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

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