Molecular hydrogen and its ions in dark interstellar clouds and star forming regions

Abstract

Fundamental observations of molecular hydrogen (H₂) in dark clouds, star forming regions, and radiation-dominated environments are presented, modeled, and interpreted. Through a weak infrared absorption line spectrum, the abundance of cold H₂ in dark molecular clouds and star forming regions is measured directly and compared with the abundance of its most commonly cited surrogate, CO. The derived abundance of CO is between 1.5 and 2.5 x 10⁻⁴ for the sample. The CO molecule thus represents about ⅓ of the total carbon budget in dense clouds. Also detected via infrared line absorption is the pivotal molecular ion H⁺₃ , yielding a direct measure of the cosmic ray ionization rate of H₂ in dark molecular clouds (between 1 and 5 x 10⁻¹⁷ s⁻¹), a process that instigates the complex ion-neutral chemical pathways that form many of the 120+ known molecular species deep inside interstellar clouds. These timely tests of theory are applied to the detailed submillimeter-wave study of the ρ Ophiuchi star forming cloud and photodissociation front, allowing partial disentanglement of the complicated physical and chemical structure of a star forming cloud. Yet H₂ and H⁺₃ continue to surprise and delight us with more mysteries. The formation, excitation and survival of molecules in unusual & hostile environments is highlighted by the discoveries of H⁺₃ in circumstellar disks of early-type stars, and of fluorescing H₂ in two harshly-irradiated filaments of the Crab Nebula. The role of H⁺₃ as a possible tracer of planet formation, and the evolution of H₂ in the interstellar medium is discussed. The study of H₂ in hostile environments is extended to the ensemble properties of extragalactic star forming regions, and applied to the Arp 299 merger system as a unique probe of the feedback of newly-formed hot stars, their fossil remains, and the molecular material which formed them.