Other
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010mwac.meet..t05r&link_type=abstract
"Midwest Astrochemistry Meeting 2010, held 5-6 November at the University of Illinois at Urbana-Champaign. http://midwest.astroc
Other
Scientific paper
When the Heterodyne Instrument for the Far Infrared (HIFI) on Herschel was first turned toward various sightlines, observers hoped to detect small amounts of species involved in the first steps of interstellar chemistry, such as OH+ and H2O+. It turns out that great amounts of these two molecules are present along these sightlines, and are even more abundant than the relatively stable ion H3O+. This is surprising because both OH+ and H2O+ react with molecular hydrogen. For diffuse regions, this can be explained by the balance between atomic and molecular hydrogen.
This explanation is not valid for denser regions (n > 103 cm-3) where there is relatively little atomic hydrogen. In the KL region, of density n > 104 cm-3, OH+ and H2O+ have been observed with estimated column densities ~ 1013 cm-2, where only an upper limit of 1012 cm-2 was established for H3O+. To account for this, the initial paper used a density of 103 cm-3, as well as an extremely high cosmic ray ionization rate of ζ > 10-14 s-1. These values achieve agreement for OH+, H2O+ and H3O+ but, among other things, underpredict the abundance of water by about 4 orders of magnitude. We consider this region in greater detail, accounting for the interplay among X-rays, cosmic rays and the very high flux of UV photons (χ of 104 in Draine units). We also include an inflow rich in water. The water will become partially ionized, providing a new formation rate for H2O+.
These effects are modeled for the KL environment with a temperature determined by thermal balance, using the Meudon code. A time-dependent onedimensional PDR code is then used to analyze the chemistry. We include in this model the observed X-ray flux and a constant ζ of 5 x 10-15 s-1. This agrees within a factor of 2 with the estimated column densities of OH+, H2O+ and H3O+, as well as water. As with the model in the observation paper, this model results in unlikely abundances for other species, such as HCO+. The model, its implications and possible future work will be discussed.
Herbst Eric
Rimmer Paul B.
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