Gas chemistry in the dust-depleted inner regions of protoplanetary disks. I. Near-infrared spectra and overtones

The molecular composition inside the dust-sublimation zones of protoplanetary disks is mostly unknown, but important to our understanding of terrestrial planet formation. A few molecules have been observed from this region, specifically , , , and . The small surface area makes observing this region difficult; hence, modeling is required to disentangle the innermost disk from regions further out. CO H2O OH SiO We aim to model a protoplanetary disk around a Herbig-type star including the dust-depleted inner region (≈ 0.1-0.3 au), investigate the chemistry of this region, and explain existing and future observations. We post-processed the dust and gas distribution of a magnetohydrostatic model with the radiation thermochemical code ProDiMo to study the chemistry and to produce observables. We find that the dust-free inner disk is a molecularly rich environment where, besides , we also find , , and . The gas-temperature profile is complex and fluctuates between 700 and 2000 K, which is warm enough to produce CO overtone line emission. Next to the CO overtone lines, we also find strong high J-level fundamental CO lines between 4.3 and 4.6 . The elemental enrichment of due to dust sublimation leads to two orders of magnitude more abundance. The gas has average temperatures of ≈ 1000 K, resulting in strong overtone emission in the spectral range between 4 and 4.3 . CO H2 H2O SiO μ m Si SiO SiO SiO μ m We predict that the gas density in the dust-depleted inner disk is high enough to allow for formation, resulting in a molecularly rich environment. For our representative Herbig model, the dust-depleted inner disk is responsible for at least 90% of the line emission for and between 1 and 28 . Next to CO overtone lines, overtone lines are expected to be an important tracer of a dust-free inner disk. H2 CO H2O μ m SiO