The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). XII. Unanchored Forked Stream in the Propagating Path of a Protostellar Outflow

Outflows are key indicators of ongoing star formation. We report the discovery of an unanchored forked stream within the propagating path of an extremely young protostellar outflow in the 70 μm-dark clump G34.74-0.12, based on Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations with an angular resolution of 1 .″ 6 (∼5000 au). This outflow originates from a 9.7 M⊙ core, exhibits a fork-shaped stream structure in its redshifted lobe, which is traced by CO (2–1), SiO (5–4), and H2CO (30,3–20,2). It has a momentum of 13 M⊙ km s−1, an energy of 107 M⊙ km2 s−2, and a dynamical timescale of ∼104 yr. Significantly, the enhanced relative abundances of SiO, H2CO, and CH3OH with respect to CO, along with the increased temperature at the forked point, indicate a collisional origin. The forked point does not coincide with any dust continuum core >0.1 M⊙. Moreover, CO (2–1) emission also traces three other outflows in this region, characterized by their masses (0.40, 0.02, and 0.15 M⊙) and momenta (5.2, 0.2, and 1.8 M⊙ km s−1), as part of the ALMA Survey of 70 μm dark High-mass clumps in Early Stages project. All the newly discovered morphological and kinematic features associated with these extremely young protostellar outflows (with timescales of 103–104 yr) suggest that the initial stages of star formation are more complicated than previously understood.