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