High Contrast Infrared Imaging and Polarimetry of Dusty Disks around Herbig Ae/Be Stars

Circumstellar disks are now known to be very common around young stars,
but actually imaging these disks remains a formidable challenge. The technical
difficulties are twofold: First, one must have sufficiently fine angular resolution
to separate the disk from the star. Adaptive optics can now routinely provide
subarcsecond resolution in the near-IR, and laser guide star AO now provides
this capability even for faint sources embedded in dusty regions.
The second challenge is achieving high enough contrast to detect faint
circumstellar material in the presence of much brighter starlight.
One powerful technique for high contrast is differential polarimetry,
which can reject unpolarized starlight and thus reveal faint polarized light
scattered from circumstellar dust. An alternate approach is to change the rules
of the game, by moving to a different wavelength at which the star is fainter
and the dust is brighter, such as the mid-infrared. In fact these two approaches
are complimentary, with near-IR polarimetry tracing dust in scattered light while
mid-IR imaging traces thermal emission. I have employed these two techniques
to survey 110 Herbig Ae/Be stars (pre-main-sequence stars of intermediate mass),
resolving extended circumstellar dust around 44 of them. The observed dust
geometries are highly varied, including circumstellar disks, bipolar envelopes,
and complex asymmetric nebulae. In this talk I will briefly describe
the AO differential polarimeter I developed at Lick Observatory,
summarize the overall results of my near-IR and mid-IR survey of Herbig stars,
and discuss in depth a few of the most interesting (and most puzzling!)
of the observed sources.