, which provides an excellent fit to I, V, and B data, might fit equally deep eclipses of W Ursae Majoris in the UV. The simulation showed that light curves at 250 and 220 nm, from Astronomical Netherlands Satellite (ANS) data by EWR, cannot be represented by an unmodified Roche model. No physical continuity is found between an ad hoc model roughly representing the ANS data and a Roche model providing an excellent fit to I, V, and B data in a separate study by Linnell (1991). A connection does exist between the two data sets in that the U data in the separate study shows a departure from the Roche model fit in the direction of the ANS data.