High helium abundance in the HII region DR-21 ?

The 36.5-GHz (H, He) 56 alpha and 22.4-GHz (H, He) 66 alpha radio combination lines were observed in DR-21 on the RT-22 radio telescope. Along with other data, this shows that the measured relative abundance of ionized helium increases with the angular beamwidth, peaking when the beamwidth is 2 - 2.5 arcmin, and then begins to decrease as the angular beamwdith increases further. This behavior is more complicated than previously thought, and can be interpreted as a blister-type nebula model, implying a high actual helium abundance of 26 percent (+ or - 11 percent) in DR-21. By modeling the observed increase in radial velocities of helium lines as the frequency decreases, the gas expansion rate away from the observer can be determined to be 5 - 20 km/s. Possible explanations of this apparently high helium abundance are discussed.