| Abstract |
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>106 K to the
warm ISM with T < 2 × 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 × 104 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 langN(C
IV)/N(Si IV)rang = 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.
Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc. under
NASA contract no. NAS5-26555. Based also on observations made with the
NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated
for NASA by the Johns Hopkins University under NASA contract NAS5-32985. |