The SOFIA Massive (SOMA) Star Formation Q-band Follow-up. II. Hydrogen Recombination Lines Toward High-Mass Protostars

Abstract

Hydrogen recombination lines (HRLs) are valuable diagnostics of the physical conditions in ionized regions around high-mass stars. Understanding their broadening mechanisms and intensity trends can provide insights into the densities, temperatures, and kinematics of HII regions. We investigate the properties of ionized gas around massive protostars by analyzing hydrogen recombination lines (H-alpha and H-beta) in the Q-band. Observations were conducted using the Yebes 40m radio telescope in the Q-band (30.5~50 GHz) toward six high-mass protostars selected from the SOMA Survey (G45.12+0.13, G45.47+0.05, G28.20-0.05, G35.20-0.74, G19.08-0.29, and G31.28+0.06). The line profiles were analyzed to assess broadening mechanisms, from which electron densities and temperatures were derived. We compared our results with Q-band data from the TianMa 65m Radio Telescope (TMRT) and ALMA Band 1 Science Verification observations of Orion KL. A total of eight H-alpha (n = 51 to 58) and ten H-beta (n = 64 to 73) lines were detected toward G45.12+0.13, G45.47+0.05, and G28.20-0.05, with non-detections in the other sources. Electron densities of ~1-5$\times$10${}^6$ cm${}^{-3}$ and temperatures of 8000-10000 K were derived. Orion KL shows one order of magnitude lower electron density, but a similar temperature. Notably, G45.12 and G28.20 show increasing intensity with frequency for both H-alpha and H-beta, in contrast to the decreasing trend in Orion KL. The observed line widths indicate contributions from both thermal and dynamical broadening, suggesting high-temperature ionized gas affected by turbulence, outflows, rotation, or stellar winds. Pressure broadening may also play a minor role. The contrasting intensity trends likely reflect differences in local physical conditions or radiative transfer effects, warranting further study through higher-resolution observations and modeling.

Accepted for publication in A&A

References in corpus (18)

• Resolution of the Distance Ambiguity for Galactic HII Regions
• Yebes 40 m radio telescope and the broad band NANOCOSMOS receivers at 7 mm and 3 mm for line surveys
• The Early Evolution of Massive Stars: Radio Recombination Line Spectra
• The Ionization of Accretion Flows in High Mass Star Formation: W51e2
• Binary system and jet precession and expansion in G35.20-0.74N
• Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology
• Internal Dynamics of the Hypercompact H II Region G28.20-0.04N
• The SOFIA Massive (SOMA) Star Formation Survey. II. High Luminosity Protostars
• ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -XIV. Properties of resolved UC Hii regions
• The SOFIA Massive (SOMA) Star Formation Survey. IV. Isolated Protostars
• Discovery of a Photoionized Bipolar Outflow towards the Massive Protostar G45.47+0.05
• The ionized warped disk and disk wind of the massive protostar Monoceros R2-IRS2 seen with ALMA
• A Q-band line survey towards Orion KL using the Tianma radio telescope
• Outflow-Confined HII regions. II. The Early Break-Out Phase
• Isolated Massive Star Formation in G28.20-0.05
• First Detection of Radio Recombination Lines of Ions Heavier than Helium
• High-Sensitivity Observations of Molecular Lines with the Arecibo Telescope
• Imaging the jet of MWC 349A with resolved Radio Recombination Line emission from ALMA