Wonju Kim
I. Physikalisches Institut, Universität zu Köln
HyGAL: Characterizing the Galactic ISM - Atomic oxygen, OH, CH, and small molecules
Understanding the interstellar medium (ISM) is the first step to understanding star formation/evolution and, ultimately, planet formation. The formation of molecular gas occurs in diffuse clouds. These clouds are also closely associated with the progenitors of molecular clouds and clumps, which are the potential birthplace of stars. In addition, the initial chemical inventory of molecular clouds is set by the composition of diffuse clouds. These initial chemical conditions are crucial to explain the observed molecular species toward star-forming molecular clouds and their chemical history. To characterize the properties of the diffuse Galactic ISM, the HyGAL Stratospheric Observatory for Infrared Astronomy legacy program surveyed six hydride molecules (ArH+, OH+, H2O+, SH, OH, and CH) and two atomic constituents (C+ and O) by observing absorption lines toward 25 bright continuum Galactic sources. As ancillary data, we also used absorption lines of small molecules (HCO+, HCN, HNC, C2H, c-C3H2, CS, and H2S) obtained with the IRAM 30m telescope. A principal component analysis (PCA) of the column densities of all the species classifies three species groups that show strong correlations or weak correlations to other groups (Group 1: C+ and hydride ions, group 2: neutral molecules and HCO+, and group 3: CH, OH, and O). Based on the PCA results, the neutral hydrides and OI lines significantly prefer to trace diffuse cloud components with intermediate conditions between high atomic gas- and high molecular gas-fraction diffuse ISM. This presentation will discuss the HyGAL legacy program results, including detailed comparisons of atomic O, OH, and CH absorption lines as tracers of H2 gas fraction and how the surrounding environments (e.g., star formation activity or supernova remnants) impact molecular abundance and column density in the transition from the cold neutral medium to molecular clouds.
