An interstellar conduction front within a Wolf-Rayet ring nebula observed with the Goddard High Resolution Spectrograph

With the High Resolution Spectrograph aboard the Hubble Space Telescope we obtained high signal-to-noise ratio (S/N ≳ 200-600 per 17 km s^-1 resolution element) spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The ring nebula S308 that surrounds this star is thought to be caused by a pressure-driven bubble bounded by circumstellar gas (most likely from a red supergiant or luminous blue variable progenitor) pushed aside by a strong stellar wind. Our observation has shown for the first time that blueshifted (≈ 70 km s^-1 relative to the star) absorption components of C IV and N V arise in a conduction front between the hot interior of the bubble and the cold shell of swept-up material. These lines set limits on models of the conduction front. Nitrogen in the shell appears to be overabundant by a factor 10. The P Cygni profiles of N V and C IV are variable, possibly because of a suspected binary companion to HD 50896.