One-Sided Fountain from a Young Star express
A young star that hasn't yet ignited its thermonuclear furnace is surrounded by a rotating disk of gas and dust. To gain mass, it needs to shed excess rotation — otherwise, material won't fall onto the star but will keep orbiting. Usually, this excess is carried away by two narrow gas jets that shoot from the star's poles in opposite directions, like two fountains.
But for the protostar HOPS 10 in the constellation Orion, the ALMA telescope saw only one fountain. The high-speed jet, moving at velocities from +44 to +66 km/s, is directed only to one side, while the other half is missing. Meanwhile, the slow gas outflow (from -20 to +30 km/s) is observed on both sides, as expected. The disk is almost edge-on to us, and there's little dust in the way, so the second jet can't simply be hidden behind a cloud. Shock wave signatures in SiO molecules confirm that the jet is real, not a random gas cloud.
Astronomers suspect that the disk's magnetic field lines are unevenly loaded with material: on one side, more gas accumulates, and that's what shoots out as a jet. It's like a double-barreled shotgun firing only one barrel — the charge was only there. Such asymmetry can reveal a lot about how young stars gather mass and lose rotation.
Original abstract
as written by the authors arXiv:2602.09404
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 ALMA Band 7 (345 GHz) and Band 6 (230 GHz) observations of CO (3-2), 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 with velocity offsets of +44 to +66 km s-1, unlike the predominantly bipolar morphology characteristic of most protostellar jets. While the low-velocity outflow, with velocity offsets of -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 magnetic field lines provides the most plausible explanation for the observed monopolarity.
The jet of HOPS 10 reaches speeds of 66 km/s — faster than a rifle bullet, but hundreds of times slower than light.