Spatial distribution of the aromatic and aliphatic carbonaceous nanograin features in the protoplanetary disk around HD 100546

Abstract

Context. Carbonaceous nanograins are present at the surface of protoplanetary disks around Herbig Ae/Be stars, where most of the ultraviolet energy from the central star is dissipated. Efficiently coupled to the gas, they are unavoidable to understand the physics and chemistry of these disks. Furthermore, nanograins are able to trace the outer flaring parts of the disk and possibly the gaps from which the larger grains are missing. However, their evolution through the disks, from internal to external regions, is only poorly understood so far. Aims. Our aim is to examine the spatial distribution and evolution of the nanodust emission in the emblematic (pre-)transitional protoplanetary disk HD 100546. This disk shows many structures (annular gaps, rings, and spirals) and reveals very rich carbon nanodust spectroscopic signatures (aromatic, aliphatic) in a wide spatial range of the disk (~20−200 au). Methods. We analysed adaptive optics spectroscopic observations in the 3–4 μm range (angular resolution of ~0.1′′) and imaging and spectroscopic observations in the 8–12 μm range (angular resolution of ~0.3′′). The hyperspectral cube was decomposed into a sum of spatially coherent dust components using a Gaussian decomposition algorithm. We compared the data to model predictions using the heterogeneous dust evolution model for interstellar solids (THEMIS), which is integrated in the radiative transfer code POLARIS by calculating the thermal and stochastic heating of micro- and nanometre-sized dust grains for a given disk structure. Results. We find that the aromatic features at 3.3, 8.6, and 11.3 μm, and the aliphatic features between 3.4 and 3.5 μm are spatially extended; each band shows a specific morphology dependent on the local physical conditions. The aliphatic-to-aromatic band ratio, 3.4/3.3, increases with the distance from the star from ~0.2 (at 0.2′′ or 20 au) to ~0.45 (at 1′′ or 100 au), suggesting UV processing. In the 8–12 μm observed spectra, several features characteristic of aromatic particles and crystalline silicates are detected. Their relative contribution changes with the distance to the star. The model predicts that the features and adjacent continuum are due to different combinations of grain sub-populations, in most cases with a high dependence on the intensity of the UV field. The model reproduces the spatial emission profiles of the bands well, except for the inner 20-40 au, where the observed emission of the 3.3 and 3.4 μm bands is, unlike the predictions, flat and no longer increases with the UV field. Conclusions. With our approach that combines observational data in the near- to mid-IR and disk modelling, we deliver constraints on the spatial distribution of nano-dust particles as a function of the disk structure and radiation field.

Radial distribution of the carbonaceous nano-grains in the protoplanetary disk around HD 169142

“…At 8.6 µm, the ratio appears to drop at small radii and increases from 0.75" (0.88 AU), but shows overall little change over 0"-1". These globally flat behaviors of the total emission/continuum ratios are different from what was observed by Habart et al (2021) in the HD 100546 disk. For HD 100546, the emission near the star is dominated by micronic grains, resulting in an important increase of the band-to-continuum ratios with distance to the star between 0" and 1", and constant ratios at larger distance.…”

“…Overall, our data provides evidence of the domination of carbonaceous nano-grains in the infrared emission of the HD 169142 disk at all radii. This is to be linked with the presence of an inner cavity in the disk below 0.17" (20 AU), resulting in a lack of hot micronic grains, which were observed to dominate the continuum emission in, for instance, HD 100546 Habart et al (2021). account for the aromatic infrared bands and showed that the spectrum is intermediate between fully neutral, for which the 11.3 µm feature dominates over the 6.2, 8, and 8.6 µm features as well as the fully ionized interstellar PAH, showing predominant 6.2, 8, and 8.6 µm features over the 11.3 µm band (Peeters et al 2002).…”

“…The a-C(:H) nano-particles, are modelled as hydrogenated aromatic-rich carbonaceous nano-particles with mixed aromatic, aliphatic, and olefinic domains. It is applied, in particular, to the modelling of dust in the diffuse and dense ISM (Ysard et al 2015(Ysard et al , 2016Saajasto et al 2021), near-infrared to sub-millimeter observations of nearby galaxies (Chastenet et al 2017;Viaene et al 2019), the near-infrared observations of protoplanetary disks (Boutéraon et al 2019;Habart et al 2021), and in photon-dominated regions (Schirmer et al 2020(Schirmer et al , 2021.…”

“…The spatial distribution of each type of dust is given in Table 5. We modeled the dust in the outer regions of the disk from 0.17" (20 AU), according to the dust model used by Habart et al (2021). It only features dust grains smaller than 5 µm as a mix of a-C 1 , a-C 2 , VSG, and Sil1 dust populations (Table 5, third column).…”

The disk of FU Orionis viewed with MATISSE/VLTI: first interferometric observations in $L$ and $M$ bands

“…The larger component is a mixture of carbonaceous material and "astronomical silicate" (Draine & Lee 1984) with a mass ratio of C:Si=1:2, and a powerlaw grain size distribution f (a) ∼ a −3.5 (Mathis et al 1977), from a min = 10 µm to a max = 1000 µm. The mass ratio was set to this value because recent studies generally indicate that there is more silicate than carbon in protoplanetary disks (e.g., Habart et al 2021). The small silicate and larger dust were then mixed with a mass ratio of f small :(1f small ), where f small is a free parameter and represents the percentage of small grains in the mixture.…”