Grain growth in the envelopes and disks of Class I protostars

TL;DR

This study uses multi-wavelength observations and radiative transfer modeling to investigate dust grain growth and disk structures in two Class I protostars, revealing early formation of millimeter-sized grains and very compact disks.

Contribution

It provides new observational evidence and modeling insights into dust grain growth and disk compactness in early-stage protostars, which was previously less understood.

Findings

Presence of unresolved components and extended envelopes in the sources.

Detection of low spectral index indicating early dust grain growth.

Disks likely very compact, with radii down to tens of AU.

Abstract

We present new 3 mm ATCA data of two Class I Young Stellar Objects in the Ophiucus star forming region: Elias29 and WL12. For our analysis we compare them with archival 1.1 mm SMA data. In the (u,v) plane the two sources present a similar behavior: a nearly constant non-zero emission at long baselines, which suggests the presence of an unresolved component and an increase of the fluxes at short baselines, related to the presence of an extended envelope. Our data analysis leads to unusually low values of the spectral index $\alpha_{\rm 1.1-3mm}$, which may indicate that mm-sized dust grains have already formed both in the envelopes and in the disk-like structures at such early stages. To explore the possible scenarios for the interpretation of the sources we perform a radiative transfer modeling using a Monte Carlo code, in order to take into account possible deviations from the Rayleigh-Jeans and optically thin regimes. Comparison between the model outputs and the observations indicates that dust grains may form aggregates up to millimeter size already in the inner regions of the envelopes of Class I YSOs. Moreover, we conclude that the embedded disk-like structures in our two Class Is are probably very compact, in particular in the case of WL12, with outer radii down to tens of AU.