HIGHLY IONIZED-GAS IN THE GUM NEBULA AND ELSEWHERE - A COMPARISON OF IUE AND COPERNICUS SATELLITE RESULTS

HD 64760 is a BO.5 Ib star situated behind the Gum nebula at a distance estimated to be 970 pc. By averaging the data from six high-dispersion IUE echelle spectra we obtain an interstellar absorption line spectrum with a S/N of about 30 and a resolution of about 25 km s-1. The interstellar lines of C IV and Si IV are very strong and broad and N V is detected. The profiles for these species and Al III are compared to the Copernicus satellite profiles for O VI. The high ionization lines toward HD 64760 are much stronger and broader than those recorded toward zeta-Pup and gamma-2 Vel, the two exciting stars of the Gum nebula. It appears that the HD 64760 sight line allows us to view absorption by highly ionized gas through the entire nebula while the zeta-Pup and gamma-2 Vel sight lines record only absorption to the central regions of the nebula. Similarities and differences between the profile shapes for Al III, Si IV, C IV, and O IV toward HD 64760 provide information about the ionization processes affecting these species. The profiles for Al III and Si IV are similar and considerably narrower than the O VI profile. An origin in photoionized Gum nebula gas is the most likely explanation for Al III and Si IV. The C IV profile has a high positive velocity wing extending to approximately + 80 km s-1 which is similar in appearance to the positive velocity portion of the O IV profile. This suggests that a substantial part of the observed C iv has an origin in the collisionally ionized gas most likely responsible for the O VI. Much of the O VI and some of the C IV absorption seen toward HD 64760 may represent absorption in the cooling gas resulting from an explosive event that occurred in the Gum nebula perhaps 10(6) yr ago. In the Appendix we report on IUE satellite observations of interstellar absorption by Al III, Si IV, and C IV toward stars previously surveyed for O VI absorption with the Copernicus satellite. Unfortunately due to the low S/N of typical IUE data, it was not possible to record absorption by C IV for the majority of the O VI survey stars. However, a comparison between absorption by O VI and C IV is possible for a number of stars where the absorption by the highly ionized gas is unusually strong. These stars include: HD 36695, HD 37128 (epsilon-Ori), HD 44506, HD 64760, HD 66811 (zeta-Pup), HD 68273 (gamma-2 Vel), HD 149881, HD 155806, and HD 219188.