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The physical processes leading to the disappearance of disks around youngstars are not well understood. A subclass of transitional disks, the so-calledcold disks with large inner dust holes, provide a crucial laboratory forstudying disk dissipation processes. IRS 48 has a 30 AU radius hole previouslymeasured 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 tocharacterize this disk's gas distribution, we obtained AO-assisted VLT CRIREShigh resolution (R ~100,000) spectra of the CO fundamental rovibrational bandat 4.7 micron. All CO emission, including that from isotopologues andvibrationally excited molecules, is off-source and peaks at 30 AU. The gas isthermally excited to a rotational temperature of 260 K and is also strongly UVpumped, showing a vibrational excitation temperature of ~5000 K. We model thekinematics and excitation of the gas and posit that the CO emission arises fromthe dust hole wall. Prior imaging of UV-excited PAH molecules, usually a gastracer, within the hole makes the large CO hole even more unexpected.

A 30 AU radius CO gas hole in the disk around the Herbig Ae star Oph IRS 48