2007AJ....133..845W

We consider optical and near-infrared images of the edge-on disk surrounding the young stellar object HH 30 taken with the Hubble Space Telescope at 18 epochs from 1994 to 2005. These images allow us to study asymmetry and variability in the disk. The lateral brightness asymmetry in the upper nebula, first seen to strongly vary in 1998, continues to show significant variability throughout the period of our observations. The lateral asymmetry is not uniformly distributed between both sides of the disk; the upper nebula appears brighter on its north-northwest side at 12 epochs, nearly symmetric at four epochs, and brighter on its east-southeast side at only two epochs. This and other evidence indicate that the lateral asymmetry has both static and variable components. The lateral asymmetry shares the overall continuum color of the nebula and is weaker in emission lines than in the continuum. We have searched for periodicity in the sense of the lateral asymmetry. While some possible periods can be excluded, there is no convincing evidence for any specific period. We also consider the lower counternebula. It is as a whole less variable than the upper nebula and shows no significant lateral variability. We discuss several possible mechanisms that might explain these phenomena. Periodic illumination or shadowing models remain viable at periods less than 1 yr, but further observations are required to firmly establish them.