Using the observations from the Atacama Large Millimeter/submillimeter Array (ALMA) of eleven Galactic protoclusters, KIAA postdoctoral fellow Tapas Baug with Ke Wang (KIAA), Tie Liu (SHAO) and collaborators showed that direction of gas flow along Galactic filaments might not always co-alliance with the orientation of protostellar accretion disks. This particular work is published in the Astrophysical Journal (Baug et al., 2020, ApJ, 890, 44).
Astronomical filaments are characterized by over-dense elongated structures of the interstellar medium compared to their surrounding, and have an aspect ratio of more than 5-10. Herschel observations revealed such filaments in almost all Galactic molecular clouds. This particular finding leads to the believe that filaments are likely to play an important role in star formation. Indeed, dense star-forming cores often form within these filaments. Two types of gas flows are typically expected in such star-forming filaments – large-scale flows from the surrounding cloud onto the short-axis of the filaments, or flow of gas from the parent cloud along the long-axis of the filament. Classically, it is expected that the angular momentum of a star-forming molecular cloud is transported to protostars via dense cores. Thus, in a non-turbulent scenario, the flow of gas along the short or long axes of a filament leads to rotation of the embedded cores either parallel or perpendicular to the parent filament. Under such conditions, if the embedded protostars within the cores inherit the angular momentum axis, they should also follow the same preferred direction of the rotation as the cores. A couple of previous studies on Galactic massive star-forming regions found outflows are typically perpendicular with respect to filaments’ long-axes.
Baug and collaborators performed a comprehensive study of eleven Galactic massive protoclusters to examine the role of filamentary flows in the protostellar rotation (vis-e-vis angular momentum axis). The angular momentum axis of a protostar can be inferred from the direction of the bipolar outflows which is typically perpendicular to the accretion disks. Using the CO (3-2) line data from the ALMA, Baug and his team identified more than hundred outflow lobes in these eleven protoclusters (see an example region in Figure 1). Among the eleven regions, seven are embedded within large-scale filamentary structures. The plane-of-sky position angles between the outflow lobes and filaments (γFil) show no preferred orientation, but they are rather oriented in a random fashion. As the plane-of-sky position angle depicts the 2-D projection of the actual 3-D angle, Baug and his collaborators also compared the distribution of γFil with the simulated (Monte-Carlo) cumulative distribution function of 2-D projections of 3-D angles. The distribution of γFil correlates more to the simulated random distribution (see Figure 2). The observed scenario could also depend on the evolutionary stage of the source and turbulence in the accreted gas along the filaments.
These results advance the idea that large-scale filamentary accretion might not always play an important role at the protostellar scale.
Figure 1: (a) Outflows toward the source IRAS 16071-5142. Blue and red contours overlaid on the continuum image are to depict the blue and red- lobes of the outflows. Outflow directions and end points are also marked. Yellow ellipse mark continuum cores that are driving outflows. (b) Cumulative histogram function of the projected plane-of-sky position angles between the directions of γFil. Green dashed line is the simulated cumulative distribution function which shows the outflow directions are purely random.