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要旨

The 2p-3s lines of Fe xvii in the X-ray spectrum of the O-type star zeta
Puppis exhibit an anomalous (3G + M2)/(3F) line ratio of similar to 1.4,
in comparison with similar to 2.4 for almost all other collisionally
excited astrophysical spectra. Based on the work of Mauche et al., we
conjectured that the strong UV field of zeta Puppis produces the
observed ratio by depopulation of metastable 3s excited states, and that
the ratio can potentially be used as an independent diagnostic of plasma
formation radius. We used the Flexible Atomic Code collisional-radiative
model to model the effect of UV photoexcitation from O stars on the Fe
xvii lines. We compared our model calculations to archival spectra of
coronal and hot stars from the Chandra HETGS and XMM-Newton RGS to
benchmark our calculations for various electron densities and UV field
intensities. Our calculations show that UV photoexcitation does not
produce a sufficiently large dynamic range in the 3F / (3F + 3G + M2)
fraction to explain the difference in the observed ratio between coronal
stars and zeta Pup. Thus, this effect likely cannot explain the observed
line ratio of zeta Pup, and its origin is still unexplained.