Stellar Photospheric Contamination as a Tool for Indirect Characterisation of Hot Jupiter Atmospheres

Alexandra Lehtmets EPSC-DPS 2025 12.09.25 Stellar Photospheric Contamination as a Tool for Indirect Characterisation of Hot Jupiter Atmospheres In collaboration with Z. Balkoova, H. Ramler, M. Kama, E. Väljamäe and C. Matt. Motivation •Challenging observations of atmospheres (e.g. faint signals, clouds, stellar activity, star-planet contrast, ...) [Seager & Deming 2010, Rukdee+ 2024] •Hot Jupiters: strong irradiation → atmospheric escape [Fossati+ 2010] •HD 209458 b: 15 ± 4 % absorption in Lyαof HI [Vidal-Madjar+ 2003] •Make a uniform study of planetary parameters - Uniform stellar parameters Tsantaki+ 2025, Ramler+ in. prep. •Escaping material may be accreted onto the star [Jermyn & Kama 2018] Stellar photospheric contamination offers a new indirect pathway to probe exoplanet atmospheres! 2EPSC-DPS 2025 Background •Jermyn and Kama 2018: mechanism of stellar photospheric contamination [Jura 2015] •One main result: Stellar convective zone depth limits the survival of exoplanet atmosphere signatures → A-F type stars are the best! ሶ 𝑀𝑈𝑉 ≈10−13𝑀𝑆𝑢𝑛 𝑦𝑟−1 HD 195689 (KELT-9b) atmospheric mass-loss (estimated): This result may still lead to a detectable stellar photospheric contamination (non-H elements). 3EPSC-DPS 2025 Questions •Under what stellar and planetary conditions can contamination occur? •Which atmospheric species are most likely to escape and be accreted? [Lehtmets+ 2025, under review] •Which ultrahot Jupiter systems are the most promising contamination candidates? →(current focus) •Are these imprints detectable with current or upcoming spectrographs? •UV, Optical and/or IR observations? 4 Sample 5 55 Ultrahot Jupiters 𝑇𝑒𝑞 ≥ 2000 K, SM𝐴 < 0.07 AU F-A stars + hot G stars 𝑇𝑒𝑓𝑓 = 6000 − 9000 K DUST SUBLIMATION ZONES –refractory species remain gaseous HILL SPHERE –stability limit for bound atmosphere & extended H atmospheres ROCHE FLUID LIMIT –direct overflow channels planetary gas to star CONTAMINATION SWEETSPOT –best region to find stellar contamination WASP-33Ab TOI-2109b HD 2685b KELT-18b KELT-20b Methods 6EPSC-DPS 2025 •Atmospheric models: P-T profiles & chemistry (observationally constrained) •Escape pathways: hydrodynamic escape, Roche overflow, stellar wind interactions, ... •Star-planet interactions: irridation, magnetic fields, ... •Mass-loss rate estimates: energy-limited and literature values •Contamination potential: compare escape with stellar convective zone depth. SAMPLE ATMOSPHERIC MODELS (observationally constrained) CONTAMINATION POTENTIAL ESCAPE SPECIES Preliminary results from CAMstar [Jermyn & Kama 2018] Best: fpℎ ~ 1 Assumed mass-loss rate: ሶ 𝑀 = 10−13 𝑀𝑆𝑜𝑙𝑎𝑟 𝑦𝑟−1 (𝑡 = 107𝑦𝑟) Several UHJs approach or exceed the photospheric contamination fraction threshold. One promising candidate! 7EPSC-DPS 2025 Evidence for Stellar accretion from Protoplanetary discs •Disc accretion around AFG-type stars alters stellar surface composition; explains abundance anomalies. [Hoppe+ 2020, Borthakur+ 2025] •Late accretion of refractory-poor gas dilutes metallicity; planet ingestino may reverse it. [Kunitomo+ 2018] •Ca II lines show refractory depletion in T Tauri innermost disc; evidence for dust trapping by forming planets. [Micolta+ 2023, 2024] 8EPSC-DPS 2025 Conclusions •Stellar contamination is a complementary indirect tool for probing exoplanet atmospheres. •Promising for hot Jupiters around F-A stars. •Opens synenergy between stellar spectroscopy & exoplanetary science. •Ongoing and next steps: •Uniform exoplanet parameters (Zuzana Balkoova) •Atmospheric models & escape (Alexandra Lehtmets) •Demographics studies (Teodora Žižak) •Star-planet interactions & uniform stellar parameters (Heleri Ramler) •Modelling stellar wind influence on atmospheric escape (Anselmo Falorca?) •... 9EPSC-DPS 2025