Low abundances of TiO and VO on the dayside of KELT-9 b: Insights from ground-based photometric observations

Publications of the Astronomical Society of Japan, 2024, 76(5), 1131–1141 https://doi.org/10.1093/pasj/psae075 Advance access publication date: 2024 September 11 Low abundances of TiO and VO on the dayside of KELT-9 b: Insights from ground-based photometric observations 1 Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan 2 Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan 3 Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 4 Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain 5 Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA 6 Department of Physics and Astronomy, UCL, Gower St, Bloomsbury, London WC1E 6BT, UK 7 Department of Physical Sciences, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan 8 Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain 9 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain 10 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 11 Astronomical Science Program, Graduate University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan 12 Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan ∗ Email: Abstract We present ground-based photometric observations of secondary eclipses of the hottest known planet KELT-9 b using MuSCAT2 and Sinistro. +199 We detect secondary eclipse signals in i and zs with eclipse depths of 373+74 −75 and 638−178 parts per million, respectively. We perform an atmospheric retrieval on the emission spectrum combined with the data from HST/WFC3, Spitzer, TESS, and CHEOPS to obtain the temperature profile and chemical abundances, including TiO and VO, which have been thought to produce temperature inversion structures in the dayside of ultra-hot Jupiters. While we confirm a strong temperature inversion structure, we find low abundances of TiO and VO with mixing ratios of .15 +0.64 log(TiO) = −7.80+0 −0.30 and log(VO) = −9.60−0.57 , respectively. The low abundances of TiO and VO are consistent with theoretical predictions for such an ultra-hot atmosphere. In such low abundances, TiO and VO have little effect on the temperature structure of the atmosphere. The abundance of e− , which serves as a proxy for H− ions in this study, is found to be high, with log(e− ) = −4.89 ± 0.06. These results indicate that the temperature inversion in KELT-9 b’s dayside atmosphere is likely not caused by TiO/VO, but rather by the significant abundance of H− ions. The best-fitting model cannot fully explain the observed spectrum, and chemical species not included in the retrieval may introduce modeling biases. Future observations with broader wavelength coverage and higher spectral resolution are expected to provide more accurate diagnostics on the presence and abundances of TiO/VO. These advanced observations will overcome the limitations of current data from HST and photometric facilities, which are constrained by narrow wavelength coverage and instrumental systematics. Keywords: opacity — planets and satellites: atmospheres — planets and satellites: individual: KELT-9 b 1 Introduction Ultra-hot Jupiters (UHJs) are gaseous planets with equilibrium temperatures exceeding about 2000 K (e.g., Lothringer et al. 2018). Temperature inversion structures in the dayside atmospheres of UHJs have been widely detected using the Spitzer Space Telescope, the Hubble Space Telescope (HST), and recently the JWST (e.g., Changeat et al. 2022; Coulombe et al. 2023; Deming et al. 2023). These temperature inversions are thought to be caused by the absorption of stellar radiation by molecules such as TiO and VO, which have large cross-sections at optical wave- lengths (Hubeny et al. 2003; Fortney et al. 2008). These molecules were detected in some of the dayside atmospheres of UHJs with temperature inversions using HST/WFC3 (e.g., Haynes et al. 2015; Changeat et al. 2024). However, there has so far been no detection in the dayside atmospheres of UHJs using ground-based high-resolution cross-correlation spectroscopy (HRCCS), except for one controversial detection reported in WASP-33b (Nugroho et al. 2017). It is worth noting that TiO and VO have been detected in the transmission spectra of WASP-189b (Prinoth et al. 2022) and WASP76b (Pelletier et al. 2023), respectively, using HRCCS. These Received: 2024 April 12; Accepted: 2024 July 31 © The Author(s) 2024. Published by Oxford University Press on behalf of the Astronomical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Yuya HAYASHI ,1 ,∗ Norio NARITA,2 ,3 ,4 Akihiko FUKUI ,2 ,4 Quentin CHANGEAT,5 ,6 Kiyoe KAWAUCHI,7 Kai IKUTA,1 Enric PALLE,4 ,8 Felipe MURGAS,4 ,8 Hannu PARVIAINEN ,4 ,8 Emma ESPARZA-BORGES,4 ,8 Alberto PELÁEZ-TORRES,4 ,8 Pedro Pablo MENI GALLARDO,4 Giuseppe MORELLO,9 Gareb FERNÁNDEZ-RODRÍGUEZ,4 ,8 Néstor ABREU GARCÍA,4 ,8 Sara MUÑOZ TORRES ,4 ,8 Yéssica CALATAYUD BORRÁS,4 ,8 Pilar MONTAÑÉS RODRÍGUEZ,4 ,8 John H. LIVINGSTON ,3 ,10 ,11 Noriharu WATANABE ,1 Jerome P. DE LEON ,1 Yugo KAWAI ,1 Keisuke ISOGAI,1 ,12 and Mayuko MORI 3 ,10 Publications of the Astronomical Society of Japan (2024), Vol. 76, No. 5 1132 Table 1. Observation summary. Telescope Instrument Start date (UT) Observing time (UT) Filter texp (s) Airmass TCS 1.52 m LCO 1 m LCO 1 m MuSCAT2 Sinistro Sinistro 2018 August 09 2023 July 15 2023 July 21 22:16–5:00 23:29–6:30 21:40–3:50 g, r, i, zs Y Y 2.0, 2.0, 5.0, 10.0 30 30 1.14-1.02-1.83 1.20-1.02-1.33 1.56-1.02-1.12 and Edwards (2021) could also potentially be explained by the mixing of these molecules from deeper layers of the atmosphere, where they are stable under chemical equilibrium conditions. In their disequilibrium retrieval result, KELT-9 b has nearly solar metallicity and C/O ratio while the TiO abundance [log(TiO) = −7.9] contradicts that of the upper limit derived by Kasper et al. (2021). Moreover, even their bestfitting model does not fully explain the HST/WFC3 spectrum. In addition, the narrow wavelength coverage of the HST might not be able to fully break the degeneracies between the abundances of key species (TiO, VO, FeH, H2O), the thermal structure, and thermal dissociation processes (H− ). When combining with other datasets, especially photometric points such as Spitzer, instrumental systematics could render the datasets incompatible (Changeat et al. 2020; Colón et al. 2020; Yip et al. 2021). This paper presents a thermal emission spectrum using gro (...truncated)