Dust Scattering Albedo at Millimeter-Wavelengths in the TW Hya Disk

TL;DR

This study measures the millimeter-wavelength dust albedo in the TW Hya disk using molecular lines to break degeneracies, revealing high albedo and constraining dust properties without assuming specific dust models.

Contribution

First direct measurement of dust albedo spectrum at 0.9-3 mm in a protoplanetary disk using molecular lines as temperature probes.

Findings

Albedo at 0.9-3 mm is high, around 0.5-0.8.

Maximum grain size estimated at ~340 μm.

Dust size distribution index > -4.1.

Abstract

Planetary bodies are formed by coagulation of solid dust grains in protoplanetary disks. Therefore, it is crucial to constrain the physical and chemical properties of the dust grains. In this study, we measure the dust albedo at mm-wavelength, which depends on dust properties at the disk midplane. Since the albedo and dust temperature are generally degenerate in observed thermal dust emission, it is challenging to determine them simultaneously. We propose to break this degeneracy by using multiple optically-thin molecular lines as a dust-albedo independent thermometer. In practice, we employ pressure-broadened CO line wings that provide an exceptionally high signal-to-noise ratio as an optically thin line. We model the CO $J=2-1$ and $3-2$ spectra observed by the Atacama Large Millimeter/sub-millimeter Array (ALMA) at the inner region ($r<6\ {\rm au}$) of the TW Hya disk and successfully derived the midplane temperature. Combining multi-band continuum observations, we constrain the albedo spectrum at $0.9-3$ mm for the first time without assuming a dust opacity model. The albedo at these wavelengths is high, $\sim0.5-0.8$, and broadly consistent with the Ricci et al. (2010), DIANA, and DSHARP dust models. Even without assuming dust composition, we estimate the maximum grain size to be $\sim 340\ \mu m$, the power law index of the grain size distribution to be $>-4.1$, and porosity to be $<0.96$. The derived dust size may suggest efficient fragmentation with the threshold velocity of $\sim 0.08\ {\rm m\ s^{-1}}$. We also note that the absolute flux uncertainty of $\sim10\%$ ($1\sigma$) is measured and used in the analysis, which is approximately twice the usually assumed value.