Context : CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks , effectively complementing ALMA observations .
Recent studies have found a relation between the ratio of lines originating from the second and first vibrationally excited state , denoted as v 2 / v 1 , and the Keplerian velocity or emitting radius of CO. Counterintuitively , in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star , and high when lines only probe gas at large radii ( more than 5 AU ) .
The v 2 / v 1 ratio is also counterintuitively anti-correlated with the near-IR ( NIR ) excess , which probes hot/warm dust in the inner disk .

Aims : We aim to find explanations for the observed trends between CO vibrational ratio , emitting radii , and NIR excess , and identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars .

Methods : First , slab model explorations in LTE and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission .
Second , we explore a grid of thermo-chemical models using the DALI code , varying gas-to-dust ratio and inner disk radius .
Line flux , line ratios and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data .

Results : Broad CO lines with low vibrational ratios are best explained by a warm ( 400–1300 K ) inner disk surface with gas-to-dust ratios below 1000 ( N _ { \mathrm { CO } } < 10 ^ { 18 } cm ^ { -2 } ) ; no CO is detected within/at the inner dust rim , due to dissociation at high temperatures .
In contrast , explaining the narrow lines with high vibrational ratios requires an inner cavity of a least 5 AU in both dust and gas , followed by a cool ( 100–300 K ) molecular gas reservoir with gas-to-dust ratios greater than 10000 ( N _ { \mathrm { CO } } > 10 ^ { 18 } cm ^ { -2 } ) at the cavity wall .
In all cases the CO gas must be close to thermalization with the dust ( T _ { \mathrm { gas } } \sim T _ { \mathrm { dust } } ) .

Conclusions : The high gas-to-dust ratios needed to explain high v 2 / v 1 in narrow CO lines for a subset of group I disks can naturally be interpreted as due to the dust traps that have been proposed to explain millimeter dust cavities .
The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets .
The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap , respectively .
The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA .
We provide simulated ELT-METIS images to directly test these scenarios in the future .