Context. 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. Aims. We probe the composition and evolution of carbon nano-grains at the surface of (pre-)transitional protoplanetary disks around Herbig stars. Methods. We present spatially resolved infrared emission spectra obtained with NAOS CONICA at the VLT in the 3-4 µm range with a spatial resolution of 0.1", which allow us to trace aromatic, olefinic andhttps://www.overleaf.com/project/5c46c1dbf80925580ab5dd2d 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). Results. 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.

Context. Dust grains are determinant for setting the chemical, physical, dynamical, and radiative properties of all the media in which they are present. Their influence depends on the grain composition, size, and geometrical structure which vary throughout the lifecycle of dust. Particularly grain growth arises in dense molecular clouds and protoplanetary disks as traced by an enhancement of the dust far-IR emissivity and by the effects of cloudshine and coreshine. Aims. Our aim is to investigate the imprint of the grain characteristics on the dust unpolarised optical properties from the visible to the far-IR wavelengths for isolated grains as well as for aggregates. Methods. Using optical constants for both carbonaceous and silicate materials, we derive the absorption and scattering efficiencies, the asymmetry factor of the phase function, the single scattering albedo, and the mass opacity for isolated grains and aggregates, using either the Mie theory or the Discrete Dipole Approximation (DDA). We investigate the effects of the size, porosity and shape of the grains, and of the monomers constituting the aggregates, on the optical properties. Besides, for aggregates we study the influence of the number of monomers and of mixing monomer sizes. Results. Grain structure changes result in optical property variations at all wavelengths. Porosity, grain elongation, as well as aggregation all produce an increase in the far-IR opacity. The spectral dependence of this increase depends on the nature of the material composing the grain: it is independent of the wavelength for insulators but not for conductors. In the case of aggregates, the far-IR increase does not depend on the monomer size and saturates for aggregates containing 6 or more monomers. In the visible and near-IR, the aggregate behaviour is reminiscent of a compact sphere of the same mass whereas at longer wavelengths, it is closer to the effect of porosity. Finally, for silicates, the mid-IR spectral feature at 18 µm is more sensitive to the details of the grain structure than the 10 µm feature. Conclusions. Dust optical properties, from the visible to the far-IR, are highly dependent upon the grain composition, size, and structure. This study provides a basis for understanding the range of variations achievable as a result of varying the grain characteristics. It emphasises the importance of considering the detailed grain structure in determining the dust optical properties and of using exact methods because approximate methods are unable to reproduce the entire range of the observed variations at all wavelengths.