Because of the large abundances of hydrogen, oxygen, and their
ions and the efficiency of the charge transfer reactions (1)
and (2), the processes are important in a variety of
astrophysical and atmospheric environments. They can influence
the ionization balance, chemistry, radiative spectrum,
cooling, and possibly heating of astrophysical
plasmas.
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Figure 7:
J-resolved charge transfer rate coefficients for a)
Reaction (1), O+ + H ![]() ![]() |
Much of the chemistry in the interstellar medium is driven by ionization.
However, atomic oxygen cannot be photoionized by the
interstellar radiation field in either dense or diffuse interstellar
clouds. The necessary source of ionization is provided by
cosmic rays which collisionally ionize H to create H+. The proton can then
capture an electron from the oxygen atom via reaction (2)
thereby initiating the oxygen chemistry. Most interstellar cloud
models adopt the standard set of reaction rate coefficients
given by Millar et al. ([1997]). For process (2)
they suggest the relation
cm3 s-1 shown in Fig. 6. This is based on the
measurement of Fehsenfeld & Ferguson ([1972]) with the oxygen
fine-structure levels in collisional equilibrium.
Black & Dalgarno
([1977]) deduced a similar value by modeling
the OH and HD abundances in a diffuse interstellar
cloud. The new reaction (2)
rate coefficients are in good agreement with the earlier estimates, though
somewhat larger than the QMOCC-FS calculations of Chambaud et al.,
and should not have a significant effect on the oxygen chemistry.
For process (1)
Millar et al. suggest the constant value of
6.8 10-10cm3 s-1 based on the measurement of Federer et al.
([1984]), which is also in agreement with the value
deduced by Black & Dalgarno ([1977]).
However, the new results and those of
Chambaud et al. ([1980]) suggest the rate coefficient
has a significant temperature dependence decreasing below
the Federer et al. measurement for T<500 K and as a result
may enhance the oxygen chemistry by reducing the O+removal efficiency.
O I and O II emission lines are observed in a variety of photoionized nebulae including classical novae (e.g., Andreä et al. [1994]), planetary nebulae (e.g., Péquignot et al. [1978]), H II regions (e.g., Baldwin et al. [1996]), and metal line systems in high redshift clouds (e.g., Petitjean et al. [1996]). Reactions (1) and (2) are important in establishing the O+/O ratio, which because of the efficiencies of the charge transfer reactions tracks the H+/H ratio. Given the available data, Kingdon & Ferland ([1996]) deduced a set of recommended rate coefficients from the Chambaud et al. ([1980]) data and an extrapolation to the 10000 K data of Field & Steigman ([1971]). Figure 6 shows that the current rate coefficients become greater than those of Kingdon & Ferland ([1996]) for T>2000 K, being about a factor of two larger at 10000 K. This may have some effect on the oxygen ionization balance. Further, Kingdon & Ferland ([1999]) have suggested that in photoionized nebulae charge transfer can be an important heating mechanism. The reaction N+ + H is the most important reaction, but process (1) can contribute about 2%. Use of the current rate coefficients in future photoionization models could enhance the effect of charge transfer on the thermal balance.
Péquignot ([1990]) performed an extensive study of the populations of the neutral oxygen fine-structure levels in gaseous nebulae. Adopting the rate coefficients of Chambaud et al., he was the first to include charge transfer in such an analysis. As he found that charge transfer had a critical influence on the fine-structure level populations, use of the current rate coefficients may modify his results for T>1000 K.
The discovery of soft X-ray emission from the Jovian aurora and measurements of energetic oxygen and sulfur ions from Io suggest that the ions are precipitating into the Jovian atmosphere and charge exchanging with the neutral atmospheric constituents H2, H, and He. The electrons are captured into highly excited states of the highly charged ions which then radiate the X-rays (see Stancil et al. [1998c] for a review). While X-rays will not be emitted by either O+ or O, reaction (1) can play an important role in the deceleration of the energetic ions and the ionization stage distribution. Recent Monte Carlo calculations of oxygen ion beam stopping in the Jovian atmosphere were performed by Kharchenko et al. ([1998]). They found that a O+ ion beam ensemble of 10 MeV would be quickly converted to an O8+ beam in less than 200 collision events. The charge and energy of the beam slowly decreases until after just 1.5 105 collisions when the beam becomes mostly O+ with an energy of 1 MeV. The cross section shown in Fig. 1 is therefore relevant to the initial and final stages of the oxygen precipitation into Jupiter.
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