Radio Recombination Lines in Orion A at 8 and 13 mm: The Ionization Structure and Effective Temperature of the Star θ1 C Ori, the Electron Temperature of the Ionized Gas and Turbulence

Abstract

The radio recombination lines (RRLs) of hydrogen, helium (H, He) and carbon (C) have been observed at several positions of the HII region Orion A with the RT-22 radio telescope (Pushchino) at 8 and 13 mm. Information about the ionization structure of the HII region has been obtained. The behavior of y+ = n(He+)/n(H+) over the nebula and model calculations suggest that the effective temperature (Teff) of the star θ1 C Ori is in the range 35 000–37 500 K, corresponding to a spectral type ≈O6.5 V, which is important for the calibration of hot O-B stars. The electron temperatures (Te) of this HII region have been measured by taking into account the departures from local thermodynamic equilibrium (LTE); their distribution over the nebula up to distances of 300 arcsec from the center has been derived. The inferred temperatures are in the range 6600–8400 K, strictly decreasing in the eastward directionwith distance from the center, also tend to drop toward the periphery in the southward and westward directions. The turbulent velocities (Vt) of the ionized gas and their distribution over the nebula have been determined. The values of Vt inferred from H RRLs are in the range 9–13 km s−1.