Gas chemistry in the dust-depleted inner regions of protoplanetary disks

Context. 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 CO, H2O, OH, and SiO. The small surface area makes observing this region difficult; hence, modeling is required to disentangle the innermost disk from regions further out. Aims. 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. Methods. 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. Results. We find that the dust-free inner disk is a molecularly rich environment where, besides CO, we also find H2, H2O, and SiO. 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 μm. The elemental enrichment of Si due to dust sublimation leads to two orders of magnitude more SiO abundance. The SiO gas has average temperatures of ≈1000 K, resulting in strong SiO overtone emission in the spectral range between 4 and 4.3 μm. Conclusions. We predict that the gas density in the dust-depleted inner disk is high enough to allow for H2 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 CO and H2O between 1 and 28 μm. Next to CO overtone lines, SiO overtone lines are expected to be an important tracer of a dust-free inner disk.