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S. Wissing, F. Bruhweiler (IACS/CUA)
The intercloud medium (ICM) constitutes the largest volume fraction of the interstellar medium, yet the nature of the gas comprising the ICM is undetermined. The Local Bubble, in which the Sun and the local cloud complex are immersed, is filled by hot gas (HG) with T ~ 106 K and n ~ 10-2.5 cm-3, but this HG may be atypical of the ICM. Diffuse Galactic H-alpha and forbidden line emission present strong evidence that the ICM, at larger distances, consists largely of a warm, ionized medium (WIM) with T ~ 104 K and n ~ 0.1 cm-3. Theory predicts ISM filling factors of 0.7 for the WIM and less than 0.2 for the HG. In a pilot study, we have used IUE and HST data to probe interstellar C IV and Si IV toward 5 runaway O stars in the Galactic plane that are also well away from recognized H II regions. The stars used in this study were HD 39680 (06 V), HD 210839 (06 I), HD 12993 (06.5 V), HD 13268 (07 V) and HD 30614 (09.5 Ia). Three of the five stars reveal column density upper limits that are factors of 100-500 below CLOUDY predictions that assume these species are due to stellar photoionization and that the O stars are embedded in an uniform WIM. The O star winds are expected to carve out evacuated cavites with denser shells or low-velocity bowshocks around the observed stars. This leads to dramatically reduced C IV and Si IV column densities. The hot cavities produced by the stellar winds of these very stars act to increase the filling factor for the HG component to the ICM. It is still premature to extrapolate from this small sample. Yet, a simple interpretation points to a much larger filling factor for the HG. Taking the observed diffuse Galactic emission and these results together implies a far more complex picture for the ICM than presently envisioned. More detailed interpretations of these results are discussed.
The author(s) of this abstract have provided an email address for comments about the abstract: bruhweiler@cua.edu