THE ORION OB1 ASSOCIATION .2. THE ORION-ERIDANUS BUBBLE

Observations of the interstellar medium in the vicinity of the Orion OB 1 association show a cavity filled with hot ionized gas, surrounded by an expanding shell of neutral hydrogen (the Orion-Eridanus Bubble). In this paper we examine this cavity and the surrounding bubble with the aid of data from the Leiden/Dwingeloo HI survey. We present neutral-hydrogen maps for the Orion-Eridanus region which allow identification of the HI filaments and arcs delineating the Bubble and derivation of its expansion velocity. The X-ray emission from the Orion Eridanus region is enhanced with respect to the Galactic background emission. Comparison with the H I data shows a detailed anti-correlation of the X-ray enhancement with kinematically-narrow features. This confirms the association of the X-ray enhancement with the cavity in H I. Comparison of the derived column densities in H I with the IRAS 100 mu m intensities shows thar the H I shell emission is optically thin. This justifies a derivation of the mass of the H I shell by direct conversion of the observed HI emission to column densities. Models of wind-blown bubbles are considered, to investigate if the cavity was formed by the stellar winds and supernovae from Orion OB I. Using a model that takes the density stratification of the Galactic H I layer into account, we show that the stellar winds and supernovae from stars in Orion OB 1 can account for the size as well as for the. expansion velocity of the HI shell. However, density inhomogeneities in the ambient interstellar medium cause significant discrepancies between our model and the observed shell.