Matryoshka Holes: Nested Emission Rings in the Transitional Disk Oph IRS
  48
  The processes that form transition disks - disks with depleted inner regions
- are not well understood; possible scenarios include planet formation, grain
growth and photoevaporation. Disks with spatially resolved dust holes are rare,
but, in general, even less is known about the gas structure. The disk
surrounding A0 star Oph IRS 48 in the nearby Rho Ophiuchus region has a 30 AU
radius hole previously detected in the 18.7 micron dust continuum and in warm
CO in the 5 micron fundamental ro-vibrational band. We present here
Submillimeter Array 880 micron continuum imaging resolving an inner hole.
However, the radius of the hole in the millimeter dust is only 13 AU,
significantly smaller than measured at other wavelengths. The nesting structure
of the disk is counter-intuitive, with increasingly large radii rings of
emission seen in the millimeter dust (12.9 +1.7/-3.4 AU), 5 micron CO (30 AU)
and 18.7 micron dust (peaking at 55 AU). We discuss possible explanations for
this structure, including self-shadowing that cools the disk surface layers,
photodissociation of CO, and photoevaporation. However, understanding this
unusual disk within the stringent multi-wavelength spatial constraints will
require further observations to search for cold atomic and molecular gas.
