[Paper Review] Structure of Herbig AeBe disks at the milliarcsecond scale A statistical survey in the H band using PIONIER-VLTI

B. Lazareff, Jean-Philippe Berger+27 more|CaltechAUTHORS (California Institute of Technology)|Nov 25, 2016

Astrophysics and Star Formation Studies92 references63 citations

TL;DR

This study uses H-band PIONIER/VLTI interferometry to probe the milliarcsecond-scale structure of 51 Herbig AeBe disks, revealing a typical inner disk thickness of z/r ≈ 0.2 and strong evidence for flared disk geometry (z/r ≈ 0.5) due to self-obscuration. The dust sublimation temperature is constrained to >1500 K, with a preferred value of 1800 K, and extended emission components are linked to disk flaring or starlight scattering, while azimuthal modulations suggest non-axisymmetric structures likely due to rim shadowing.

ABSTRACT

Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. The aims of this article are to infer the radial and vertical structure of these disks at scales of order one au, and the properties of the dust grains. Methods. The program objects (51 in total) were observed with the H-band (1.6micron) PIONIER/VLTI interferometer. The largest baselines allowed us to resolve (at least partially) structures of a few tenths of an au at typical distances of a few hundred parsecs. Dedicated UBVRIJHK photometric measurements were also obtained. Spectral and 2D geometrical parameters are extracted via fits of a few simple models: ellipsoids and broadened rings with azimuthal modulation. Model bias is mitigated by parallel fits of physical disk models. Sample statistics were evaluated against similar statistics for the physical disk models to infer properties of the sample objects as a group. Results. We find that dust at the inner rim of the disk has a sublimation temperature Tsub~1800K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide delta_r>=0.5. A wide ring favors a rim that, on the star-facing side, looks more like a knife edge than a doughnut. The data are also compatible with a the combination of a narrow ring and an inner disk of unspecified nature inside the dust sublimation radius. The disk inner part has a thickness z/r~0.2, flaring to z/r~0.5 in the outer part. We confirm the known luminosity-radius relation; a simple physical model is consistent with both the mean luminosity-radius relation and the ring relative width; however, a significant spread around the mean relation is present. In some of the objects we find a halo component, fully resolved at the shortest interferometer spacing, that is related to the HAeBe class.

Motivation & Objective

• To characterize the radial and vertical structure of Herbig AeBe disks at ~1 au scales using high-angular-resolution interferometry.
• To infer dust grain properties, particularly sublimation temperature, from near-infrared excess emission.
• To investigate the presence of extended or non-axisymmetric structures in the inner disk region.
• To assess the role of disk flaring and self-obscuration in shaping observed visibility and closure phase profiles.
• To evaluate the viability of alternative models (e.g., hot inner gaseous components) for explaining visibility curve shapes.

Proposed method

• High-contrast, long-baseline interferometric observations were conducted using the PIONIER instrument at the VLTI in the H band (1.6 μm).
• Visibility and closure phase measurements were extracted from interferometric data to probe spatial structure at milliarcsecond scales.
• Radiative transfer models were fitted to the observed visibility and closure phase data to infer disk geometry, inclination, and dust properties.
• Model comparison included axisymmetric models with and without azimuthal modulation to test for deviations from symmetry.
• Statistical analysis of visibility curves and closure phases was used to assess the significance of non-axisymmetric features and extended components.
• Sublimation temperature was constrained by comparing observed flux and visibility profiles with models that account for temperature spread and fitting biases.

Experimental results

Research questions

• RQ1 What is the radial and vertical structure of Herbig AeBe disks at ~1 au scales, as revealed by milliarcsecond-resolution interferometry?
• RQ2 What is the dust sublimation temperature in these systems, and how does it relate to grain properties?
• RQ3 Is there evidence for extended emission components, and what physical mechanisms could explain them?
• RQ4 Are there detectable deviations from axisymmetry in the inner disk, and what causes them?
• RQ5 Can the observed visibility curves be explained by a standard flared disk model, or do they require additional components such as hot gas or refractory grains inside the dust sublimation radius?

Key findings

• The inner region of Herbig AeBe disks has a typical thickness of z/r ≈ 0.2, indicating a moderately flared geometry close to the star.
• The observed distribution of inclinations is consistent with a flared disk model (z/r ≈ 0.5) due to self-obscuration, with no systems observed at high inclinations (cos i < 0.45).
• The dust sublimation temperature is constrained to be greater than 1500 K, with a preferred value of 1800 K, based on radiative transfer modeling and bias correction.
• A significant fraction of the sample shows extended emission components resolved at baselines corresponding to angular scales larger than 40 mas, correlated with Herbig group classification and infrared colors.
• Azimuthal modulations in visibility and closure phase data suggest non-axisymmetric structures, likely due to self-shadowing of an inclined rim, with phase closure maxima reaching ~10° at resolution ρ′ ≈ 0.5.
• While some models (IN) reproduce the observed closure phase amplitude (~10°), others (THM) fall short (~2°), indicating a need for improved modeling of inner disk physics and asymmetries.