The BP Tau disk: A missing link between Class II and III objects?

We present new single-dish (30-m) observations and an improved analysis of our earlier interferometric observations (Simon et al. 2000) of BP Tau*. Our analysis yields a detailed description of the properties of the circumstellar disk. The disk is small (outer radius 120 AU) and marginally optically thick in the (CO)-C-12 J = 2 --> 1 line. It is also relatively hot, about 50 K at 100 AU. The mm continuum emission is consistent with a thermal emission from circumstellar dust with an emissivity index beta similar or equal to 0.7, and a disk mass about 1.2 x 10(-3) M.. The anomalously low (CO)-C-12 to dust emission ratio implies that either CO is depleted by a factor of the order of 150 with respect to H-2, or has a very low gas to dust ratio, or highly anomalous dust properties. The disk exhibits direct evidence of Keplerian rotation, but because of a combination of insufficient resolution and optically thin CO emission, the mass derived for the star remains inaccurate (between 0.6 and 1.3 M. for 140 pc). The unusual properties of the circumstellar disk suggest that BP Tau may be a transient object in the process of clearing its disk.