The discovery of OI atoms and CII ions in the upper atmosphere of HD 209458b,
made with the Hubble Space Telescope Imaging Spectrograph (STIS) using the
G140L grating, showed that these heavy species fill an area comparable to the
planet's Roche lobe. The derived ~10% transit absorption depths require
super-thermal processes and/or supersolar abundances. From subsequent Cosmic
Origins Spectrograph (COS) observations, CII absorption was reported with
tentative velocity signatures, and absorption by SiIII ions was also claimed in
disagreement with a negative STIS G140L detection. Here, we revisit the COS
dataset showing a severe limitation in the published results from having
contrasted the in-transit spectrum against a stellar spectrum averaged from
separate observations, at planetary phases 0.27, 0.72, and 0.49. We find
variable stellar SiIII and CII emissions that were significantly depressed not
only during transit but also at phase 0.27 compared to phases 0.72 and 0.49.
Their respective off-transit 7.5 and 3.1% flux variations are large compared to
their reported 8.2+/-1.4% and 7.8+/-1.3% transit absorptions. Significant
variations also appear in the stellar line shapes, questioning reported
velocity signatures. We furthermore present archive STIS G140M transit data
consistent with no SiIII absorption, with a negative result of 1.7+/-18.7
including ~15% variability. Silicon may still be present at lower ionization
states, in parallel with the recent detection of extended magnesium, as MgI
atoms. In this frame, the firm detection of OI and CII implying solar or
supersolar abundances contradicts the recent inference of potential x20-125
subsolar metallicity for HD 209458b.Comment: Accepted for publication in Ap
