FUNDAMENTAL PROPERTIES OF THE HIGHLY IONIZED PLASMAS IN THE MILKY WAY

The cooling transition temperature gas in the interstellar medium ( ISM), traced by the high ions, Si IV, C IV, N V, and O VI, helps to constrain the flow of energy from the hot ISM with T > 10(6) K to the warm ISM with T < 2 x 104 K. We investigate the properties of this gas along the lines of sight to 38 stars in the Milky Way disk using 1.5-2.7 km s(-1) resolution spectra of Si IV, C IV, and N V absorption from the Space Telescope Imaging Spectrograph, and 15 km s(-1) resolution spectra of O VI absorption from the Far Ultraviolet Spectroscopic Explorer. The absorption by Si iv and C iv exhibits broad and narrow components while only broad components are seen in N V and O VI. The narrow components imply gas with T < 7 x 10(4) K and trace two distinct types of gas. The strong, saturated, and narrow Si iv and C iv components trace the gas associated with the vicinities of O-type stars and their supershells. The weaker narrow Si iv and C iv components trace gas in the general ISM that is photoionized by the EUV radiation from cooling hot gas or has radiatively cooled in a non-equilibrium manner from the transition temperature phase, but rarely the warm-ionized medium probed by Al III. The broad Si iv, C IV, N V, and O VI components trace collisionally ionized gas that is very likely undergoing a cooling transition from the hot ISM to the warm ISM. The cooling process possibly provides the regulation mechanism that produces < N(C IV)/N(Si IV)> = 3.9 +/- 1.9. The cooling process also produces absorption lines where the median and mean values of the line widths increase with the energy required to create the ion.