Exploring the Grain Properties in the Disk of HL Tau with an Evolutionary Model

TL;DR

This study models the dust grain properties in the HL Tau disk using evolutionary models to explain observed millimeter emission profiles, revealing different grain sizes in the atmosphere and midplane and suggesting dust deficits in disk gaps.

Contribution

It introduces a detailed modeling approach combining disk evolution and dust properties to interpret millimeter observations of HL Tau, highlighting the importance of dust size distribution and dust deficits in gaps.

Findings

Models with 100 μm grains in the atmosphere and 1 cm in the midplane fit observations.

Dust deficits in gaps better reproduce emission profiles than opacity effects alone.

A global dust-to-gas ratio twice the ISM value is required to match the 7.8 mm emission.

Abstract

We model the ALMA and VLA millimeter radial profiles of the disk around HL Tau to constrain the properties of the dust grains. We adopt the disk evolutionary models of Lynden-Bell \& Pringle and calculate their temperature and density structure and emission. These disks are heated by the internal viscosity and irradiated by the central star and a warm envelope. We consider a dust size distribution $n(a) da \propto a^{-3.5} da $, and vary the maximum grain size in the atmosphere and the midplane, $a_{\rm max}=100\ \mu$m, 1 mm, and 1cm. We also include dust settling and vary the dust-to-gas mass ratio from 1 to 9 times the ISM value. We find that the models that can fit the observed level of emission along the profiles at all wavelengths have an atmosphere with a maximum grain size $a_{\rm max} = 100 \ \mu$m, and a midplane with $a_{\rm max}=1$ cm. The disk substructure, with a deficit of emission in the gaps, can be due to dust properties in these regions that are different from those in the rings. We test an opacity effect (different $a_{\rm max}$) and a dust mass deficit (smaller dust-to-gas mass ratio) in the gaps. We find that the emission profiles are better reproduced by models with a dust deficit in the gaps, although a combined effect is also possible. These models have a global dust-to-gas mass ratio twice the ISM value, needed to reach the level of emission of the 7.8 mm VLA profile.