Modifications of the relation between cosmic ray ionization rate ζ and H3+ column density in the central molecular zone of the galactic center

Oka, Takeshi

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https://hdl.handle.net/2142/96823 Copy

Description

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

Issue Date

2017-06-20

Keyword(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_

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/96823

DOI

https://doi.org/10.15278/isms.2017.TA08

Copyright and License Information

Copyright 2017 Takeshi Oka

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