Re-visit of HST FUV observations of hot-Jupiter system HD 209458: No Si III detection and the need for COS transit observations

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