Dramatic Evolution of the Disk-Shaped Secondary in the Orion Trapezium Star Theta1 Ori B1 (BM Ori): MOST Satellite Observations

Abstract

The eclipsing binary Theta1 Orionis B1, variable star designation BM Ori, is the faintest of the four well-known Trapezium stars at the heart of the Orion Nebula. The primary is a B3 star (~6Msun) but the nature of the secondary (~2Msun) has long been mysterious, since the duration and shape of primary eclipse are inappropriate for ordinary stars. Here we report nearly continuous photometric observations obtained with the MOST satellite over ~4 cycles of the 6.47d binary period. The light curve is of unprecedented quality, revealing a deep, symmetric primary eclipse as well as a clear reflection effect and secondary eclipse. In addition, there are other small disturbances, some of which repeat at the same phase over the four cycles monitored. The shape of the primary light curve has clearly evolved significantly over the past 40 yr. While its overall duration and depth have remained roughly constant, the slopes of the descent and ascent phases are significantly shallower now than in the past and its distinctive flat-bottomed "pseudo-totality" is much less obvious or even absent in the most recent data. We further demonstrate that the primary eclipse was detected at X-ray wavelengths during the Chandra COUP study. The light curve continues to be well modeled by a self-luminous and reflective disk-shaped object seen nearly edge-on orbiting the primary. The dramatic change in shape over four decades is modeled as an opacity variation in a tenuous outer envelope or disk of the secondary object. We presume that the secondary is an extremely young protostar at an earlier evolutionary phase than can be commonly observed elsewhere in the Galaxy and that the opacity variations observed are related to its digestion of some accreted matter over the last 50-100 yr. Indeed, this object deserves continued observational and theoretical attention as the youngest known eclipsing binary system.