IRTF Data Archive Program Information

# # Program information file # PROGRAM_ID 2022B027 PROGRAM_TITLE Cosmic-ray ionization rates inferred from H3+: testing predictions of cosmic-ray transport theories PROGRAM_INV1 Nick Indriolo PROGRAM_INV2 Alexei Ivlev PROGRAM_INV3 Kedron Silsbee PROGRAM_INV4 Paola Caselli PROGRAM_INV5 David Neufeld PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG The cosmic-ray ionization rate [CRIR] is an important parameter that controls chemical complexity and heating in the ISM. Evidence shows that the ionization rate decreases from diffuse clouds to dense clouds as cosmic-ray particles lose energy from interactions with the ambient medium, but how quickly the ionization rate decreases at low column densities [i.e., in diffuse clouds alone], remains uncertain. We aim to answer this question by taking a two-pronged approach to study cosmic-ray ionization rates in diffuse molecular clouds. The first approach is to determine ionization rates in a sample of sight lines where H2 column densities have been directly measured from UV observations. These sight lines provide the smallest uncertainties on the CRIR, and such precision estimates enable the best study of CRIR vs total column density. The second approach is to determine ionization rates in a sample of sight lines where OH+ column densities have been measured. OH+ itself provides an estimate of the CRIR, but chemical models indicate that this molecule preferentially probes the mostly atomic outer layers of interstellar clouds. We will infer the CRIR by making observations of H3+, a molecule that preferentially probes the mostly molecular interiors of interstellar clouds. In the sight lines where H3+ and OH+ are observed, we will infer ionization rates from both species to determine how the CRIR changes between the outer and inner portions of individual clouds. In the sight lines where H3+ and H2 are observed, we will use precision estimates of the cosmic-ray ionization rate to study the relationship between CRIR and total cloud column density among a sample of diffuse molecular clouds. Approaching this question from two different angles---relationships within individual clouds and amongst a sample of clouds---provides a unique opportunity to better understand the behavior of the CRIR in diffuse molecular clouds. PROGRAM_ABSTRACT_END