Variability of the Great Disk Shadow in Serpens
  We present multi-epoch Hubble Space Telescope imaging of the Great Disk
Shadow in the Serpens star-forming region. The near-infrared images show strong
variability of the disk shadow, revealing dynamics of the inner disk on time
scales of months. The Great Shadow is projected onto the Serpens reflection
nebula by an unresolved protoplanetary disk surrounding the young
intermediate-mass star SVS2/CK3/EC82. Since the shadow extends out to a
distance of at least 17,000 au, corresponding to a light travel time of 0.24
years, the images may reveal detailed changes in the disk scale height and
position angle on time scales as short as a day, corresponding to the angular
resolution of the images, and up to the 1.11 year span between two observing
epochs. We present a basic retrieval of temporal changes in the disk density
structure, based on the images. We find that the inner disk changes position
angle on time scales of months, and that the change is not axisymmetric,
suggesting the presence of a non-axisymmetric dynamical forcing on $\sim$1\,au
size scales. We consider two different scenarios, one in which a quadrupolar
disk warp orbits the central star, and one in which an unequal-mass binary
orbiting out of the disk plane displaces the photo-center relative to the
shadowing disk. Continued space-based monitoring of the Serpens Disk Shadow is
required to distinguish between these scenarios, and could provide unique, and
detailed, insight into the dynamics of inner protoplanetary disks not available
through other means.
