A Young Star’s Jet Left a Hydrogen Radio Clue
An archival ALMA observation revealed a broad hydrogen radio line near a massive young star. The signal may trace gas escaping at the base of its jet, not a photograph of the jet itself.
The Discovery
A massive young star is surrounded by activity we cannot simply zoom in and watch. In an October 6 Astronomy & Astrophysics study, researchers report the first detection toward IRAS 18162-2048 of thermal emission from the hydrogen H30α radio recombination line. They found it while examining archival ALMA observations at approximately 1.3 millimetres.
The star drives the known jet associated with HH 80–81. The detected line is a spectral signature of ionised gas, not a visual snapshot of a star throwing material into space.
What Makes It Strange
The emission was not spatially resolved in these observations. The authors estimate its region to be at most roughly 50 astronomical units across, given their adopted distance of 1.4 kiloparsecs. Yet its line profile is broad, about 82 kilometres per second at half maximum, and shifted blueward relative to the surrounding cloud's systemic velocity.
- Measured: a resolved radio spectrum containing the H30α line.
- Inferred: a compact region of ionised gas near the young star.
- Proposed interpretation: outflowing material in the already known radio jet.
The Implications
Ionised emission close to a forming massive star could in principle arise from several environments, including photoionised gas, a disk or a jet base. The authors say the blue shift and broad profile suggest an outflow. That is a testable interpretation, not a direct map of the emitting gas. Further observations might constrain the jet's motion and physical conditions in a region that remains hard to resolve.
A line in a spectrum can reveal movement where a photograph sees only a point.