Discovery of thermal radio recombination line emission towards the high-mass protostar IRAS 18162–2048

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Abstract
Context. The physical conditions within the innermost few tens of astronomical units of high-mass protostars ( M * > 8 M ⊙ ) remain largely unexplored. At these scales, ionised emission can trace photoionised gas, accretion disks, or the base of protostellar jets. Aims. We search for ionised emission in the form of radio recombination lines (RRLs) in high-mass star-forming regions. Methods. We used archival data from the Atacama Millimeter/submillimeter Array (ALMA) interferometer in band 6 (∼1.3 mm) with a resolution of θ beam ∼ 53 mas × 42 mas to examine the high-mass protostar IRAS 18162−2048, the driving source of the Herbig-Haro (HH) objects HH80-81. Results. We detect, for the first time in this protostar, the thermal emission of the RRL H30 α (rest frequency = 231.90093 GHz). The ionised emission is spatially unresolved in our observations, but we estimate an emitting region of ≲50 au at the source distance of 1.4 kpc. The H30 α line profile is broad and spectrally resolved with a local standard of rest velocity of ∼ − 14 km s −1 (i.e. ∼ − 26 km s −1 blueshifted with respect to the systemic velocity of the cloud) and a full width at half maximum of ∼82 km s −1 . These observations suggest that the H30 α line is tracing outflowing blueshifted material in the well-known radio jet from this object. This detection opens the possibility of detailed studies of the kinematics and physical properties of ionised jets.

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