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

TL;DR

This paper investigates the earliest stages of high-mass star formation in the 70 μm-dark clump G34.74-0.12 using ALMA Band-6 data, revealing four very young protostellar outflows and a novel unanchored forked stream in Outflow I. By combining CO (2-1) with SiO (5-4), H2CO and CH3OH line maps and LVG-based temperature estimates, the authors characterize outflow kinematics, physical properties, and chemical enrichment at shock sites, notably at knot R2 where the fork originates. They argue that the forked morphology results from interaction with ambient dense gas or a filament (potential jet–cloud collision) and/or ambient pressure gradients, highlighting how the earliest outflow phases are shaped by environmental conditions. Overall, the work demonstrates that initial star formation in such clumps is dynamically complex and environment-dependent, with multiple outflows persisting on timescales of $10^{3}$–$10^{4}$ years and leaving a chemical and thermal imprint on their surroundings.

Abstract

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 $M_{\odot}$ core, exhibits a fork-shaped stream structure in its red-shifted lobe, which is traced by CO (2-1), SiO (5-4), and H$_2$CO (3$_{0,3}$-2$_{0,2}$). It has a momentum of 13 $M_{\odot}$ km s$^{-1}$, an energy of 107 $M_{\odot}$ km$^{2}$ s$^{-2}$, and a dynamical timescale of ~10$^{4}$ yr. Significantly, the enhanced relative abundances of SiO, H$_2$CO, and CH$_3$OH 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 $M_{\odot}$. Moreover, CO (2-1) emission also traces three other outflows in this region, characterized by their masses (0.40, 0.02 and 0.15 $M_{\odot}$) and momenta (5.2, 0.2, 1.8 $M_{\odot}$ 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 10$^3$ - 10$^4$ years) suggest that the initial stages of star formation are more complicated than previously understood.

Figures (15)

• Figure 1: CO 2-1 outflow contours overlaid on 1.34 mm continuum grey map. The black contours show the emission of the continuum with levels starting from 3 $\sigma$ and increase in steps of 2 $\sigma$ (1 $\sigma$ = 0.3 mJy beam$^{-1}$ km s$^{-1}$). The blue and red contours represent the blue- and red-shifted components, respectively. The integrated velocity ranges of the blue- and red-shifted components are shown in the upper right corner ($V_{\rm sys,lsr}$ is 79.0 km s$^{-1}$). The contour levels for the blue-shifted component start at 8 $\sigma$, while those for the red-shifted component start at 24 $\sigma$, both increasing in increments of 16 $\sigma$ (1 $\sigma$ = 0.05 Jy beam$^{-1}$ km s$^{-1}$). The green, blue, orange, and magenta bipolar arrows indicate the directions of the outflows, with the outflow IDs labeled. The green and cyan crosses indicate the positions of the identified cores, with the green cross representing the location of the most massive core 1 (9.7 $M_{\odot}$). Core IDs labeled are taken from Morii2023ApJ...950..148M. The synthesized beam is given in the bottom left.
• Figure 2: The intensity-weighted velocity map (moment 1) of SiO (5-4), H$_2$CO (3$_{0,3}$-2$_{0,2}$), H$_2$CO (3$_{2,2}$-2$_{2,1}$), H$_2$CO (3$_{2,1}$-2$_{2,0}$), CH$_3$OH (4$_{2}$-3$_1$), and HC$_3$N (24-23). Black contours represent their integrated intensity levels. SiO (5-4): start from 6 $\sigma$ and increase in steps of 16 $\sigma$ (1 $\sigma$ = 0.028 Jy beam$^{-1}$ km s$^{-1}$); H$_2$CO (3$_{0,3}$-2$_{0,2}$): start from 6 $\sigma$ and increase in steps of 6 $\sigma$ (1 $\sigma$ = 0.025 Jy beam$^{-1}$ km s$^{-1}$); H$_2$CO (3$_{2,2}$-2$_{2,1}$): start from 4 $\sigma$ and increase in steps of 4 $\sigma$ (1 $\sigma$ = 0.020 Jy beam$^{-1}$ km s$^{-1}$); H$_2$CO (3$_{2,1}$-2$_{2,0}$): start from 4 $\sigma$ and increase in steps of 4 $\sigma$ (1 $\sigma$ = 0.022 Jy beam$^{-1}$ km s$^{-1}$); CH$_3$OH (4$_{2}$-3$_1$): start from 4 $\sigma$ and increase in steps of 4 $\sigma$ (1 $\sigma$ = 0.026 Jy beam$^{-1}$ km s$^{-1}$); HC$_3$N (24-23): start from 4 $\sigma$ and increase in steps of 1.5 $\sigma$ (1 $\sigma$ = 0.024 Jy beam$^{-1}$ km s$^{-1}$). The blue and red contours represent the blue- and red-shifted lobes of outflow I, II, III, and IV, respectively, as labeled in Fig. \ref{['fig:CO21_outflow']}. The green cross indicates the position of the most massive core 1, and the cyan crosses indicate the position of core 2, 4, and 10. The orange arrow in SiO (5-4) map represents the path for the outflow I. Yellow crosses mark the positions of knots B1, B2, B3, R1, and R2. The synthesized beam is given in the bottom left.
• Figure 3: The left panel shows the position-velocity diagram of SiO (5-4) along the east–west direction (orange path shown in Fig. \ref{['fig:velocity_maps']}) of outflow I with a slice width of 1$^{\prime \prime}$.4. The contour levels start from 8 $\sigma$ and increase in steps of 5 $\sigma$ (1 $\sigma$ = 0.004 Jy beam$^{-1}$). The vertical white dashed line indicates the position of central protostar, the white horizontal dashed line indicates the systemic velocity. Panel on the right shows the line profiles of SiO (5-4) extracted from a synthesized beam towards positions R2, core 1, and B2. The vertical red dashed line represents the systemic velocity (79.0 km s$^{-1}$) of G34.74-0.12.
• Figure 4: Channel map of the CO (2-1) display the forked stream structure. The contour levels are 20 $\sigma$, 75 $\sigma$, and 110 $\sigma$ (1 $\sigma$ = 0.05 Jy beam$^{-1}$). The green dashed lines indicate the paths along two streams of outflow I, while the blue dashed line indicates the path of outflow II. The green cross indicates the position of core 1, and the cyan crosses indicate the position of core 2, 4, and 10, as labeled in Fig. \ref{['fig:CO21_outflow']}. The synthesized beam is given in the bottom right.
• Figure 5: The temperature maps obtained from the Non-LTE method using RADEX. The black contours represent the intensity of H$_2$CO (3$_{2,1}$-2$_{2,0}$), starting at 4 $\sigma$ and increasing in steps of 4 $\sigma$ (1 $\sigma$ = 0.022 Jy beam$^{-1}$ km s$^{-1}$). The black cross indicates the position of the core 1, 2, 4, and 10. The synthesized beam is given in the bottom right.