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WR 55

Title: NANTEN 12CO (J=1-0) observations around the star WR 55
Authors: N. U. Duronea, E. M. Arnal, J. C. Testori

Context: A complete study of the molecular and ionised gas in the environs of the nebula RCW 78 around WR 55 is presented.
Aims: The aim of this work is to investigate the spatial distribution, physical characteristics, and kinematical properties of the molecular gas linked to the galactic nebula RCW 78 to achieve a better understanding of its interaction with the star and with the ionised gas.
Methods: This study was based on 12CO(1-0) fully sampled observations of a region of ~0.45 in size around the star WR 55 and the nebula RCW 78 obtained with the 4-m NANTEN telescope, radio continuum archival data at 1.4 and 4.85 GHz, obtained from SGPS and PMNRAO Southern Radio Survey, respectively, and available infrared MIPSGAL images at 24 microns.
Results: A molecular gas component in the velocity range from ~ -58 to -45 km s-1, compatible with the velocity of the ionised gas, was found to be associated with the optical nebula. Adopting a distance of ~ 5 kpc, the mass of this molecular component is about 3.4 x 10^4 Msun. The analysis of the molecular data revealed the presence of a velocity gradient, in agreement with the Halpha line. New radiocontinuum flux density determinations confirm the thermal nature of RCW 78. This indicates that the ionised gas in RCW 78 arises from photoionisation of the molecular gas component in the velocity range from -58 km s-1 to -45 km s-1. A molecular concentration at a velocity of -56.1 km s-1 (identified as C1) is very likely associated with the star HD 117797 and with a collection of candidate YSOs, lying at a distance of 3.9 kpc, while the rest of the molecular gas at velocities between -56 and -46 km s-1 constitute an incomplete ring-like structure which expands around WR 55 at a velocity of about ~ 5 km s-1. Mechanical energy and time requirements indicate that WR 55 is very capable of sustaining the expansion of the nebula.

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