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 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.
We analyzed archival spectra acquired with the Hubble Space Telescope for a study of interstellar C2. Absorption from the electronic transitions, D ^1Sigma^+_u -- X ^1Sigma^+_g (0,0) as well as F ^1Pi_u -- X ^1Sigma^+_g (0,0) and (1,0), was the focus
of the study. Our profile syntheses revealed that the lines of the F-X bands were broadened as a result of a perturbation involving the upper levels. Further evidence for the perturbation came from anomalies in line strength and position for the F-X (0,0) band. The perturbation likely arises from a combination of triplet-singlet interactions involving spin-orbit mixing between ^3Pi_u states and F ^1Pi_u and an avoided crossing between the ^3Pi_u states. Tunneling through a potential barrier caused by the 3 and 4 ^1Pi_u states and spin-orbit mixing with other close-lying triplet states of ungerade symmetry are less likely. Except for the broadening, lines in the F-X (1,0) band appear free from anomalies and can be used to study interstellar C2; new results for 10 sight lines are presented.
We present an analysis of results on absorption from Ca II, Ca I, K I, and the molecules CH+, CH, C2, and CN that probes gas interacting with the supernova remnant IC443. The eleven directions sample material across the visible nebula and beyond its
eastern edge. Most of the neutral material, including the diatomic molecules, is associated with the ambient cloud detected via H I and CO emission. Analysis of excitation and chemistry yields gas densities that are typical of diffuse molecular gas. The low density gas probed by Ca II extends over a large range in velocities, from -120 to +80 km/s in the most extreme cases. This gas is distributed among several velocity components, unlike the situation for the shocked molecular clumps, whose emission occurs over much the same range but as very broad features. The extent of the high-velocity absorption suggests a shock velocity of 100 km/s for the expanding nebula.
