2014ApJ...781L...8P

Near ultraviolet observations of OH⁺ and OH in diffuse molecular clouds reveal a preference for different environments. The dominant absorption feature in OH⁺ arises from a main component seen in CH⁺ (that with the highest CH⁺/CH column density ratio), while OH follows CN absorption. This distinction provides new constraints on OH chemistry in these clouds. Since CH⁺ detections favor low-density gas with small fractions of molecular hydrogen, this must be true for OH⁺ as well, confirming OH⁺ and H₂O⁺ observations with the Herschel Space Telescope. Our observed correspondence indicates that the cosmic ray ionization rate derived from these measurements pertains to mainly atomic gas. The association of OH absorption with gas rich in CN is attributed to the need for a high enough density and molecular fraction before detectable amounts are seen. Thus, while OH⁺ leads to OH production, chemical arguments suggest that their abundances are controlled by different sets of conditions and that they coexist with different sets of observed species. Of particular note is that non-thermal chemistry appears to play a limited role in the synthesis of OH in diffuse molecular clouds.