![]() We detect no significant cross-correlation signal due to TiO, though artificial injection of our template spectra into the data indicates a sensitivity down to a volume-mixing ratio of ∼10-10. ![]() We subsequently co-added the correlation signal in time, taking the change in Doppler shift due to the orbit of the planet into account. We created model transmission spectra that include absorption by TiO, and cross-correlated them with the residual spectral data after removal of the dominating stellar absorption features. We used archival data from the 8.2 m Subaru telescope taken with the High Dispersion Spectrograph of a transit of HD 209458b in 2002. Gaseous TiO has been suggested as the cause of the thermal inversion layer invoked to explain the dayside spectrum of this planet. In this work, we aim to detect TiO in the optical transmission spectrum of HD 209458b. During these observations, the radial component of the planet's orbital velocity changes, resulting in a significant Doppler shift that allows its spectral features to be extracted. The spectral signature of an exoplanet can be separated from the spectrum of its host star using high-resolution spectroscopy. Assuming a clear atmosphere, we can rule out a full-disc dayside inversion layer in the pressure range 1 bar to 1 mbar.Ĭontext. That there is no signal indicates that HD 209458b either has a nearly isothermal atmosphere or that the signal is heavily muted. The non-detection of CO in the dayside spectrum of HD 209458b is interesting in light of a previous CO detection in the transmission spectrum. Furthermore, we found no evidence of absorption or emission from H 2O at these wavelengths. Models with strong CO emission lines show a weak anti-correlation with similar or lower significance levels. We do not detect any significant absorption or emission of CO in the dayside spectrum of HD 209458b, although cross-correlation with template spectra either with CO absorption lines or with weak emission at the core of the lines show a low-significance correlation signal with asignal - to - noiseratioof ~3-3.5. Thermal inversions give rise to emission features, which means that detecting emission lines in the planetary spectrum, as opposed to absorption lines, would be direct evidence of a region in which the temperature increases with altitude. We observed the thermal dayside emission of HD 209458b with the CRyogenic Infra-Red Echelle Spectrograph (CRIRES) on the Very Large Telescope during three nights, targeting the carbon monoxide band at 2.3 μm. This bright, transiting hot-Jupiter has long been considered the gold standard for a hot Jupiter with an inversion layer, but this has been challenged in recent publications. ![]() We apply high-resolution (R = 100 000) infrared spectroscopy to probe the temperature-pressure profile of HD 209458b. This question cannot be answered without reliable identification of thermal inversions. Furthermore, the primary mechanisms that drive the inversion layers in hot-Jupiter atmospheres are unknown. Broadband secondary-eclipse measurements of strongly irradiated hot Jupiters have indicated the existence of atmospheric thermal inversions, but their presence is difficult to determine from broadband measurements because of degeneracies between molecular abundances and temperature structure.
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