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 \mu m. 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 \sim 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 \sim 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 .