Enshrouding & Star-planet Interactions in Close-orbiting Systems

Doherty, James (2020). Enshrouding & Star-planet Interactions in Close-orbiting Systems. PhD thesis The Open University.


The OU-SALT survey of the chromospheric activity of close-orbiting, transiting exoplanet hosts is unique. Emission flux in stellar Ca II H & K line cores is measured using the same telescope, spectrograph and calibration. I present homogeneous log(R'$_{HK}$) values for 104 bright southern hemisphere stars, ∼90% of which host hot Jupiters. Errors associated with systematic offsets between instruments are avoided. Uncertainties are on average three times smaller than in prior state-of-the-art planetary research. Approximately one third of the OU-SALT main sequence sub-sample is less active than a quiet F, G or K star that is devoid of spots, faculae and active regions (corresponding to log(R'$_{HK}$) <−5.1). Only 2% of field stars display such sub-basal activity. Absorption in the interstellar medium can not account for these depressed activity measurements. This provides compelling evidence of atmospheric escape from highly irradiated planets, and the absorption of emission flux by diffuse enshrouding discs. I find that such discs form only around stars with effective temperatures of 5200 K ≲ T$eff$ ≲ 6600 K and masses of M⋆ ≳ 0.8 M$\odot$.

I present evidence of star-planet interactions (SPI) in the OU-SALT sample with 4 σ certainty. I compare two multivariate models that predict activity: one that incorporates a proxy term for SPI (M1) and one that does not (M2). M1 consistently outperforms M2, providing a 0.003 − 0.3 % probability that massive, close-orbiting planets do not influence host activity. A systematic correlation study reveals that log(R'$_{HK}$) is correlated with planetary surface gravity (Spearman’s coefficient ρ = 0.41 ± 0.02), semi-major axis (ρ = −0.43 ± 0.01), and planet mass (ρ = 0.52 ± 0.01), as well as several SPI proxies. This provides further evidence of atmospheric escape and SPI in the OU-SALT sample. I confirm that ultra-hot Jupiter host WASP-43 has anomalously high activity (log(R′$_{HK}$) = -4.18 $_{-0.11}^{+0.09}$), likely caused by tidal spin-up. Significant activity variation is identified in several systems, notably KELT-11. A novel method to search for orbitally-modulated SPI in planet host samples is formulated.

I search for the spot-modulated rotation period (P$_{rot}$) of terrestrial planet host candidate HD 184960 using the Lomb-Scargle periodogram, string-length method, and phase dispersion minimisation. I constrain the star’s photometric amplitude to be less than 0.018 mag and estimate that P$_{rot}$ ≈ 5 − 6 d.

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