PAH emission in the proplyd HST10: what is the mechanism behind photoevaporation?

TL;DR

This study uses mid-infrared imaging and spectroscopy to analyze PAH emissions in the proplyd HST10, revealing low PAH abundance and implications for photoevaporation mechanisms around young stars.

Contribution

First spatially resolved analysis of PAHs in a proplyd, showing their low abundance and impact on photoevaporation models.

Findings

PAHs are mostly neutral in HST10.

PAH abundance is at least 50 times lower than in the diffuse ISM.

Reduced PAH abundance implies lower photoelectric heating efficiency.

Abstract

Proplyds are photodissociation region (PDR)-like cometary cocoons around young stars which are thought to originate through photo-evaporation of the central protoplanetary disk by external UV radiation from the nearby OB stars. This letter presents spatially resolved mid-infrared imaging and spectroscopy of the proplyd HST10 obtained with the VLT/VISIR instrument. These observations allow us to detect Polycyclic Aromatic Hydrocarbons (PAH) emission in the proplyd photodissociation region and to study the general properties of PAHs in proplyds for the first time. We find that PAHs in HST10 are mostly neutral and at least 50 times less abundant than typical values found for the diffuse ISM or the nearby Orion Bar. With such a low PAH abundance, photoelectric heating is significantly reduced. If this low abundance pertains also to the original disk material, gas heating rates could be too low to efficiently drive photoevaporation unless other processes can be identified. Alternatively, the model behind the formation of proplyds as evaporating disks may have to be revised.