APERTURE SYNTHESIS OBSERVATIONS OF W51 AT 151 MHZ

Radio continuum observations at 151 MHz of the galactic H II region complex W51 were made with the Cambridge Low Frequency Synthesis Telescope. The data were used to estimate the electron temperature of G49.5 - 0.4 and G49.4 - 0.3, two of the main components of W51. The values of electron temperature derived from radio continuum observations (almost-equal-to 3500 K) are significantly lower than those obtained from recombination lines (almost-equal-to 6400 K), which indicates the existence of internal variations of electron temperature in these nebulae. The existence of electron temperature gradients in H II regions would have an important effect on the chemical abundance determinations. The nature of the shell-like component W51C, a possible supernova remnant, was also studied. At 151 MHz, W51C appears very distinctly as a large incomplete shell obviously connected to W51B. This shell is an X-ray source and no recombination line has been detected from it. A turnover in the spectrum of W51C was found by comparison of the present data with previous observations at higher frequencies. The shell W51C (plus part of W51B) is likely to be a supernova remnant surrounded or obstructed by a low-density ionized envelope, possibly related to an evolved H II region in the complex or simply on the line-of-sight, which would explain the turnover of the radio continuum spectrum at low frequencies, the integrated apparently thermal spectrum of the whole W51 complex, and could be responsible for a partial depolarization of the radio continuum.