# Nano carbon dust emission in proto-planetary disks: the aliphatic-aromatic components

## TL;DR

This study investigates the composition and spatial distribution of nano-sized carbon grains in protoplanetary disks around Herbig stars using infrared spectroscopy, revealing ongoing replenishment of these grains despite their vulnerability to destruction.

## Contribution

It provides spatially resolved infrared spectra of nano-carbon grains in disks, analyzing their composition and proposing a model for their evolution and replenishment.

## Key findings

- Detection of aromatic and aliphatic hydrocarbon features from 10 to 100 au
- Constant intensity ratios suggest a common carrier for different hydrocarbon types
- Evidence of continuous replenishment of nano-grains despite high destruction rates

## Abstract

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.