On the history of the interplay between HD 56925and NGC 2359

NGC 2359is an optical nebula excited by the powerful wind and the radiation of the Wolf-Rayet star HD 56925. We have investigated the interaction between this massive star and the surrounding neutral gas by analyzing the large-scale 21 cm-Hiemission and by mapping the nebula in the $J=$ $1 \rightarrow 0$and the $J=$ $2 \rightarrow 1$lines of CO. We found a conspicuous ($70 \times 37$ pc) Hishell, expanding at 12 km s -1 , likely produced during the main-sequence phase of the star. The molecular gas towards NGC 2359shows three velocity components. Two of these components, A1 and A2, have narrow linewidths (1-2 km s -1 ) and radial velocities of 35-38 and 64-68 km s -1 , respectively. The third component is detected at radial velocities between 50 and 58 km s -1 and has a broader profile (up to 5.5 km s -1 ). Furthermore, this component is morphologicaly related with the nebula and has a velocity gradient of a few km s -1 . We have also estimated the physical parameters of the molecular gas by means of a LVG modelling of the CO emission. The gas projected onto the southern Hii region of the nebula has low CO column density and is rather hot, probably up to 80 K. Several profiles of the 13 CO $J=$ $1 \rightarrow 0$line near the peak of the emission, together with a weak emission bridge between the broad and one of the narrow components (component A2), suggest the presence of a shock front acting in the southern part of the nebula. This shock was likely produced in a previous RSG/LBV phase of HD 56925.

[1]  E. M. Arnal,et al.  The Interstellar Medium around Galactic WN Stars: WR 2, WR 128, and WR 151 , 1999 .

[2]  W. Goss,et al.  A Study of Neutral and Ionized Gas of the Wolf-Rayet Ring Nebula NGC 2359 , 1999 .

[3]  J. Black,et al.  Molecular Gas Associated with the Ejecta Ring Nebula around the Wolf-Rayet Star WR 16 , 1999 .

[4]  D. Garnett,et al.  Physical Structure of Small Wolf-Rayet Ring Nebulae , 1998, astro-ph/9812076.

[5]  P. Benaglia,et al.  The Interstellar Medium in the Environs of O-Type Stars , 1998 .

[6]  R. Doyon,et al.  Molecular Hydrogen Emission in the Wolf-Rayet Nebula NGC 2359 , 1998 .

[7]  Frank J. Kerr,et al.  Atlas of Galactic Neutral Hydrogen , 1997 .

[8]  A. Marston.  Large IRAS Shells Around Galactic Wolf-Rayet Stars and the O Star Phase of Wolf-Rayet Evolution , 1996 .

[9]  G. Dubner,et al.  The Interaction of NGC 6888 and HD 192163 With the Surrounding Interstellar Medium , 1996 .

[10]  E. M. Arnal,et al.  THE H I DISTRIBUTION IN THE ENVIRONMENT OF THE WR STAR HD 50896 , 1996 .

[11]  G. García-Segura,et al.  Wolf-Rayet Bubbles. I. Analytic Solutions , 1995 .

[12]  T. Wilson,et al.  Abundances in the interstellar medium , 1992 .

[13]  F. Lockman,et al.  A Survey of Radio H II Regions in the Northern Sky (Lockman+ 1989) , 1989 .

[14]  A. Stark,et al.  The rotation curve of the Milky Way to 2 R(0) , 1989 .

[15]  D. Buren.  Kinetic efficiencies of stellar wind bubbles , 1986 .

[16]  Y. Chu,et al.  Galactic ring nebulae associated with Wolf-Rayet stars. VIII: Summary and atlas , 1983 .

[17]  B. L. Ulich,et al.  Recommendations for calibration of millimeter-wavelength spectral line data. , 1981 .

[18]  R. Weaver,et al.  Interstellar bubbles. II - Structure and evolution , 1977 .

[19]  A. Marston.  Far-infrared observations and the structure of Wolf-Rayet ring nebulae , 1991 .

[20]  M. Silkey,et al.  Abundances in H II regions at the edge of the Galaxy , 1991 .

[21]  J. Melnick,et al.  Structure and Dynamics of the Interstellar medium , 1989 .

[22]  J. Dyson.  Interstellar wind-blown bubbles , 1989 .

[23]  R. McCray.  Stellar Winds and the Interstellar Medium , 1983 .

[24]  A. H. Barrett,et al.  The Stellar Wind Bubble NGC2359 - Part One - CO VLA and Optical Observations , 1981 .