Expanded Very Large Array observations of the H66{\alpha} and He66{\alpha} recombination lines toward MWC 349A

TL;DR

This study used the Expanded Very Large Array to observe recombination lines and continuum emission from MWC 349A, revealing details about its ionized gas, including temperature, composition, and maser activity.

Contribution

First high-resolution observations of H66α and He66α lines toward MWC 349A with the EVLA, clarifying line emission mechanisms and ionized gas properties.

Findings

H66α line is not strongly masing, but moderate maser contribution possible.

Ionized helium to hydrogen ratio y+ = 0.12 ± 0.02.

Electron temperature of ionized gas is 6,300 ± 600 K.

Abstract

We have used the greatly enhanced spectral capabilities of the Expanded Very Large Array to observe both the 22.3 GHz continuum emission and the H66{\alpha} recombination line toward the well-studied Galactic emission-line star MWC 349A. The continuum flux density is found to be 411 $\pm$ 41 mJy in good agreement with previous determinations. The H66{\alpha} line peak intensity is about 25 mJy, and the average line-to-continuum flux ratio is about 5%, as expected for local thermodynamic equilibrium conditions. This shows that the H66{\alpha} recombination line is not strongly masing as had previously been suggested, although a moderate maser contribution could be present. The He66{\alpha} recombination line is also detected in our observations; the relative strengths of the two recombination lines yield an ionized helium to ionized hydrogen abundance ratio y+ = 0.12 $\pm$ 0.02. The ionized helium appears to share the kinematics of the thermally excited ionized hydrogen gas, so the two species are likely to be well mixed. The electron temperature of the ionized gas in MWC 349A deduced from our observations is 6,300 $\pm$ 600 K.