Multiwavelength investigation of the temperature of the cold neutral medium

Abstract

We present measurements of the H i spin temperatures (Ts) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H2 ortho-para temperatures (T01) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na i/K i/C i) observed towards these same stars to make a plausible identification of the CNM corresponding to the H2 absorption. We then find that T01 and Ts match within observational uncertainties for lines-of-sight with H2 column density above 1015.8 cm−2, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM Ts tracks the kinetic temperature due to collisions and that T01 is driven towards the kinetic temperature by proton exchange reactions.