Helium abundance and ionization structure in the Orion nebula: radio recombination lines observations

Results of the Ori A HII region mapping based on hydrogen (H), helium (He) and carbon (C) Radio Recombination lines (RRL) are presented. Observations were made with the same angular resolution ($2'$) using the 32 m VLBI dish of Medicina (Italy, 22.4 GHz) and the Pushchino RT-22 dish (Russia, 36.5 GHz). The behaviour of the ionized helium abundance, y + , with distance from the center shows that the He + zone size is smaller than that of H + . Such a behaviour is different for the core and for the envelope, as well as for different directions from the center. The helium abundance, $N({\rm He})/N({\rm H})=10.0(\pm 0.8)$%, is measured. Derived line radial velocities, their widths and y + data support the well-known “blister-type” structure of this HII region. LTE electron temperatures ($7800{-}9600$ K) are also measured.