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

TL;DR

This study re-evaluates HST FUV observations of HD 209458b, highlighting variability and uncertainties in detecting Si III and C II, and emphasizes the need for dedicated COS transit observations to clarify atmospheric composition.

Contribution

It critically revisits previous COS data, demonstrating the importance of consistent stellar baseline measurements for accurate exoplanet atmospheric analysis.

Findings

Stellar SiIII and CII emissions vary significantly, affecting transit absorption measurements.

Archive STIS data show no SiIII absorption, challenging previous claims.

Detection of OI and CII supports solar or supersolar atmospheric abundances.

Abstract

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.