Discovery of thermal radio recombination line emission toward the high-mass protostar IRAS18162−2048
Abstract
The physical conditions within the innermost few tens of au of high-mass protostars (M*>8 M⊙) remain largely unexplored. At these scales, ionised emission may trace photoionised gas, accretion disks, or the base of protostellar jets. We search for ionised emission in the form of radio recombination lines (RRLs) in high-mass star-forming regions. We use archival data from the Atacama Millimeter/submillimeter Array (ALMA) interferometer in band 6 (∼1.3 mm) with a resolution of θbeam∼53 mas×42 mas to examine the high-mass protostar IRAS18162−2048, driving source of the Herbig-Haro (HH) objects HH80-81. We detect, for the first time in this protostar, the thermal emission of the RRL H30α (rest frequency=231.90093 GHz). The ionised emission is spatially unresolved in our observations, but we estimate an emitting region ≲50 au at the source distance of 1.4 kpc. The H30α 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 blue-shifted 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 H30α line is tracing outflowing blue-shifted 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.
Versions
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BibTeX
@misc{fedriani2026discovery,
title = {{Discovery of thermal radio recombination line emission toward the high-mass protostar IRAS18162$-$2048}},
author = {R. Fedriani and J. F. Gomez and Y. Cheng and M. Osorio and G. Anglada and C-Y. Law and J. M. Masque and A. F. Placinta-Mitrea and G. Blazquez-Calero and G. Cosentino},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.03343},
eprint = {2610.03343},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.03343},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Discovery of thermal radio recombination line emission toward the high-mass protostar IRAS18162−2048
AU - R. Fedriani
AU - J. F. Gomez
AU - Y. Cheng
AU - M. Osorio
AU - G. Anglada
AU - C-Y. Law
AU - J. M. Masque
AU - A. F. Placinta-Mitrea
AU - G. Blazquez-Calero
AU - G. Cosentino
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2610.03343
UR - https://arxiv.org/abs/2610.03343
AB - The physical conditions within the innermost few tens of au of high-mass protostars (M_*>8 M_⊙) remain largely unexplored. At these scales, ionised emission may trace photoionised gas, accretion disks, or the base of protostellar jets. We search for ionised emission in the form of radio recombination lines (RRLs) in high-mass star-forming regions. We use 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 IRAS18162−2048, driving source of the Herbig-Haro (HH) objects HH80-81. We detect, for the first time in this protostar, the thermal emission of the RRL H30α (rest frequency=231.90093 GHz). The ionised emission is spatially unresolved in our observations, but we estimate an emitting region ≲50 au at the source distance of 1.4 kpc. The H30α 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 blue-shifted 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 H30α line is tracing outflowing blue-shifted 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.
ER -