A Monopolar Jet from Protostar HOPS 10: Evidence for Asymmetric Magnetized Launching

Abstract A fundamental challenge in star formation is understanding how a protostar accretes mass from its circumstellar disk while removing excess angular momentum. Protostellar jets are widely invoked as the primary channels for angular-momentum removal, yet the mechanism by which they are launched and extract angular momentum remains poorly constrained. Here, we report high-resolution Atacama Large Millimeter/submillimeter Array Band 7 (345 GHz) and Band 6 (230 GHz) observations of 12 CO (3–2), 12 CO (2–1), and SiO (5–4) emission from the protostar HOPS 10 (G209.55−19.68S2). The combined data trace both the entrained outflow and the collimated jet with excellent spatial and velocity resolution, revealing a uniquely monopolar protostellar jet—the clearest example reported to date. The system exhibits a distinctly unipolar high-velocity jet (velocity offset, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">HV</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">off</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">observed</mml:mi> </mml:mrow> </mml:msub> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">systemic</mml:mi> </mml:mrow> </mml:msub> </mml:math> = +44 to +66 km s −1 ), unlike the predominantly bipolar morphology characteristic of most protostellar jets. While the low-velocity outflow (velocity offset, V LV,off = V observed − V systemic = −20 to +30 km s −1 ) is detected in both directions, the high-velocity jet appears only on one side, and this monopolarity is consistent across all tracers. Given the nearly edge-on geometry and low submillimeter extinction, comparable emission would normally be expected from both lobes. The shock tracer SiO emission confirms a genuine, highly collimated jet rather than cloud contamination, and no ambient structure is capable of obscuring a counterjet. We argue that intrinsically asymmetric mass loading along the disk’s magnetic field lines provides the most plausible explanation for the observed monopolarity.