A 30 AU radius CO gas hole in the disk around the Herbig Ae star Oph IRS
  48
  The physical processes leading to the disappearance of disks around young
stars are not well understood. A subclass of transitional disks, the so-called
cold disks with large inner dust holes, provide a crucial laboratory for
studying disk dissipation processes. IRS 48 has a 30 AU radius hole previously
measured from dust continuum imaging at 18.7 micron. Using new optical spectra,
we determine that IRS 48 is a pre-main sequence A0 star. In order to
characterize this disk's gas distribution, we obtained AO-assisted VLT CRIRES
high resolution (R ~100,000) spectra of the CO fundamental rovibrational band
at 4.7 micron. All CO emission, including that from isotopologues and
vibrationally excited molecules, is off-source and peaks at 30 AU. The gas is
thermally excited to a rotational temperature of 260 K and is also strongly UV
pumped, showing a vibrational excitation temperature of ~5000 K. We model the
kinematics and excitation of the gas and posit that the CO emission arises from
the dust hole wall. Prior imaging of UV-excited PAH molecules, usually a gas
tracer, within the hole makes the large CO hole even more unexpected.
