Faint optical emission lines from the interstellar medium - Limits on the temperature and ionization state of the emitting gas

Abstract

In order to investigate the physical state of the gas that produces the faint, diffuse, galactic H..cap alpha.. emission, we made scans of H..cap alpha.., (N II) lambda6584, (N I) lambda5201, and (O III) lambda5007 in a direction (l=184degree.4, b=-0degree.6) near the galactic anticenter, using a two-etalon, 150 mm diameter Fabry-Perot spectrometer. Analysis of the H..cap alpha.. and (N II) line widths and the (N I)/(N II) and (O III)/H..cap alpha.. intensity ratios implies a temperature of 9600 +- 2000 K, a nitrogen ionization ratio n (N/sup +/)/n (N/sup 0/) >6, and a low abundance of O III in the emitting gas. These results suggest that the primary source of the ionization is UV (14 to 60 eV) radiation. If this radiation originates primarily from O and early B stars, then the stars which contribute to most of the ionization in this direction must be at large distances (more than 100 pc) from the line of sight. (AIP)