Synthetic observations of highly clumped formation of CO and CI clouds in the turbulent interstellar medium; Implications on the seed CO clouds and the CO-dark H₂ gas

We carried out synthetic observations of a theoretical model of interstellar carbon in CO, C I and C II including H2 formation. The theoretical model based on numerical simulations of H I gas converging at a velocity of 20 km/s by Inoue & Inutsuka (2012) is employed in the present study. The gas model, which is fairy realistic and consistent with the observations of the local gas located at high b, is turbulent and inhomogeneous with magnetic field. We focus on the evolution of the rare species, CO, C I and C II from 0.3 Myr to 9 Myr since the converging flows collided. In the early phase within 1 Myr, the Cold Neutral Medium (CNM) clumps of sub-pc scales are the formation sites of H2, CO and C I, and in the late phase from 3 Myr to 9 Myr the CNM clumps merge together to form pc-scale molecular clouds. The results show that C I and CO are well mixed at pc scales, and mark a significant disagreement with the classic picture which predicts layered C I and CO distributions according to AV . We suggest that the faint CO clouds in the Oph North (Tachihara et al. 2012) and the Pegasus loop (Yamamoto et al. 2006) are promising candidates for such small CO clouds. The total mass budget of the local ISM indicates the fraction of H I is about 70% and the CO-dark H2 is small, less than a few percent, contrary to the usual assumption.