Measuring the Dust Masses of Protoplanetary Disks in Lupus with ALMA: Evidence that Disks can be Optically Thick at 3 mm

TL;DR

This study models the spectral energy distributions of 41 protoplanetary disks in Lupus, revealing that many are optically thick at 3 mm, which impacts accurate disk mass measurements crucial for understanding planet formation.

Contribution

It demonstrates that most disks are optically thick at millimeter wavelengths, challenging the common assumption of optically thin emission for mass estimates.

Findings

Disk dust masses are 1.5-6 times higher when accounting for optical thickness.

Most disks remain optically thick at 3 mm, even up to this wavelength.

Longer wavelength observations are necessary for accurate mass measurements.

Abstract

Accurate disk mass measurements are necessary to constrain disk evolution and the timescale of planet formation, but such measurements are difficult to make and are very dependent on assumptions. Here we look at the assumption that the disk is optically thin at radio wavelengths and the effect of this assumption on measurements of disk dust mass. We model the optical to radio spectral energy distributions (SEDs) of 41 protoplanetary disks located in the young (~1-3 Myr old) Lupus star-forming region, including 0.89 mm, 1.33 mm, and 3 mm flux densities when available. We measure disk dust masses that are ~1.5-6 times higher than when using the commonly adopted disk dust mass equation under the assumption of optically thin emission in the (sub-)millimeter. The cause of this discrepancy is that most disks are optically thick at millimeter wavelengths, even up to 3 mm, demonstrating that observations at longer wavelengths are needed to trace the fully optically thin emission of disks.