YSOVAR: the first sensitive, wide-area, mid-IR photometric monitoring of
  the ONC
  We present initial results from time series imaging at infrared wavelengths
of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we
obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days.
We extracted light curves with ~3% photometric accuracy for ~2000 ONC members
ranging from several solar masses down to well below the hydrogen burning mass
limit. For many of the stars, we also have time-series photometry obtained at
optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined
to determine stellar rotation periods, identify new pre-main-sequence (PMS)
eclipsing binaries, search for new substellar Orion members, and help better
determine the frequency of circumstellar disks as a function of stellar mass in
the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron
variability information to improve our understanding of inner disk processes
and structure in the Class I and II young stellar objects (YSOs). In this
paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall
2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow
dips in flux, most probably due to disk density structures passing through our
line of sight. Detailed follow-up observations are needed in order to better
quantify the nature of the obscuring bodies and what this implies for the
structure of the inner disks of YSOs.
