The HD 189733 system offers the possibility to study these various interactions (Table 1). It is a binary system with a K2V dwarf (HD 189733 A, hereafter HD 189733) and a M4V dwarf (HD 189733 B; Bakos et al. 2006) at a mean separation of ∼220 au. The bright primary (V = 7.7) hosts a transiting hot Jupiter at 0.03 au (Bouchy et al. 2005), whose strong irradiation and large occultation area make it particularly favourable for atmospheric characterization (see Pino et al. 2018 and references inside for observations of the lower atmospheric layers). Recent transit observations in the near-infrared (Salz et al. 2018) revealed absorption by helium in an extended but compact thermosphere. Transit observations in the far-ultraviolet (FUV) previously revealed absorption by a dense and hot layer of neutral oxygen at higher altitudes (Ben-Jaffel & Ballester 2013). The close distance of HD 189733 to the Sun (19.8 pc) enables observations of the stellar Lyman α line with the Hubble Space Telescope (HST). Atmospheric escape of neutral hydrogen was first detected in the unresolved line with the HST Advanced Camera for Surveys (ACS) in two out of three epochs (Lecavelier des Etangs et al. 2010), and in the line resolved with the HST Space Telescope Imaging Spectrograph (STIS) in one out of two epochs (Lecavelier des Etangs et al. 2012; Bourrier et al. 2013). These observations provided the first indication of temporal variations in the physical conditions of an evaporating planetary atmosphere. Bourrier & Lecavelier des Etangs (2013) attributed the high-velocity, blueshifted absorption signature detected by Lecavelier des Etangs et al. (2012) to intense charge-exchange between the planet exosphere and the stellar wind, proposing that an X-ray flare observed before the transit increased the atmospheric mass-loss and/or increased the density of the stellar wind. The latter scenario is favoured by thermal escape simulations from Chadney et al. (2017), who found that the energy input from a flare would not increase sufficiently the mass-loss. Interestingly the tentative detection of absorption by ionized carbon (Ben-Jaffel & Ballester 2013) and excited hydrogen (Jensen et al. 2012; Cauley et al. 2015, 2016; Cauley, Redfield & Jensen 2017; Kohl et al. 2018) before and during the transit of HD 189733b could be explained by the interaction of the stellar wind with the planetary magnetosphere or escaping material. The observed temporal variability in the atmospheric escape of neutral and excited hydrogen is likely linked to the high-level of activity from the host star, which results in a fast-changing radiation, particle, and magnetic environment for the planet. Enhanced activity in the stellar chromosphere and transition region have also been observed after the planetary eclipse, and attributed to signatures of magnetic star–planet interactions (Pillitteri et al. 2010, 2011, 2014, 2015). Evidence for modulation in the Ca ii lines at the orbital period of the planet, during an epoch of strong stellar magnetic field, further supports this scenario (Cauley et al. 2018). The detectability of star–planet interactions in the HD 189733 system however remain uncertain, and intrinsic stellar variability and inadequate sampling has been proposed to explain the observed variations (Route 2019). These results show the need to study contemporaneously and in different epochs the upper atmosphere of HD 189733b and its high-energy environment.
Citation: Lecavelier des Etangs et al. 2010
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