A Detached Protostellar Disk around a ∼0.2 M _o Protostar in a Possible Site of a Multiple Star Formation in a Dynamical Environment in Taurus

We report ALMA observations in 0.87 mm continuum and ¹²CO (J = 3-2) toward a very low-luminosity (<0.1 L _o) protostar, which is deeply embedded in one of the densest cores, MC27/L1521F, in Taurus with an indication of multiple star formation in a highly dynamical environment. The beam size corresponds to ∼20 au, and we have clearly detected blueshifted/redshifted gas in ¹²CO associated with the protostar. The spatial/velocity distributions of the gas show there is a rotating disk with a size scale of ∼10 au, a disk mass of ∼10^-4 M _o, and a central stellar mass of ∼0.2 M _o. The observed disk seems to be detached from the surrounding dense gas, although it is still embedded at the center of the core whose density is ∼10⁶ cm^-3. The current low-outflow activity and the very low luminosity indicate that the mass accretion rate onto the protostar is extremely low in spite of a very early stage of star formation. We may be witnessing the final stage of the formation of ∼0.2 M _o protostar. However, we cannot explain the observed low luminosity with the standard pre-main-sequence evolutionary track unless we assume cold accretion with an extremely small initial radius of the protostar (∼0.65 ). These facts may challenge our current understanding of the low mass star formation, in particular the mass accretion process onto the protostar and the circumstellar disk.