CO and OH Absorption toward the Low-Latitude Radio Source 2023+336

April 1995 • 1995ApJ...443L..45K

Authors • Kobulnicky, Henry A. • Dickey, John M. • Akeson, Rachel L.

Abstract • We report interferometric observations of ^12^CO (0 -> 1) 115 GHz and OH 1667 MHz absorption toward the extragalactic radio continuum source 2023 + 336. Our data show two partially blended absorption features near + 2 km s^-1^ LSR which probably originate in local molecular clouds in the Cygnus rift, ~0.7 kpc away. The CO lines are optically thin, with τ_CO_ = 0.5 +/- 0.1 while τ_OH_ = 0.030 +/- 0.004. The detected absorption features have a combined velocity width of 3.0 km s^-1^ FWHM, greater than the width of the corresponding ^12^CO emission features, FWHM = 2.3 km s^-1^, indicating that the absorbing gas in a ~1 AU diameter column shows a greater velocity range than the gas in the 0.2 pc diameter emission beam. CO excitation temperatures, T_ex_ range from 2.7 to 7.0 K across the line profile, implying that the gas along this line of sight is rotationally quite cold. By measuring the ^12^CO excitation temperature and optical depth, we derive an abundance ratio, [CO]/[H_2_], of 6 x 10^-6^, which is more than an order of magnitude lower than is seen elsewhere. Our low observed CO optical depth is inconsistent with the ratio of ^12^CO to ^13^CO emission line integrals, 16:1, which implies that the ^12^CO is optically thick. As a solution to these anomalies, we propose that the CO absorption and emission arise from preferentially different parts of the molecular medium. Other solutions would require an anomalous ^12^CO/^13^CO abundance ratio and an anomalous CO/H_2_ abundance ratio, or much lower excitation temperatures. Since we find no absorption at negative velocities with τ > 0.23(2.5 σ), there is no compelling evidence for the existence of an extensive population of cold molecular gas in the outer Galaxy along this line of sight.

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Rachel Akeson

IPAC Deputy Director