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

TL;DR

This study reveals a large 30 AU radius gas hole in the disk around the Herbig Ae star IRS 48, with detailed spectroscopic analysis showing gas distribution and excitation mechanisms, providing insights into disk dissipation processes.

Contribution

First high-resolution spectroscopic characterization of gas distribution and excitation in the large dust hole of IRS 48, linking gas and dust disk structures.

Findings

CO gas peaks at 30 AU, coinciding with the dust hole

Gas is thermally excited at 260 K and UV pumped at ~5000 K

Gas distribution suggests the CO emission arises from the dust hole wall

Abstract

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.