No evidence of the significant grain growth but tentative discovery of disk substructure in a disk around the Class I Protostar L1489 IRS

For revealing the first step of the plant formation, it is important to understand how and when dust grains become larger in a disk around a protostar. To investigate the grain growth, we analyze dust continuum emission toward a disk around the Class I protostar, L1489 IRS at 0.9 and 1.3 mm wavelengths obtained by the Atacama Large Millimeter/submillimeter Array. The dust continuum emission extends to a disk radius ($r$) of $r\sim 300$ au, and the spectral index ($\alpha$) is derived to be $\alpha \sim 3.6$ at a radius of $r\sim 100-300$ au, as similar to the interstellar dust. Therefore, the grain growth does not occur significantly in the outer disk ($r\sim 100-300$ au). Furthermore, we tentatively identify a ring-like substructure at $r\sim 90$ au even though the spatial resolution and sensitivity are not enough to determine this structure. If this is the real ring structure, the ring position and small dust in the disk outer part are consistent with the idea of the growth front. These results suggest that the L1489 protostellar disk may be the beginning of the planet formation.