No Evidence of the Significant Grain Growth but Tentative Discovery of Disk Substructure in a Disk around the Class I Protostar L1489 IRS

Abstract

For revealing the first step of planet 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 ∼ 300 au, and the spectral index (α) is derived to be α ∼ 3.6 at a radius of r ∼ 100–300 au, similar to the interstellar dust. Therefore, the grain growth does not occur significantly in the outer disk (r ∼ 100–300 au). Furthermore, we tentatively identify a ring-like substructure at r ∼ 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 planet formation.

Early Planet Formation in Embedded Disks (eDisk). I. Overview of the Program and First Results

“…More details of the continuum maps obtained in the eDisk program will be presented in subsequent papers. Ring-like structures in continuum emission around these two sources have been suggested by previous studies as well (Segura-Cox et al 2020;Ohashi et al 2022), although our map of L1489 IRS seems to show ring-like structures more clearly than in the previous work (Ohashi et al 2022). The continuum emission around IRAS 04169+2702 seems to show a bean-like structure with brighter emission on its southwest side, which could also be a ring-like structure.…”

Early Planet Formation in Embedded Disks (eDisk). IV. The Ringed and Warped Structure of the Disk around the Class I Protostar L1489 IRS

“…The ring location of 56 au found in our observations is consistent with this expectation. Ohashi et al (2022a) also tentatively detected the ring-like structure at ∼90 au in the L1489 IRS disk; the difference with our result is likely due to their coarser spatial resolution. If the dust ring is produced by the dust growth front mechanism, dust grains in the inner 60 au have already grown to millimeter size or even larger (Ohashi et al 2021).…”

“…Outside the dust growth front, the dust particles have not grown and remain in their initial states (micrometer size), while inside the growth front, they have grown to millimeter size or larger and drifted radially, resulting in a local maximum of the surface density profile (i.e., ring-like structure) at the growth front. The expected radial location of the ring matched the observed dust ring locations in young protostellar disks well (Ohashi et al 2021(Ohashi et al , 2022a, suggesting that the dust growth front mechanism might explain the origin of dust rings, particularly in young disks. Ohashi et al (2021) constructed an equation that predicts the radial location of the ring that is formed by this mechanism based on radiative transfer modeling.…”

“…Dust continuum images obtained with ALMA (Yen et al 2014;Sai et al 2020) and the Combined Array for Research in Millimeter-wave Astronomy (CARMA; Sheehan & Eisner 2017) also show a disk-like structure elongated along the northeast-southwest direction. Furthermore, Ohashi et al (2022a) tentatively detect a ring-like structure at r ∼ 90 au in the 1.3 mm dust continuum emission, which may be a hint of dust growth, or even planet formation.…”

Early Planet Formation in Embedded Disks (eDisk). V. Possible Annular Substructure in a Circumstellar Disk in the Ced110 IRS4 System

“…On the other hand, observations toward younger disks around Class 0 and I protostars, which are still embedded in surrounding envelopes, have shown that the embedded disks have sufficient mass to enable giant planet formation without the need for unrealistically high efficiency of planet formation (Tychoniec et al 2018;Tobin et al 2020). Moreover, several embedded disks exhibit substructures (Sheehan & Eisner 2017, 2018Teague et al 2019;Segura-Cox et al 2020;Sheehan et al 2020;Ohashi et al 2022), which could be a more direct signature of planet formation. However, how common such substructures are among embedded disks is still unclear as few protostars have been imaged on scales of ∼5-10 au.…”