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An empirical sequence of disk gap opening revealed by rovibrational CO

The fundamental rovibrational band of CO near 4.7 $μ$m is a sensitive tracer of the presence and location of molecular gas in the planet-forming region of protoplanetary disks at 0.01--10 AU. We present a new analysis of a high-resolution spectral survey (R$\,\sim\,$96,000, or $\sim3.2\,\rm km\,s^{-1}$) of CO rovibrational lines from protoplanetary disks spanning a wide range of stellar masses and of evolutionary properties. We find that the CO emission originates in two distinct velocity components. Line widths of both components correlate strongly with disk inclination, as expected for gas in Keplerian rotation. By measuring the line flux ratios between vibrational transitions $F_{v=2-1}/F_{v=1-0}$, we find that the two velocity components are clearly distinct in excitation. The broad component ($\rm FWHM=50-200\,km\,s^{-1}$) probes the disk region near the magnetospheric accretion radius at $\approx0.05$ AU, where the gas is hot ($800-1500$ K). The narrow component ($\rm FWHM=10-50 km\,s^{-1}$) probes the disk at larger radii of 0.1--10\,AU, where the gas is typically colder (200--700 K). CO excitation temperatures and orbital radii define an empirical temperature-radius relation as a power law with index $-0.3\pm0.1$ between 0.05--3 AU. The broad CO component, co-spatial with the observed orbital distribution of hot Jupiters, is rarely detected in transitional and Herbig Ae disks, providing evidence for an early dissipation of the innermost disk. An inversion in the temperature profile beyond 3 AU is interpreted as a tracer of a regime dominated by UV pumping in largely devoid inner disks, and may be a signature of the last stage before the disk enters the gas-poor debris phase.