Proplyds are protoplanetary disks surrounded by cocoons of ionized gas with a cometary morphology. These disks are commonly found in the nearest star-forming clusters, and their morphologies indicate that protoplanetary disks are influenced by radiative feedback from nearby massive stars. In this talk, I will present new ALMA observations that, for the first time, detect hydrogen and helium radio recombination lines from proplyds in the Orion Nebula Cluster (ONC).
We have imaged the ONC at 3.1 mm with a spectral setup that covers the n = 42 to n = 41 transitions of hydrogen (H41a) and helium (He41a). We detect H41a from 17 proplyds. The detected H41a emission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41a emission is produced by gas that has been photoevaporated off the disk and externally ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41a lines, and find line fluxes of ~40-600 mJy km/s and line widths of ~35-90 km/s. The derived line widths imply that the broadening of proplyd H41a emission is dominated by bulk gas motions associated with disk photoevaporation. The derived line fluxes, when compared with measurements of free-free emission, imply that proplyds have electron temperatures of 6,000 to 8,000 K and electron densities of 10^6 to 10^7 cm^-3. Finally, we detect He41a towards one H41a-detected proplyd and find that the He41a to H41a line ratio of this proplyd implies a helium abundance that is two times greater than the canonical helium abundance for the Orion Nebula. This suggests that either proplyds are enriched in helium relative to the Orion Nebula, or that the Orion Nebula is more abundant in helium than currently thought.
Our study demonstrates that radio recombination lines are readily detectable in externally ionized protoplanetary disks, i.e., proplyds. This essentially opens a new way to study planet formation in clustered star-forming regions.