2007A&A...464..995P

Result. 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 32m VLBI dish of Medicina (Italy, 22.4GHz) and the Pushchino RT-22 dish (Russia, 36.5GHz). 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(He)/N(H)=10.0(±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.