A Detached Protostellar Disk around a $\sim$0.2$M_{\odot }$ protostar in a Possible Site of a Multiple Star Formation in a Dynamical Environment in Taurus

Abstract

We report ALMA observations in 0.87 mm continuum and ${}^{12}$CO ($J$ = 3--2) toward a very low-luminosity ($<$0.1 $L_{\odot }$) protostar, which is deeply embedded in one of the densest core MC27/L1521F, in Taurus with an indication of multiple star formation in a highly dynamical environment. The beam size corresponds to $\sim$20 AU, and we have clearly detected blueshifted/redshifted gas in ${}^{12}$CO associated with the protostar. The spatial/velocity distributions of the gas show there is a rotating disk with a size scale of $\sim$10 AU, a disk mass of $\sim$10${}^{-4}$ $M_{\odot }$ and a central stellar mass of $\sim$0.2 $M_{\odot }$. 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 $\sim$10${}^6$ 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 $\sim$0.2 $M_{\odot }$ 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 ($\sim$0.65 $R_{\odot }$). 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.

arXiv comment9 pages, 4 figures, accepted for publication in The Astrophysical Journal

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