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

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.

💡 Research Summary

In this paper the authors present a detailed ALMA study of the Class 0 protostar HOPS 10 (G209.55‑19.68S2) in Orion, using simultaneous Band 7 (345 GHz) and Band 6 (230 GHz) observations of 12CO (3‑2), 12CO (2‑1) and SiO (5‑4). The data achieve a uniform angular resolution of ~0.16″ (≈ 160 AU) and a velocity resolution of 2 km s⁻¹, allowing the authors to separate the outflow into two distinct kinematic components: a low‑velocity (LV) wide‑angle wind (−20 to +30 km s⁻¹) that is bipolar, and a high‑velocity (HV) collimated jet (+44 to +66 km s⁻¹) that appears only on the red‑shifted (north‑east) side. The SiO emission, a well‑known shock tracer, is detected exclusively in the HV component, confirming that the red‑shifted jet is a genuine, highly collimated molecular jet rather than a line‑of‑sight artifact or ambient cloud contamination.

The authors carefully rule out observational bias as the cause of the apparent monopolarity. HOPS 10 is viewed nearly edge‑on (inclination i ≈ 20°) and suffers little sub‑millimeter extinction, so a counter‑jet would be expected to be detectable if present. The sensitivity and uv‑coverage are identical for both sides, yet no HV emission is seen in the blue‑shifted lobe, nor is any SiO counterpart present. The surrounding CO maps show no dense clump that could obscure a counter‑jet, and the systemic velocity derived from 13CO (3‑2) is well constrained (V_sys ≈ 8.3 km s⁻¹).

Physical conditions of the jet are estimated using the CO (3‑2)/CO (2‑1) line ratio under the LTE assumption. The derived excitation temperature is low (T_ex ≈ 9 K, range 8–20 K) and the optical depths are modest (τ_2‑1 ≈ 0.5, τ_3‑2 ≈ 1.1), indicating that the HV jet is partially optically thin and relatively cool compared with typical atomic jets. The authors acknowledge that the two bands were observed 2.15 years apart, so proper motion and time variability introduce additional uncertainties.

Mass‑loss rates are calculated separately for the LV wind and the HV jet. Assuming optically thin CO emission, a CO/H₂ abundance of 10⁻⁴, and an excitation temperature of 50 K for the LV component, the authors obtain \dot{M}wind ≈ (2.7 ± 0.9) × 10⁻⁶ M⊙ yr⁻¹ for the red lobe and \dot{M}wind ≈ (3.2 ± 1.1) × 10⁻⁶ M⊙ yr⁻¹ for the blue lobe, after correcting for inclination. For the HV jet, a rough estimate yields \dot{M}jet ≈ 1 × 10⁻⁶ M⊙ yr⁻¹, roughly 30 % of the wind’s mass‑loss rate, consistent with magneto‑centrifugal disk‑wind models that predict a fast, dense jet embedded within a slower, broader wind.

To explain the monopolarity, three scenarios are examined: (1) environmental asymmetry (density or pressure gradients), (2) differential extinction or obscuration, and (3) intrinsically asymmetric mass loading along the disk magnetic field lines. The first two are dismissed based on the lack of dense material on the blue side and the negligible extinction at the observed wavelengths. The authors therefore favor the third explanation, arguing that the magnetic field geometry on the disk surface can become asymmetric, leading to preferential mass loading on one side. This interpretation aligns with recent MHD simulations that show how non‑uniform ionization or warped field lines can produce one‑sided jets.

The paper also discusses methodological limitations. The LTE and optically thin assumptions may not hold throughout the jet, especially given the modest optical depths derived. The line‑ratio analysis is constrained by having only two CO transitions; attempts at non‑LTE RADEX modeling were inconclusive. Moreover, the temporal offset between the Band 7 and Band 6 datasets could blur proper‑motion signatures. The authors propose future work involving higher‑J CO and SiO transitions, polarized dust observations to map the magnetic field directly, and multi‑epoch monitoring to capture jet variability.

In summary, this study provides the clearest observational evidence to date of a truly monopolar protostellar jet. By combining high‑resolution, multi‑band ALMA data with careful kinematic and radiative‑transfer analysis, the authors demonstrate that the asymmetry is intrinsic rather than observational. The findings have important implications for jet‑launching theories, suggesting that asymmetric magnetized launching—driven by uneven mass loading along disk magnetic field lines—may be a natural outcome in some star‑forming environments.