The properties of the inner disk around HL Tau: Multi-wavelength modeling of the dust emission

TL;DR

This study uses multi-wavelength radiative transfer modeling to analyze the dust properties and structure of HL Tau's inner disk, revealing dust evolution and grain trapping in early disk stages.

Contribution

It introduces a detailed multi-wavelength modeling approach that constrains the dust density distribution and grain size variation in HL Tau's inner disk for the first time.

Findings

Most of the disk becomes optically thin at 7mm, enabling dust density constraints.

A shallow grain size distribution fits observations better than a homogeneous one.

Larger grains are trapped in the first bright ring, indicating dust evolution.

Abstract

We conducted a detailed radiative transfer modeling of the dust emission from the circumstellar disk around HL Tau. The goal of our study is to derive the surface density profile of the inner disk and its structure. In addition to the Atacama Large Millimeter/submillimeter Array images at Band 3 (2.9mm), Band 6 (1.3mm), and Band 7 (0.87mm), the most recent Karl G. Jansky Very Large Array (VLA) observations at 7mm were included in the analysis. A simulated annealing algorithm was invoked to search for the optimum model. The radiative transfer analysis demonstrates that most radial components (i.e., >6AU) of the disk become optically thin at a wavelength of 7mm, which allows us to constrain, for the first time, the dust density distribution in the inner region of the disk. We found that a homogeneous grain size distribution is not sufficient to explain the observed images at different wavelengths simultaneously, while models with a shallower grain size distribution in the inner disk work well. We found clear evidence that larger grains are trapped in the first bright ring. Our results imply that dust evolution has already taken place in the disk at a relatively young (i.e., ~1Myr) age. We compared the midplane temperature distribution, optical depth, and properties of various dust rings with those reported previously. Using the Toomre parameter, we briefly discussed the gravitational instability as a potential mechanism for the origin of the dust clump detected in the first bright ring via the VLA observations.