Analysis of CCD Photometry for the Very Short Period Eclipsing Binary BM Ursae Majoris

CCD images of the fourteenth-magnitude variable BM Ursae Majoris were taken in the spring of 1991 at Lowell Observatory. An RCA CCD camera system and a standard PMT were used. Four times of minimum light were determined and improved linear and quadratic ephemerides were calculated. A period study, spanning over a thirty-year interval, shows that the system is undergoing a small but continuous period decrease of ~ 5×10⁻ɸ d/yr. This may be due to angular momentum loss (AML) caused by stellar winds. The VRI light curves formed from the present precision observations, show that BM UMa is a W-type W UMa system. The first synthetic light curve solution of BM UMa is also presented. This solution reveals that BM UMa is a contact binary consisting of two early-K spectral-type components with a fill out of ~20% and a mass ratio of ~0.5. A substantial temperature difference of ~400 was determined. Both unspotted and spotted models were calculated. The spotted model indicates the presence of a region of enhanced brightness in the neck of the secondary component. This may be attributed to fluid dynamics of mass in transit rather than to magnetic activity.