Interstellar Extinction for \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\lambda \geq 1100$ \end{document} A and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\mathrm{H}\,^{+}_{2}$ \end{document}

Abstract

The wavelength-dependent photoabsorption cross section σ(λ) for H by a transition from a discrete rotation-vibration (RV) state in the 1sσg electronic orbital to a continuum state of internuclear motion in the 2pσu electronic orbital is discussed. The predicted extinction depends strongly on the, as yet, unknown occupation numbers of the initial RV states appropriate to various parts of the interstellar medium. A simple method for adjusting the occupation numbers is introduced. It is applied to three formulae representing the variety observed in interstellar extinction for this or other galaxies. The agreement is quantitative for wavelengths λ ≥ 2700 A. Agreement in the ubiquitous λ ~ 2175 A bump region becomes less accurate as the observed relative height increases and the width decreases. The agreement is poor for λ ≤ 1400 A. The occupation numbers found for these cases are acceptable on physical grounds and bear a resemblance to those predicted and observed for the ionization of cold H2 by energetic radiation. The predicted shape would allow this mechanism to be the sole carrier for interstellar medium (ISM) extinction for λ ≥ 2700 A and important for 2700 A ≥ λ ≥ 1400 A. However, the [H] required by this mechanism is considerably larger than predicted by current ISM models. It is suggested that this mechanism be considered, along with the established dust mechanism and perhaps others, as a contributor to the ISM extinction.