  HD 169142 is part of the class of (pre-)transitional protoplanetary disks
showing multiple carbon nanodust spectroscopic signatures (aromatic, aliphatic)
dominating the infrared spectrum. Precise constraints on the spatial
distribution and properties of carbonaceous dust particles are essential to
understanding the physics of the disk. The HD 169142 disk is seen almost
face-on and thus offers a unique opportunity to study the dust radial
evolution. We investigate the spatial distribution and properties of the
carriers of several dust aromatic emission features in the disk across a broad
spatial range (10-200 AU). We analysed imaging and spectroscopic observations
in the 8-12 microns range from VLT/VISIR, as well as adaptive optics
spectroscopic observations in the 3-4 microns range from VLT/NACO. The data
probes the spatial evolution of the 3.3, 8.6, and 11.3 microns aromatic bands.
To constrain the radial distribution of carbonaceous nano-grains, the
observations were compared to models using The Heterogeneous dust Evolution
Model for Interstellar Solids (THEMIS), integrated into the POLARIS radiative
transfer code by calculating the thermal and stochastic heating of
sub-micrometer dust grains. Our data show predominant nano-particle emission at
all radii (resolution of about 0.1", 12 AU at 3 microns and 0.3", 35 AU at 10
microns) in the HD 169142 disk. This unambiguously shows that carbonaceous
nano-grains dominate radiatively the infrared spectrum in most of the disk, as
suggested by previous studies. In order to account for both VISIR and NACO
emission maps, we show the need for aromatic particles distributed within the
disk from the outermost regions to a radius of 20 AU, corresponding to the
outer limit of the inner cavity derived from previous observations. In the
inner cavity, these aromatic particles might be present but their abundance
would then be significantly decreased.
