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Title:MODIFICATIONS OF THE RELATION BETWEEN COSMIC RAY IONIZATION RATE ζ AND H3+ COLUMN DENSITY IN THE CENTRAL MOLECULAR ZONE OF THE GALACTIC CENTER
Author(s):Oka, Takeshi
Subject(s):Astronomy
Abstract:In deriving the simple formula, $zeta$$L$=2$k_mathrm{e}N$(H$_{3}^{+}$)($n_mathrm{C}$/$n_mathrm{H}$)$_mathrm{SV}$$R$/$f$(H$_{2}$), used to estimate cosmic ray H$_{2}$ ionization rate $zeta$ from observed H$_3^+$ column density $N$(H$_3^+$) in the Central Molecular Zone (CMZ) of the Galactic center (GC),footnote{Oka, T., Geballe, T. R., Goto, M., Usuda, T., McCall, B. J. 2005, ApJ, 632, 882} the following two effects were neglected: (1) the charge exchange reaction H$_2^+$ + H $rightarrow$ H$_2$ + H$^+$footnote{Indriolo, N., McCall, B. J. 2012, ApJ, 745:91} which significantly reduces H$_3^+$ production rate if the fraction of molecular hydrogen $f$(H$_2$) is much lower than 1, and (2) the production of electrons from ionization of H$_2$ and H which greatly increases the H$_3^+$ destruction rate if $zeta$ is much higher than 10$^{-15}$ s$^{-1}$. (Only electrons from VUV first ionization of C atoms had been considered). Recent more extensive analysis using the Meudon PDR code by Le Petit et al.footnote{Le Petit, F., Ruaud, M., Bron, E., Godard, B., Roueff, E., Languignon, D., Le Bourlot, J. 2016, A&A, 585, A105} has indicated that these effects are not negligible in the CMZ. _x000d_ _x000d_ While an extensive chemical model calculation is beyond the scope of our analysis, we have attempted to use our simple model considering only hydrogenic species and electrons to take these two effects into account. When (1) is introduced, the rate of H$_3^+$ production is approximated to be $zeta$$n_mathrm{H}$[$f$(H$_2$)]$^2$,footnote{Oka, T. 2013, Chem. Rev. 113, 8738} which is $sim$ 3 times lower than the previous value for $f$(H$_2$) = 0.6 reported by Le Petit et al.$^{c}$ When (2) is taken into account, the electron number density is approximated to be $n_mathrm{e}$ = $n_mathrm{C}R$ + $zeta$$n_mathrm{H}$/[2$k_mathrm{e}$$n$(H$_3^+$)] where the first and second term represents electrons from the C atoms and those from H$_2$ and H, respectively. The first term (in which $R$ represents the increase of metallicity from the solar vicinity to the GC, $R$ $geq$ 3) has the electron fraction $x_mathrm{e}$ = 5 $times$ 10$^{-4}$ and the second term becomes significant at $zeta$ $sim$ 10$^{-15}$ s$^{-1}$. This introduces a non-linearity between $zeta$ and $N$(H$_3^+$) and the latter reaches a maximum at $zeta$ $sim$ 10$^{-14}$ s$^{-1}$ and decreases as $zeta$ increases further. Application of the results to the observed $N$(H$_3^+$) will be discussed._x000d_ _x000d_
Issue Date:6/20/2017
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Genre:CONFERENCE PAPER/PRESENTATION
Type:Text
Language:English
URI:http://hdl.handle.net/2142/96823
DOI:10.15278/isms.2017.TA08
Date Available in IDEALS:2017-07-27


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