Electron Temperatures in W51 Complex from High Resolution, Low Frequency   Radio Observations
  W51 is a giant radio complex lying along the tangent to the Sagitarius arm at a distance of about 7kpc from Sun, with an extension of about 1deg in the sky. It is divided into three components A,B,C where W51A and W51B consist of many compact HII regions while W51C is a supernova remnant. We have made continuum radio observations of these HII regions of the W51 complex at 240,610,1060,1400 MHz using GMRT with lower resolution 20''x15'' at the lowest frequency. The observed spectra of the prominent thermal subcomponents of W51 have been fitted to a free-free emission spectrum and their physical properties like electron temperatures and emission measures have been estimated. The electron temperatures from continuum spectra are found to be lower than the temperatures reported from radio recombination line (RRL) studies of these HII regions indicating the need for a filling factor even at this resolution. Also, the observed brightness at 240MHz is found to be higher than expected from the best fits suggesting the need for a multicomponent model for the region. 
P.K.Srivastava, Pramesh Rao