Nano carbon dust emission in proto-planetary disks: the
  aliphatic-aromatic components
  In the interstellar medium, carbon (nano-)grains are a major component of
interstellar dust. This solid phase is more vulnerable to processing and
destruction than its silicate counterpart. It exhibits a complex,
size-dependent evolution due to interactions within different radiative and
dynamical environments. Infrared signatures of these nanocarbon grains are seen
in a large number of disks around Herbig HAeBe stars. We probe the composition
and evolution of carbon nano-grains at the surface of (pre-)transitional
protoplanetary disks around Herbig stars. We present spatially resolved
infrared emission spectra obtained with NAOS CONICA at the VLT in the 3-4
$\mu$m range with a spatial resolution of 0.1", which allow us to trace
aromatic, olefinic and aliphatic bands which are attributed to sub-nanometer
hydrocarbon grains. We apply a gaussian fitting to analyse the observed
spectral signatures. Finally, we propose an interpretation in the framework of
the The Heterogeneous dust Evolution Model of Interstellar Solids (THEMIS). We
show the presence of several spatially extended spectral features, related to
aromatic and aliphatic hydrocarbon material in disks around Herbig stars, from
~ 10 to 50-100 au, and even in inner gaps devoided of large grains. The
correlation and constant intensity ratios between aliphatic and aromatic CH
stretching bands suggest a common nature of the carriers. Given their expected
high destruction rates due to UV photons, our observations suggest that they
are continuously replenished at the disk surfaces.
