A POSSIBLE DETECTION OF OCCULTATION BY A PROTO-PLANETARY CLUMP IN GM Cephei

Abstract

GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an\nabrupt variable and to have a circumstellar disk with very active accretion.\nOur monitoring observations in 2009-2011 revealed the star to show sporadic\nflare events, each with brightening of < 0.5 mag lasting for days. These\nbrightening events, associated with a color change toward the blue, should\noriginate from an increased accretion activity. Moreover, the star also\nunderwent a brightness drop of ~1 mag lasting for about a month, during which\nthe star became bluer when fainter. Such brightness drops seem to have a\nrecurrence time scale of a year, as evidenced in our data and the photometric\nbehavior of GM Cep over a century. Between consecutive drops, the star\nbrightened gradually by about 1 mag and became blue at peak luminosity. We\npropose that the drop is caused by obscuration of the central star by an\norbiting dust concentration. The UX Orionis type of activity in GM Cep\ntherefore exemplifies the disk inhomogeneity process in transition between\ngrain coagulation and planetesimal formation in a young circumstellar disk.\n