Interstellar Highly-ionized Species toward Cygnus: A More-detailed Look

We examine archival Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) data, complemented with International Ultraviolet Explorer (IUE) data to delineate interstellar absorption of highly-ionized species, C IV, Si IV, & N V, as well as neutral and low ionization abundant species in the lines-of-sight toward four stars in Cygnus; HD 190918, HD 191610, HD 192639, and HDE 226868 (Cyg X-1). This work builds upon previous UV studies of interstellar C IV and Si IV, and investigations of the ionized regions in Cygnus, using both HST and IUE data. We derive atomic and ionic column densities for each resolved velocity component along the line-of-sight towards HD190918 and identify several common velocity components for the observed lines-of-sight, and extend this study to neighboring stars where HST/STIS data are available. There are two major results. First, in agreement with previous IUE studies, there are two major velocity complexes for the C IV and Si IV, which extend spatially over a large region of Cygnus. The high negative velocity complex (~-60 to -40 km/s) has no corresponding low ionization species at these velocities unlike the low velocity (~+0 km/s) C IV and Si IV. This suggests that this high negative velocity complex is embedded in a more extended low-velocity H II region. Second, interstellar N V is present in the spectra of both HD 190918 and Cyg X-1. In both cases the N V is seen at the velocities of the C IV and Si IV. The N V toward the W-R binary, HD 190918 (WN5+O7Iab), is possibly due to enhanced EUV and X-ray ionization produced by this system. Even though the data are of low signal-to-noise in Cyg X-1, where the UV is highly attenuated by a dust ridge in front of Cyg X-1, interstellar N V is strong. We speculate that the N V toward Cyg X-1 is a result of X-ray Auger ionization from the strong x-ray source. We expect the highly-ionized species seen at higher negative velocities throughout Cygnus indicate shells driven by stellar winds and/or supernova shocks. By comparing theoretical models and known physical processes to our derived velocities and column densities, we attempt to develop a more detailed model for this complex region.