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

TL;DR

This study presents a Q-band HRL survey toward six massive protostars from the SOMA sample using the Yebes 40 m telescope, detecting 18 HRLs toward three sources and deriving electron temperatures around 8,000–10,000 K and electron densities of roughly 1–5×10^6 cm^-3, higher than those found in Orion KL. By analyzing line widths with thermal, pressure, and dynamical broadening contributions, the authors find that thermal broadening alone cannot explain the observed widths (∼30–40 km s^-1) and that dynamical broadening likely plays a significant role, with pressure broadening being secondary. They report contrasting frequency trends in HRL intensities, with G45.12 and G28.20 showing increasing intensity with frequency while Orion KL shows a decrease, suggesting differences in local physical conditions, radiative transfer effects, or beam dilution. LTE-like line ratios for the HRLs across the detected transitions imply that the gas is largely near LTE, though non-LTE effects and compact source structure could influence specific lines. The work highlights how HRLs in the Q-band can constrain the density, temperature, and kinematics of ionized regions around forming massive stars, and it underscores the need for higher-resolution observations and detailed radiative-transfer modeling to interpret the observed trends fully.

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.

Figures (8)

• Figure 1: The observed and Gaussian fitted spectra of H$\alpha$ and H$\beta$ lines towards G45.12+0.13. The black line represents observed H$\alpha$ and H$\beta$ lines toward G45.12+0.13, and the red line depicts the Gaussian fitted spectra.
• Figure 2: The observed and Gaussian fitted spectra of H$\alpha$ and H$\beta$ lines towards G45.47+0.05. The black line represents observed H$\alpha$ and H$\beta$ lines toward G45.47+0.05, and the red line depicts the Gaussian fitted spectra.
• Figure 3: The observed and Gaussian fitted spectra of H$\alpha$ and H$\beta$ lines towards G28.20-0.05. The black line represents observed H$\alpha$ and H$\beta$ lines toward G28.20-0.05, and the red line depicts the Gaussian fitted spectra.
• Figure 4: Comparison of the observed intensity of H$\alpha$ and H$\beta$ towards our target sources and Orion KL.
• Figure 5: Comparison between observed and theoretical line widths of H$\alpha$ lines for different electron densities and temperatures. In each panel, the blue dotted line represents the contribution from pressure broadening, the orange dashed line represents thermal broadening, and the green dash-dotted line depicts the combined contribution from thermal and pressure broadening. Line width data of Orion KL is taken from liu2022.