Multi-Wavelength Dust Characterization of the HL Tau Disk and Implications for Planet Formation

TL;DR

This study models the HL Tau disk across multiple wavelengths to determine dust properties, revealing implications for planet formation mechanisms like pebble accretion or gravitational instability.

Contribution

It provides a comprehensive multi-wavelength analysis of the HL Tau dust disk using MCMC, constraining dust composition, size, and structure, and discusses implications for planet formation theories.

Findings

Dust in the outer disk likely has a maximum size of ~100 μm if compact.

Porous, organics-rich dust could support pebble accretion rates sufficient for planet formation.

Amorphous-carbon-rich dust makes pebble accretion unlikely, suggesting alternative formation mechanisms.

Abstract

We present a comprehensive analysis of the HL Tau dust disk by modeling its intensity profiles across six wavelengths (0.45 to 7.9 mm) with a resolution of 0.05 arcsec ($\sim7$ au). Using a Markov Chain Monte Carlo (MCMC) approach, we constrain key dust properties including temperature, surface density, maximum grain size, composition, filling factor, and size distribution. The full fitting, with all parameters free, shows a preference for organics-rich dust with a low filling factor in the outer region ($r \gtrsim 40$ au), where the spectral index is $\sim3.7$, but amorphous-carbon-rich dust also reasonably reproduces the observed intensity profiles. Considering the scattering polarization observed at 0.87 mm, compact, amorphous-carbon-rich dust is unlikely, and moderately porous dust is favored. Beyond 40 au, the maximum dust size is likely $\sim100~{\rm \mu m}$ if dust is compact or amorphous-carbon rich. However, if the dust is moderately porous and organics-rich, both the predicted dust surface density and dust size can be sufficiently large for the pebble accretion rate to reach $\sim10M_{\oplus}~{\rm Myr^{-1}}$ in most regions, suggesting that pebble accretion could be a key mechanism for forming planets in the disk. In contrast, if the dust is amorphous-carbon-rich, forming a giant planet core via pebble accretion is unlikely due to the combined effects of low dust surface density and small dust size required to match the observed emission, suggesting other mechanisms, such as disk fragmentation due to gravitational instability, may be responsible for planet formation in the HL Tau disk.