Cosmic Ray--induced Chemistry toward Perseus OB2

New measurements of absorption from interstellar OH at 3078 and 3081 Å are presented for three directions toward the Perseus OB2 association. The measured equivalent widths are in excellent agreement with previous determinations for ο and ζ Per. Our measurements also provide stringent upper limits for the line of sight to ζ Per. The available data on λ3078 toward ο and ζ Per with published equivalent widths for the (0, 0) band of the D-X electronic transition near 1222 Å are used to determine the oscillator strength for the D-X band; the value agrees with theory. The first ground-based detections of absorption from interstellar Ti II λ3072 are discussed in an appendix.

Cosmic-ray ionization followed by ion-molecule reactions is found to be the primary means of producing OH and HD in the environment probed by our observations. Our OH data are used in conjunction with a simple but adequate chemical model to obtain estimates of the cosmic-ray ionization rate. These estimates then yield a value for the local interstellar D/H ratio of (1.5±0.5) x 10^-5 from available observations of HD that is consistent with more direct measures. Furthermore, the derived rate allows us to place limits on the production of NH through gas-phase processes.