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

Abstract

The molecular composition inside the dust sublimation zones of protoplanetary disks is mostly unknown but important to understanding 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. We model a protoplanetary disk around a Herbig-type star including the dust depleted inner region (approx. 0.1-0.3 au) and aim to investigate the chemistry of this region and explain existing and future observations. Methods. We post-process 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 molecular 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 micron. The elemental enrichment of Si due to dust sublimation leads to 2 orders of magnitude more SiO abundance. The SiO gas has average temperatures of approx. 1000 K resulting in strong SiO overtone emission in the spectral range between 4 and 4.3 micron. We predict that the gas density in the dust depleted inner disk is high enough to allow for H2 formation, resulting in an molecular 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 micron. Next to CO overtone lines, SiO overtone lines are expected to be an important tracer of a dust free inner disk.