Radio Recombination Line Observations Toward the Massive Star Forming Region W51 IRS1

TL;DR

This study presents radio recombination line observations of the W51 IRS1 star-forming region, analyzing spectral lines to determine physical properties like velocity, temperature, and emission measure of the ionized gas.

Contribution

First detailed radio recombination line analysis of W51 IRS1, providing new measurements of its physical conditions and ionized gas properties.

Findings

Measured source velocity at 56.6 km/s (LSR)

Estimated electron temperature at 8500 K

Derived emission measure of 5.4 x 10^6 pc cm^-6

Abstract

We observed radio recombination lines (RRLs) toward the W51 molecular cloud complex, one of the most active star forming regions in our Galaxy. The UV radiation from young massive stars ionizes gas surrounding them to produce HII regions. Observations of the W51 IRS1 HII region were made with the Arecibo 305 m telescope. Of the full 1-10 GHz database, we have analyzed the observations between 4.5 and 5 GHz here. The steps involved in the analysis were: a) bandpass calibration using on-source/off-source observations; b) flux density calibration; c) removing spectral baselines due to errors in bandpass calibration and d) Gaussian fitting of the detected lines. We detected alpha, beta and gamma transitions of hydrogen and alpha transitions of helium. We used the observed line parameters to 1) measure the source velocity (56.6 $\pm$ 0.3 km s$^{-1}$) with respect to the Local Standard of Rest (LSR); 2) estimate the electron temperature (8500 $\pm$ 1800 K) of the HII region and 3) derive the emission measure (5.4 $\pm$ 2.7 $\times$ 10$^{6}$ pc cm$^{-6}$) of the ionized gas.