Protostellar Outflows Shed Light on the Dominant Close Companion Star Formation Pathways

Abstract

Understanding the formation pathway for close-companion protostars is central to unraveling the processes that govern stellar multiplicity and very early star formation. We analyze a large sample of 51 Class 0/I close-companion protostellar systems, of which 38 show detectable outflows, yielding 42 measured outflows used in our analysis. We use ALMA observations of 11 systems in Perseus and 40 systems in Orion. These companions formed either directly at these small scales ($\lesssim 500$ au separations) via disk fragmentation or at larger scales ($> 1000$ au separations) via turbulent fragmentation followed by inward migration. Because of differences in formation mechanism, the former is expected to have preferentially aligned disks and outflows, whereas the latter is expected to show no preferred alignment. The relative prevalence of these formation pathways remains uncertain, yet it is critical to forming a comprehensive picture of star formation. We examine the distribution of position angles of companion protostars relative to the position angles of their molecular outflows. The outflow, as traced by $^{12}$CO ($J=2\rightarrow1$), is a useful proxy for the angular momentum of the system, expected to be orthogonal to the binary orbital plane. We use a simple model to account for random sampling of inclination and orbital phase in each system, finding that the observations are consistent with a distribution in which the outflows are preferentially orthogonal to the companions. Based on this analysis, we suggest disk fragmentation is the dominant formation pathway for close-companion protostellar systems.

Figures (17)

• Figure 1: A subset of two typical close binary systems with one outflow. These are integrated intensity (moment 0) maps of $^{12}$CO ($J=2\rightarrow1$). The axes are relative to the center of the image which is given in Table \ref{['table:by_field']} along with the integrated velocities. Below each is a zoom-in panel of the continuum (2 GHz band centered at 232.5 GHz) sources, with a box size of 3" by 3". The white arrows indicate the binary separation PA, and the cyan arrow(s) indicates the measured outflow PA(s) in the direction the blue-shifted lobe. All images are shown in Appendix \ref{['sec:appendix']}.
• Figure 2: A subset of two close-multiple systems with more complex outflow phenomena. These are integrated intensity (moment 0) maps of $^{12}$CO ($J=2\rightarrow1$). The axes are relative to the center of the image which is given in Table \ref{['table:by_field']} along with the integrated velocities. Below each is a zoom-in panel of the continuum (2 GHz band centered at 232.5 GHz) sources, with a box size of 3" by 3". The white arrows indicate the binary separation PA, and the cyan arrow(s) indicates the measured outflow PA(s) in the direction the blue-shifted lobe. All images are shown in Appendix \ref{['sec:appendix']}. We categorize HOPS-288 (left) as a hierarchical triple with two outflows. We categorize HOPS-290 (right) as a binary with two outflows.
• Figure 3: The observed distribution of $\Delta$PA values. The distribution includes 42 values for $\Delta$PA: 34 single outflow binaries and 4 where each binary component drives its own outflow.
• Figure 4: The cumulative frequency distribution of $\Delta$PA. We plot the observed data along with the best fit model and four other models for reference. The observations most closely match the simulated distribution of 94% orthogonal outflows.
• Figure 5: All Fields (1 of 13). Same as Figure \ref{['fig:fig_1']}, but for all fields. Fields with no measured outflows are displayed with maximum intensity (moment 8) maps, as labeled in the map title.