In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH + , H 2 O + , and H 3 O + begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H 2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζ H) and molecular hydrogen fraction (f H 2). We present observations targeting transitions of OH + , H 2 O + , and H 3 O + made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH + and H 2 O + are detected in absorption in multiple velocity components along every sight line, but H 3 O + is only detected along 7 sight lines. From the molecular abundances we compute f H 2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH + and H 2 O + primarily reside in gas with low H 2 fractions. We also infer ζ H throughout our sample, and find a lognormal distribution with mean log(ζ H) = −15.75 (ζ H = 1.78 × 10 −16 s −1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H + 3 observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.
Icy bodies may have delivered the oceans to the early Earth, yet little is
known about water in the ice-dominated regions of extra-solar planet-forming
disks. The Heterodyne Instrument for the Far-Infrared on-board the Herschel
Space Observatory has detected emission from both spin isomers of cold water
vapor from the disk around the young star TW Hydrae. This water vapor likely
originates from ice-coated solids near the disk surface hinting at a water ice
reservoir equivalent to several thousand Earth Oceans in mass. The water's
ortho-to-para ratio falls well below that of Solar System comets, suggesting
that comets contain heterogeneous ice mixtures collected across the entire
solar nebula during the early stages of planetary birth.Comment: 18 pages, 2 figures. Corrected typo in reported mass (in g) of
detected water vapor reservoir. All conclusions are unchange
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