Scanning Disk Rings and Winds in CO at 0.01-10 au: A High-resolution M-band Spectroscopy Survey with IRTF-iSHELL

We present an overview and first results from a M-band spectroscopic survey of planet-forming disks performed with iSHELL on the Infrared Telescope Facility, using two slits that provide resolving power R approximate to 60,000-92,000 (5-3.3 km s(-1)). iSHELL provides a nearly complete coverage at 4.52-5.24 mu m in one shot, covering >50 lines from the R and P branches of (CO)-C-12 and (CO)-C-13 for each of multiple vibrational levels, and providing unprecedented information on the excitation of multiple emission and absorption components. Some of the most notable new findings of this survey are: (1) the detection of two CO Keplerian rings at <2 au (in HD 259431), (2) the detection of H2O rovibrational lines at 5 mu m (in AS 205 N), and (3) the common kinematic variability of CO lines over timescales of 1-14 yr. By homogeneously analyzing this survey together with a previous survey of cooler stars, we discuss a unified view of CO spectra where emission and absorption components scan the disk surface across radii from a dust-free region within dust sublimation out to approximate to 10 au. We classify two fundamental types of CO line shapes interpreted as emission from Keplerian rings (double-peak lines) and a disk surface plus a low-velocity part of a wind (triangular lines), where CO excitation reflects different emitting regions (and their gas-to-dust ratio) rather than just the irradiation spectrum. A disk+wind interpretation for the triangular lines naturally explains several properties observed in CO spectra, including the line blueshifts, line shapes that turn into narrow absorption at high inclinations, and the frequency of disk winds as a function of the stellar type.

Automated summary

This study presents the results of a spectroscopic survey of planet-forming disks using iSHELL. The survey provides new insights into the excitation and emission of CO and H2O in these disks and offers a unified view of the spectra. The findings suggest the presence of Keplerian rings and disk winds in these systems.