Occultation by a protoplanetary clump in the young abrupt variable GM Cepheid

GM Cephei (GM Cep) is an active T Tauri star in the young (∼ 4 Myr) open cluster Trumpler 37, showing abrupt UX Orionis type of photometric variability. Chen et al. (2012) reported frequent, sporadic brightening events, each of < 0.5 mag and lasting for days, which must have been originated from circumstellar accretion. In addition, the star also showed a brightness drop of ∼ 1 mag lasting continuously for about a month, during which time the star became bluer when fainter. Moreover, the brightness drops seem to have a recurrence timescale of a year. It is proposed that the drop is the consequence of obscuration of the central star by an orbiting dust concentration, exemplifying disk inhomogeneity in transition between grain coagulation and planetesimal formation in a young circumstellar disk. The star was found to exhibit a few percent polarization in the optical wavelengths, and an enhanced level of polarization during the occultation.