2025ApJ...983...81B

We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered the n = 42 → 41 transitions of hydrogen (H41α) and helium (He41α). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ∼200 protoplanetary disks. We detect H41α from 17 disks, all of which are HST-identified “proplyds.” The detected H41α emission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41α emission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41α lines and find line fluxes of ∼30-800 mJy km s^–1 and line widths of ∼30-90 km s^–1. The derived line widths indicate that the broadening of proplyd H41α emission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free-free flux, imply that the ionization fronts of H41α-detected proplyds have electron temperatures of ∼6000-11,000 K and electron densities of ∼10⁶-10⁷ cm^–3. Finally, we detect He41α toward one H41α-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions.