Carbonaceous nano-dust emission in proto-planetary discs: the aliphatic-aromatic components

T Boutéraon,N Ysard,E Dartois,T Pino,E Habart,A P Jones

doi:10.1051/0004-6361/201834016

Abstract

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 that is due to interactions within different radiative and dynamical environments. Infrared signatures of these carbon nano-grains are seen in a large number of discs around Herbig HAeBe stars. Aims. We probe the composition and evolution of carbon nano-grains at the surface of (pre-)transitional proto-planetary discs around Herbig stars. Methods. We present spatially resolved infrared emission spectra obtained with the Nasmyth Adaptive Optics System (NAOS) Near-Infrared Imager and Spectrograph (CONICA) at the Very Large Telescope (VLT) in the 3–4 μm range with a spatial resolution of 0.1′′, which allowed us to trace aromatic, olefinic, and aliphatic bands that are attributed to sub-nanometer hydrocarbon grains. We applied 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 that are related to aromatic and aliphatic hydrocarbon material in discs around Herbig stars, from ~10 to 50–100 au, and even in inner gaps that are devoid of large grains. The correlation and constant intensity ratios between aliphatic and aromatic CH stretching bands suggests a common nature of the carriers. Given their expected high destruction rates through UV photons, our observations suggest that they are continuously replenished at the disc surfaces.

Highlights

Carbonaceous dust is a major component of the solid matter in the interstellar medium (ISM)
Because we focused on the carbonaceous dust emission evolution, we extracted the features from the original data by subtracting an underlying continuum
These data allowed us for the first time to spatially resolve the disc emission in the bands related to aromatic and aliphatic materials

Summary

Introduction

Carbonaceous dust is a major component of the solid matter in the interstellar medium (ISM) It is observed in a wide variety of environments in emission and extinction. A-C(:H) materials, which includes both H-poor a-C and H-rich a-c:H, are a broad family of compounds that include various proportions of polyaromatic units of various sizes that are linked by olefinic and aliphatic bridges. These components, when observed in emission, which are associated with the aromatic phase, should exhibit a series of bands between 3.4 and 3.6 μm, in addition to the aromatic 3.3 μm band These components, when observed in emission, which are associated with the aromatic phase, should exhibit a series of bands between 3.4 and 3.6 μm, in addition to the aromatic 3.3 μm band (e.g. Geballe et al 1985; Jourdain de Muizon et al 1990a,b; The reduced spectra (Fig. 2) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/623/A135

Objectives

Results

Conclusion