Early Planet Formation in Embedded Disks (eDisk). VIII. A Small Protostellar Disk around the Extremely Low-Mass and Young Class 0 Protostar, IRAS 15398-3359

Abstract

Protostellar disks are a ubiquitous part of the star formation process and the future sites of planet formation. As part of the Early Planet Formation in Embedded Disks (eDisk) large program, we present high-angular resolution dust continuum ($\sim 40$mas) and molecular line ($\sim 150$mas) observations of the Class 0 protostar, IRAS 15398-3359. The dust continuum is small, compact, and centrally peaked, while more extended dust structures are found in the outflow directions. We perform a 2D Gaussian fitting to find the deconvolved size and $2\ddot{I}ƒ$ radius of the dust disk to be $4.5\times 2.8\mathrm{au}$ and $3.8\mathrm{au}$, respectively. We estimate the gas+dust disk mass assuming optically thin continuum emission to be $0.6-1.8M_{\mathrm{jup}}$, indicating a very low-mass disk. The CO isotopologues trace components of the outflows and inner envelope, while SO traces a compact, rotating disk-like component. Using several rotation curve fittings on the PV diagram of the SO emission, the lower limits of the protostellar mass and gas disk radius are $0.022M_{\odot }$ and $31.2\mathrm{au}$ from our Modified 2 single power-law fitting. A conservative upper limit of the protostellar mass is inferred to be $0.1M_{\odot }$. The protostellar mass-accretion rate and the specific angular momentum at the protostellar disk edge are found to be between $1.3-6.1\times 10^{-6}{M}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ and $1.2-3.8\times 10^{-4}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{pc}$, respectively, with an age estimated between $0.4-7.5\times 10^4$yr. At this young age with no clear substructures in the disk, planet formation would likely not yet have started. This study highlights the importance of high-resolution observations and systematic fitting procedures when deriving dynamical properties of deeply embedded Class 0 protostars.

28 pages, 16 figures. Accepted for publication in ApJ as one of the first-look papers of the eDisk ALMA Large Program