First MATISSE L-band observations of HD 179218

Е С Кокоулина ,J Duprat,F Allouche,W C Danchi,P Cruzalèbes,S Lagarde,L Klarmann,W Jaffe,E Habart,M Heininger,J Drevon,A Soulain,P Antonelli,F Vakili,M Dannhoff,F Millour,G Yoffe,A Meilland,A Bensberg,V Gámez Rosas,S Robbe-Dubois,V Hocdé,C Dominik,Felix Bettonvil ,P Ábrahám ,G Zins,B López ,Romain G Petrov ,L B F M Waters,R Van Boekel,U Graser,J Woillez,James Leftley ,Sebastian Wolf ,Ch Leinert ,M Delbó ,J Varga,G Weigelt,M Wittkowski,K Meisenheimer,N Ysard,Ph Stee,D Schertl,Philippe Bério ,J W Isbell,A Matter,E Dartois,J.-C Augereau,Paul Bristow ,J Hron,K.-H Hofmann,Lucas Labadie ,M R Hogerheijde ,Th Henning,Claudia Paladini ,Michael Lehmitz ,E Pantin,U Beckmann,A P Jones ,Isabelle Percheron ,C A Hummel

doi:10.1051/0004-6361/202141175

Abstract

Context. Carbon is one of the most abundant components in the Universe. While silicates have been the main focus of solid phase studies in protoplanetary discs (PPDs), little is known about the solid carbon content especially in the planet-forming regions (~0.1–10 au). Fortunately, several refractory carbonaceous species present C-H bonds (such as hydrogenated nano-diamond and amorphous carbon as well as polycyclic aromatic hydrocarbons), which generate infrared (IR) features that can be used to trace the solid carbon reservoirs. The new mid-IR instrument MATISSE, installed at the Very Large Telescope Interferometer (VLTI), can spatially resolve the inner regions (~1–10 au) of PPDs and locate, down to the au-scale, the emission coming from carbon grains. Aims. Our aim is to provide a consistent view on the radial structure, down to the au-scale, as well as basic physical properties and the nature of the material responsible for the IR continuum emission in the inner disk region around HD 179218. Methods. We implemented a temperature-gradient model to interpret the disk IR continuum emission, based on a multiwavelength dataset comprising a broadband spectral energy distribution and VLTI H-, L-, and N-bands interferometric data obtained in low spectral resolution. Then, we added a ring-like component, representing the carbonaceous L-band features-emitting region, to assess its detectability in future higher spectral resolution observations employing mid-IR interferometry. Results. Our temperature-gradient model can consistently reproduce our dataset. We confirmed a spatially extended inner 10 au emission in H- and L-bands, with a homogeneously high temperature (~1700 K), which we associate with the presence of stochastically heated nano-grains. On the other hand, the N-band emitting region presents a ring-like geometry that starts at about 10 au with a temperature of 400 K. Moreover, the existing low resolution MATISSE data exclude the presence of aromatic carbon grains (i.e., producing the 3.3 μm feature) in close proximity tothe star (≲1 au). Future medium spectral resolution MATISSE data will confirm their presence at larger distances. Conclusions. Our best-fit model demonstrates the presence of two separated dust populations: nano-grains that dominate the near- to mid-IR emission in the inner 10 au region and larger grains that dominate the emission outward. The presence of such nano-grains in the highly irradiated inner 10 au region of HD 179218 requires a replenishment process. Considering the expected lifetime of carbon nano-grains from The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS model), the estimated disk accretion inflow of HD 179218 could significantly contribute to feed the inner 10 au region in nano-grains.Moreover, we also expect a local regeneration of those nano-grains by the photo-fragmentation of larger aggregates.

Highlights

The great diversity of exoplanetary systems detected so far1 has raised numerous questions about the composition of planets. protoplanetary disks (PPDs) are the environments in which planets are formed and dust grains are their building blocks
– From our temperature-gradient modeling, we confirmed that the H-band and L-band emitting region is spatially extended (∼10 au) and has a homogeneously high temperature (∼1700 K)
We confirmed the ring-like shape observed in the N-band continuum emitting region, which starts at about 10 au and with a temperature of ∼400 K, expected for usual μm-sized grains located at such distance

Summary

Introduction

The great diversity of exoplanetary systems detected so far has raised numerous questions about the composition of planets. protoplanetary disks (PPDs) are the environments in which planets are formed and dust grains are their building blocks. One of the ambitions of our work is to check if the extended inner emission, attributed to stochastically heated particles, can come from solid carbonaceous nanometric species like PAHs. We aim: (1) to determine if features associated with carbonaceous nano-grains are already detectable in our low resolution (R ∼ 30) L-band MATISSE data ( the aromatic feature at 3.3 μm); and (2) to assess the feasibility of detection and characterization (position, size, rough composition) of those carbonaceous nano-particles in the inner 10 au region from future MATISSE observations in medium spectral resolution (R ∼ 500). The comparison of the low spectral resolution visibility data with the model visibilities, which assumes a contribution of 30% from the C-CH disk at 3.3 μm, suggests that the region emitting the carbonaceous features cannot start from within a too small inner radii (≤1 au) Such models produce a visibility excess around 3.3 μm, for most of the baselines, that is not seen in the measured visibilities. Future MATISSE MR data may help to distinguish between the two different intensity profiles

Findings

Discussion

AU-scale location and characterization of the nano-grains