[Paper Review] The complex structure of the disk around HD100546: the inner few astronomical units

M. Benisty, E. Tatulli+2 more|arXiv (Cornell University)|Jan 14, 2010

Astrophysics and Star Formation StudiesPhysics and Astronomy32 references59 citations

TL;DR

This study reveals a complex, gap-structured disk around the 10 Myr-old Herbig Be star HD 100546 using VLTI/AMBER and MIDI interferometry, combined with a 3D Monte Carlo radiative transfer model. It identifies a tenuous inner disk of micron-sized dust at ~0.26 AU, where 40% of the K-band flux arises from scattering, and confirms a massive, flared outer disk with vertical dust stratification, consistent with grain growth and settling, while the gap extends to ~13 AU with a total dust mass of ~6×10²³ g inside it.

ABSTRACT

Disclosing the structure of disks surrounding Herbig AeBe stars is important to expand our understanding of the formation and early evolution of stars and planets. We aim at revealing the sub-AU disk structure around the 10 Myr old Herbig Be star HD100546 and at investigating the origin of its near and mid-infrared excess. We used AMBER/VLTI observations to resolve the K-band emission and to constrain the location and composition of the hot dust in the innermost disk. Combining AMBER observations with photometric and MIDI/VLTI measurements from the litterature, we revisit the disk geometry using a passive disk model based on 3D radiative transfer. We propose a model that includes a tenuous inner disk made of micron-sized dust grains, a gap, and a massive optically thick outer disk, that successfully reproduces the interferometric data and the SED. We locate the bulk of the K-band emission at ~0.26 AU. Assuming that this emission originates from silicate, we show that micron-sized grains are required to enable the dust to survive at such a distance from the star. As a consequence, more than 40% of the K-band flux is related to scattering, showing that direct thermal emission is not sufficient to explain the near-infrared excess. In the massive outer disk, large grains in the mid-plane are responsible for the mm emission while a surface layer of small grains allows the mid and far infrared excesses to be reproduced. Such vertical structure may be an evidence for sedimentation. The observations are consistent with a model that includes a gap until ~13 AU and a total dust mass of ~0.008 lunar mass inside it. These values together with the derived scale height (~2.5 AU) and temperature (~220 K) at the inner edge of the outer disk (r=13 AU), are consistent with recent CO observations.

Motivation & Objective

• To resolve the sub-AU structure of the circumstellar disk around HD 100546, a 10 Myr-old Herbig Be star, to understand planet formation processes in hot, dense environments.
• To explain the origin of the strong mid-infrared excess and weak near-infrared excess in the SED, which are atypical for Herbig AeBe stars.
• To determine the geometry, dust composition, and radial density profile of the inner disk using interferometric and photometric data.
• To investigate whether vertical dust stratification (sedimentation) and grain growth are occurring in the evolved disk.
• To constrain the dust mass, gap size, and inner disk structure using a physically self-consistent 3D radiative transfer model.

Proposed method

• Utilized new AMBER/VLTI K-band interferometric observations to resolve the innermost disk structure at sub-AU scales.
• Combined AMBER data with archival photometric and MIDI/VLTI measurements to constrain the full spectral energy distribution (SED).
• Applied a 3D Monte Carlo radiative transfer model with full anisotropic scattering to simulate dust emission and scattering in a disk with radial and vertical structure.
• Modeled the disk as a three-component system: a tenuous inner disk of micron-sized grains, a gap from ~0.26 AU to ~13 AU, and a massive, flared outer disk with vertical stratification.
• Used the model to reproduce both the interferometric visibilities and the SED, including near-, mid-, and far-infrared excesses.
• Varied grain size distributions (0.05–3000 µm), scale heights, and dust mass to find the best-fit configuration consistent with observations.

Experimental results

Research questions

• RQ1 What is the radial and vertical structure of the inner disk around HD 100546, particularly within the first few astronomical units?
• RQ2 Why does HD 100546 exhibit a strong mid-infrared excess but a weak near-infrared excess, and what physical processes explain this SED peculiarity?
• RQ3 To what extent is the near-infrared excess due to thermal emission versus scattering from hot dust grains?
• RQ4 Is there evidence for grain growth and vertical settling (sedimentation) in the disk, and how does this affect the observed SED and interferometric signatures?
• RQ5 What is the size of the gap in the disk, and what is the total dust mass within the inner cavity?

Key findings

• The bulk of the K-band emission originates from a tenuous inner disk at ~0.26 AU, located at the dust sublimation radius for silicate grains at ~1500 K.
• More than 40% of the K-band flux is due to scattering, indicating that thermal emission alone cannot explain the near-infrared excess, and micron-sized grains are required for stability at this distance.
• The disk model includes a gap extending from ~0.26 AU to ~13 AU, with the outer disk starting at ~13 AU and having a scale height of ~2.5 AU at its inner edge, consistent with CO observations.
• The outer disk contains large grains in the mid-plane (responsible for mm emission) and a surface layer of small grains (responsible for mid- and far-infrared excesses), indicating vertical dust stratification consistent with grain growth and settling.
• The total dust mass within the gap is ~6×10²³ g (~0.008 lunar masses), with a gas-to-dust ratio of ~50, which is higher than previous estimates and highly model-dependent.
• The model is consistent with prior observations, including STIS H₂ and [OI] line data, which support a central cavity extending to ~13 AU, and with CO observations showing a hole at ~11–13 AU.