The ALMA Survey of 70 $μ$m 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 $μ$m-dark clump G34.74-0.12, based on ALMA 1.3 mm observations with an angular resolution of 1''.6 (~ 5000 au). This outflow originate from a 9.7 M_ core, exhibits a fork-shaped stream structure in its red-shifted 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, 1.8 M_ km s⁻¹), as part of the ALMA Survey of 70 $μ$m dark High-mass clumps in Early Stages (ASHES) project. All the newly discovered morphological and kinematic features associated with these extremely young protostellar outflows (with timescales of 10³ - 10⁴ years) suggest that the initial stages of star formation are more complicated than previously understood.