Limits to the broadening of radio recombination lines by electron collisions

Observations of hydrogen 92 α recombination line profiles suggest that the inner regions of Orion A and W3A have low electron densities. If a constant electron density, Nsub(e), is assumed, then the observed profiles give 5 x 103 cm-3 >= Nsub(e) >= 5 x 102 cm-3 for the inner region of Orion A, and 4 x 104 cm-3 >Nsub(e) >= 4 x 103 cm-3 for the inner region of W3A. For the volume observed in Orion A, forbidden line measurements give an average electron density of Nsub(e) > 104 cm-3. Although the application of non-LTE theory to other radio frequency line data suggests that Nsub(e) = 2.5 x 104 cm-3 and Nsub(e) = 7.7 x 104 cm-3, respectively, for Orion A and W3A, variable density models based upon all previous radio data give Nsub(e) = 5 x 103 cm-3 for the observed volume in Orion A. The observations suggest that variable density models with even lower electron densities may be appropriate. (author)