Grain Growth and Dust Segregation Revealed by Multi-wavelength Analysis of the Class I Protostellar Disk WL 17

TL;DR

This study uses multi-wavelength ALMA data to reveal early-stage grain growth, dust segregation, and potential planet formation processes in a Class I protostellar disk, providing new insights into the initial stages of planet formation.

Contribution

It presents the first detailed multi-wavelength analysis of grain growth and dust segregation in a Class I protostellar disk, suggesting early planet formation activity.

Findings

Detection of different disk substructures in two ALMA bands.

Asymmetric spectral index indicating grain growth and dust segregation.

Evidence of a massive, gravitationally unstable disk potentially forming a Jupiter-mass protoplanet.

Abstract

The first step toward planet formation is grain growth from (sub-)micrometer to millimeter/centimeter sizes. Grain growth has been reported not only in Class II protoplanetary disks but also in Class 0/I protostellar envelopes. However, early-stage grain growth occurring in Class 0/I stages has rarely been observed on the protostellar disk scale. Here we present the results from the ALMA Band 3 ($\lambda$ = 3.1 mm) and 7 ($\lambda$ = 0.87 mm) archival data of the Class I protostellar disk WL 17 in the $\rho$ Ophiuchus molecular cloud. Disk substructures are found in both bands, but they are different: while a central hole and a symmetric ring appear in Band 3, an off-center hole and an asymmetric ring are shown in Band 7. Furthermore, we obtain an asymmetric spectral index map with a low mean value of $\alpha$ = 2.28 $\pm$ 0.02, suggestive of grain growth and dust segregation on the protostellar disk scale. Our radiative transfer modeling verifies these two features by demonstrating that 10 cm-sized large grains are symmetrically distributed, whereas 10 $\mu$m-sized small grains are asymmetrically distributed. Also, the analysis shows that the disk is expected to be massive and gravitationally unstable. We thus suggest a single Jupiter-mass protoplanet formed by gravitational instability as the origin of the ring-like structure, grain growth, and dust segregation identified in WL 17.