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

¹ INAF – Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Str. 54, 09012 Capoterra (CA), Italy e-mail: spoppi@ca.astro.it
² P.N Lebedev Physical Institute, Academy of Sciences of Russia, Moscow, Russia
³ Isaac Newton Institute of Chile, Pushchino Branch, Pushchino, 142290 Russia
⁴ INAF – IASF Sezione di Bologna, via P. Gobetti 101, 40129 Bologna, Italy
⁵ Università degli Studi, Dipartimento di Astronomia, 40129 Bologna, Italy

Received: 5 November 2004
Accepted: 27 November 2006

Abstract

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 () 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, %, 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 ( K) are also measured.