Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution $M$-band spectroscopy survey with IRTF-iSHELL

TL;DR

This high-resolution M-band spectroscopic survey of planet-forming disks reveals new CO emission features, disk wind signatures, and kinematic variability, providing insights into disk structure and gas dynamics within 10 au.

Contribution

First high-resolution survey covering 4.52-5.24 μm with unprecedented spectral detail, detecting CO rings, water lines, and variability, advancing understanding of disk gas excitation and kinematics.

Findings

Detection of two CO Keplerian rings within 2 au

Observation of H2O ro-vibrational lines at 5 μm

Identification of CO line variability over 1-14 years

Abstract

We present an overview and first results from a $M$-band spectroscopic survey of planet-forming disks performed with iSHELL on IRTF, using two slits that provide resolving power R $\approx$ 60,000-92,000 (5-3.3 km/s). 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 $^{12}$CO and $^{13}$CO 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 H${_2}$O ro-vibrational lines at 5 $\mu$m (in AS 205 N), and 3) the common kinematic variability of CO lines over timescales of 1-14 years. By homogeneously analyzing this survey together with a previous VLT-CRIRES 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 $\approx10$ 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 blue-shifts, line shapes that turn into narrow absorption at high inclinations, and the frequency of disk winds as a function of stellar type.