A Study of Physical Conditions, Radiative Transfer and Shocks in the Interstellar Medium.

Two basic physical problems due to radiative transfer and time-dependent shocks are explored. In the first problem, a detailed statistical equilibrium model dealing with the radiative transfer in the interstellar molecular clouds is developed. An improved treatment of the radiation field, including the 2.73 K background radiation field, is incorporated. This model is applied in an analysis of ultraviolet high dispersion (R ~ 8.6 times 10^4) echelle observations of molecular CO obtained with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. Specifically, for the xi Per clouds, synthetic spectra were computed and fitted to the observed CO (2-0) and (3-0) bands in the A ^1Pi - X ^1Sigma ^+ system. Derived populations of the C I ground-state fine-structure levels and the Co ground -state rotational levels were used to derive densities of two of the three, and possibly four, detected cloud components. The velocity component displaying the strongest C I absorption reveals extraordinarily high pressure (P/k > 4.0 times 10^4 cm^{-3} K). For the zeta Oph clouds, the computed relative populations of the 6 lowest rotational levels of CO, based upon the statistical equilibrium model, together with multiple Voigt profile fits were determined to reproduce successfully the observations of the rotational structure of the (5 -0) and (6-0) bands in ¹²C ¹⁶O and ^ {13}C¹⁶O. The abundance of Co is found to be (1.88 +/- 0.25) times 10^{15 } cm^{-2}, and the isotope ratio of ¹² C¹⁶O/ ¹³C^{16 }O is ~79 +/- 22. In the second interstellar shock problem, we find that the photoionization by the EUV/UV (including soft X-ray) flux from both the central Of star and the surrounding hot gas plasma is a significant mechanism in determining the dynamical processes in the interstellar bubble, where the He II emission should be observable. Shock ionization and photoionization by X-rays originating from the central hot O star are responsible for producing this emission. Furthermore, from an expanding supershell model, we find that the local cloud (<=10 pc) at the periphery of Loop I is affected by a recent (<=10^5 years) supernova, and the shock velocity (the expansion velocity) is ~15 km s^ {-1}. This cloud is substantially ionized where the ionization fraction of hydrogen is ~ 66% and that of helium is 36%.