2014A&A...567A..32M

We present new 3mm ATCA data of two Class I young stellar objects (YSOs) in the Ophiucus star forming region: Elias29 and WL12. For our analysis we compare them with archival 1.1mm 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 α_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 I YSOs are probably very compact, in particular in the case of WL12, with outer radii down to tens of AU.