2003ApJ...592..964K

We present Far-Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph observations of high-ionization interstellar absorption toward HD 14434 (l=135°.1, b=-3°.8; d∼2.3 kpc), an O5.5 V star in the Perseus OB1 association. Intermediate-velocity interstellar Si IV and C IV absorption is present at V_LSR=-67 km/s, while low-ionization gas associated with the Perseus arm is detected at about -50 km/s. Neither N V nor O VI is detected at V_LSR=-67 km/s, although Al III and Fe III, tracers of warm ionized gas, are seen. The high-ion column densities in the -67 km/s component are log[N(CIV)]=13.92±0.02 cm^–2, log[N(SiIV)]=13.34±0.02 cm^–2, log[N(NV)]≤12.65 cm^–2, and log[N(OVI)]≤13.73 cm^–2 (3 σ limits). The observed C IV/Si IV ratio of 3.8±0.3 in this intermediate-velocity cloud (IVC) is similar to the Galactic average (4.3±1.9). Our analysis of the Si IV and C IV line widths yields a temperature of T∼10,450±3400 K for this component. At this low temperature, neither Si IV nor C IV can be produced via collisions. We investigate several photoionization models to explain the intermediate-velocity Si IV and C IV absorption toward HD 14434. Photoionization models employing cooling of a hot (T∼10⁶ K) diffuse plasma as the source of ionizing radiation reproduce the observed properties of the IVC toward HD 14434 quite well. The hot plasma responsible for the ionizing radiation in these models may be attributed to hot gas contained in a supershell in or near the Perseus arm or from a more generally distributed hot ionized medium.