A multiwavelength investigation of the temperature of the cold neutral medium

Abstract

We present measurements of the H I spin temperatures (T_s) 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 ultraviolet UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of the corresponding 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 T_s match within observational uncertainties for lines of sight with H₂ column density above 10^15.8 cm^-2, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM T_s tracks the kinetic temperature due to collisions and that T₀₁ is driven towards the kinetic temperature by proton exchange reactions.