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Issue A&A
Volume 464, Number 3, March IV 2007
Page(s) 995 - 1002
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20042323



A&A 464, 995-1002 (2007)
DOI: 10.1051/0004-6361:20042323

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

S. Poppi1, A. P. Tsivilev2, 3, S. Cortiglioni4, G. G. C. Palumbo5, and R. L. Sorochenko2

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 (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.


Key words: ISM: abundances -- ISM: HII regions -- radio lines: ISM -- ISM: individual objects: Orion Nebula



© ESO 2007


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