Fe XVII $2p$-$3s$ line ratio diagnostic of shock formation radius in O stars

TL;DR

This study investigates the Fe XVII $2p$-$3s$ line ratio in O stars, especially $$ Puppis, to understand if UV photoexcitation affects plasma diagnostics, but finds the effect insufficient to explain observed anomalies.

Contribution

The paper models UV photoexcitation effects on Fe XVII lines using FAC and compares with stellar spectra, challenging previous assumptions about the line ratio diagnostic.

Findings

UV photoexcitation does not account for the observed line ratio difference.

Model calculations show limited impact of UV fields on the line ratio.

The origin of the anomalous line ratio in $$ Puppis remains unexplained.

Abstract

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 $\sim$1.4, in comparison with $\sim$2.4 for almost all other collisionally excited astrophysical spectra. Based on the work of Mauche et al. (2001), 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 (FAC) 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.