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Astron. Astrophys. 334, L26-L28 (1998)

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3. Conclusion

In conclusion, we have calculated cross sections for the shake-up processes accompanying the K-shell photoionization of CII, NIII, OIV, FV and NeVI, in a close-coupling approximation using the R-matrix method. We have covered the photon energy interval X= [1.2-3.2] and find that the total cross section for these processes ranges from about 15% to about 3.5% of the single inner-shell photoionization cross section, when going from CII to NeVI. The full shake-up regime is reached for X [FORMULA] 2 due to the strong 2s [FORMULA] 2p conjugate shake-up transition. We find that the 2s [FORMULA] 3s excitation gives the dominant contribution since the 2p screening is weak and this favours the 2s excitation. This feature is not limited to the case of BI but it is present, for all ions studied. This result also suggests a strong shake-off process leading mostly to the 1s2s2p adjacent ion terms for the entire boron isoelectronic series. These features, strong 2s and 2p excitations, are of interest in astrophysical plasmas undergoing soft-X ray irradiation (Petrini and Da Silva 1996).

More experimental and theoretical work is needed, especially to confirm the strong 2s [FORMULA] 3s excitation and to obtain accurate relative shake-up probabilities for the 1s2 2s2 [FORMULA] second row atoms and ions. For carbon and nitrogen, the 2s [FORMULA] 3s excitation is expected to weaken substantially when compared to the 2p [FORMULA] 3p one. These processes (including shake-off and double Auger process) are of great importance in low-density astrophysical plasmas since highly-ionized species are produced perturbing the ionization equilibrium and giving rise directly to excited states. This will be critical for diagnosing spectra from the next generation high-resolution X-ray satellites XMM and AXAF, for example.

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998