Is TiO emission present in the ultra-hot Jupiter WASP-33b? A reassessment using the improved ExoMol TOTO line list

Abstract

Context. Efficient absorption of stellar ultraviolet and visible radiation by TiO and VO is predicted to drive temperature inversions in the upper atmospheres of hot Jupiters. However, very few inversions or detections of TiO or VO have been reported, and results are often contradictory. Aims. Using the improved ExoMol Toto line list, we searched for TiO emission in the dayside spectrum of WASP-33b using the same data in which the molecule was previously detected with an older line list at 4.8σ. We intended to confirm the molecular detection and quantify the signal improvement offered by the ExoMol Toto line list. Methods. Data from the High Dispersion Spectrograph on the Subaru Telescope was extracted and reduced in an identical manner to the previous study. Stellar and telluric contamination were then removed. High-resolution TiO emission models of WASP-33b were created that spanned a range of molecular abundances using the radiative transfer code petitRADTRANS, and were subsequently cross-correlated with the data. Results. We measure a 4.3σ TiO emission signature using the ExoMol Toto models, corresponding to a WASP-33b orbital velocity semi-amplitude of K p = 252.9 +5.0 −5.3 km s −1 and a system velocity of v sys = −23.0 +4.7 −4.6 km s −1. Injection-recovery tests using models based on the new and earlier line lists indicate that if the new models provide a perfect match to the planet spectrum, the significance of the TiO detection should have increased by a factor of ∼2. Conclusions. Although the TiO signal we find is statistically significant, comparison with previous works makes our result too ambiguous to claim a clear-cut detection. Unexpectedly, the new ExoMol Toto models provide a weaker signal than that found previously, which is offset in K p-v sys space. This sheds some doubt on both detections, especially in light of a recently published TiO non-detection using a different dataset.

Detection of Fe and evidence for TiO in the dayside emission spectrum of WASP-33b

“…Very recently, Serindag et al (2021) reassessed the TiO observation from Nugroho et al (2017), but did not confirm the results from this previous study. In line with Serindag et al (2021), we used the same line list ToTo ExoMol and adopted a similar approach by assuming a common optimal number of SYSREM iterations. They find a significant TiO detection with a K p that is similar to our value, but different from the value published by Nugroho et al (2017).…”

“…Otherwise, we cannot exclude the presence of a spurious signal. This aspect is also discussed by Serindag et al (2021), who suggest that previous claims of TiO may be false positives. The contradicting results of different studies on TiO in the atmosphere of WASP-33b (Haynes et al 2015;Nugroho et al 2017;Herman et al 2020) highlight the critical role of accurate line lists and the potential need of different data reduction techniques and spectral models in further investigations.…”

“…However, Herman et al (2020) reported a non-detection of TiO at high spectral resolution. More recently, Serindag et al (2021) reassessed the presence of TiO in the spectra from Nugroho et al (2017) by using an improved TiO line list, but they could not find a detection at the same orbital parameters as the previous work. Also the existence of TiO and VO in WASP-121b could not be confirmed by high-resolution spectroscopy observations (Merritt et al 2020).…”

“…However, the line lists of TiO suffer from inaccuracies, which reduce the detection sensitivity of the molecule (Hoeijmakers et al 2015;Nugroho et al 2017). The calculation of models leading to a detection of TiO in planetary atmospheres remains a challenging task (Herman et al 2020;Serindag et al 2021). We attempted to mitigate this issue by using the new line database ToTo ExoMol (McKemmish et al 2019) to generate our TiO model spectrum.…”

Silicon in the dayside atmospheres of two ultra-hot Jupiters

“…The presence of these so-called temperature inversions (i.e., temperature increasing with altitude) was initially explained via strong absorption of the incoming stellar radiation by TiO and VO (Hubeny et al 2003;Fortney et al 2008). However, the presence of TiO is under debate due to the conflicting results of different studies (Evans et al 2016;Nugroho et al 2017;Herman et al 2020;Edwards et al 2020;Serindag et al 2021;Cont et al 2021). Besides, the search for VO remains elusive at high spectral resolution (Merritt et al 2020).…”