ALMA observations show the compact source SMA3 with a narrow, high-speed bipolar jet, according to an arXiv preprint. Its silicon monoxide, or SiO, emission peaks at radial speeds of about 60 km/s in opposite directions. After accounting for the jet’s angle to our line of sight, the analysis estimates a total speed of roughly 80±18 km/s.
The study examines SMA3 in three molecular lines: SiO J = 5-4, SiO J = 6-5 and CS J = 5-4. It focuses on one compact source, so its conclusions are specific to SMA3.
A jet with a distinctive pattern
The red SiO lobe has a collimation ratio of 4.75, calculated by comparing its full extent with its greatest width. The geometry analysis gives the jet an opening angle of 54.5 degrees and adopts an inclination of 48.6±15 degrees, although the flow-based part of that estimate was measured by eye.
The SiO position-velocity diagrams, which show how gas speed changes along the jet, have triangular shapes on both sides of SMA3. High-velocity gas appears at all distances, while low-velocity gas does not persist far from the source. A spread of velocities at the flow base is also present. The authors interpret this combination as more consistent with a disk wind than with an X-wind model.
The observed SiO lobe extends 2.15 arcseconds in the plane of the sky, corresponding to 3,830 astronomical units at the adopted distance of 1.78 kiloparsecs. Correcting for the inferred inclination gives a length of 5,800±1,700 astronomical units and a kinematic age of 343±181 years.
Different molecules trace different parts of the flow
The two main molecular tracers do not outline the same part of the outflow. SiO follows the highest-velocity spine of the jet, while CS becomes more apparent farther along the flow after the SiO emission weakens. The paper treats this chemical and velocity separation as evidence supporting a structured, magnetically regulated disk-wind system, while noting that the interpretation depends on the tracers and the model used.
The analysis estimates SiO excitation with a two-level rotation diagram under assumptions of local thermodynamic equilibrium and optically thin emission. In plain terms, the method infers how molecules are distributed among different energy states. The reported mean excitation temperature is 42 K after excluding a region near SMA3 with temperatures above 200 K and high uncertainty.
Signs of a repeating engine
Emission peaks recur at roughly 0.2 arcsecond intervals. The analysis translates that spacing into knots about 540 astronomical units apart and an inferred ejection timescale of about 32 years. Under a Keplerian-disk interpretation, it estimates a radius of roughly 9 astronomical units for a derived mass of 0.7 solar masses. The paper treats that scale as more likely related to disk variability than to the jet’s launch region, which is expected to lie below 1 astronomical unit, and says the physical cause of the pattern remains unresolved.
Line intensity and channel-by-channel column-density estimates were used to calculate the mass, momentum and energy in each lobe. The red and blue lobe masses are reported as 2.183±0.008 and 2.129±0.008 in units of 10⁻³ solar masses. Their momenta are 1.111±0.004 and -0.899±0.004 in units of 10⁻¹ solar masses km/s, with opposite signs for the two lobes. The corresponding energies are 6.11±0.03 and 4.23±0.02 in units of 10⁴³ ergs.
The reported mass-flow rate is 0.6±0.3 in units of 10⁻⁵ solar masses per year for each lobe. The forces are 3±2 and 3±1 in units of 10⁻⁴ solar masses km/s per year, while the powers are 18±9 and 12±7 in units of 10⁴⁰ ergs per year. These derived rates use the estimated age of 343±181 years and depend on assumptions including local thermodynamic equilibrium, optically thin emission and a characteristic temperature of 42 K.
What the observation can and cannot settle
The evidence supports a disk-wind interpretation for SMA3, but it does not identify that mechanism as the only possible launch process. The inclination remains uncertain because the two estimates rely on different geometric assumptions, including a thin disk in one method and an opening-angle proxy in another. The source may instead contain a geometrically thick envelope.
The age estimate is treated as an upper limit because it assumes the emitting gas initially traveled at the observed speed. A CS-based age may also be affected by internal shocks that reduce the flow velocity. In addition, the highest blue-shifted SiO J = 6-5 emission lies outside the observed bandpass, so that transition does not provide a complete measure of the blue lobe’s extent.
The document is an arXiv version 1 preprint dated 28 August 2026. The underlying observations are stated to be available through the ALMA Science Archive under project code 2023.1.01346.S. The acknowledgements identify ALMA data and software resources but report no grant funding.
Paper data and sources
Original title: A High Velocity SiO Jet from S255N SMA3
Authors: P. D. Klaassen, M. Reiter, Y. Zhang et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: Not available
Original paper · Full text