Abstract
The high-energy X-ray to ultraviolet (XUV) irradiation of close-in planets by their host star influences their evolution and might be responsible for the existence of a population of ultra-short period planets eroded to their bare core. In orbit around a bright, nearby G-type star, the super-Earth 55 Cnc e offers the possibility to address these issues through transit observations at UV wavelengths. We used the Hubble Space Telescope to observe the transit in the far-ultraviolet (FUV) over three epochs in April 2016, January 2017, and February 2017. Together, these observations cover nearly half of the orbital trajectory in between the two quadratures, and reveal significant short- and long-term variability in 55 Cnc chromospheric emission lines. In the last two epochs, we detected a larger flux in the C III, Si III, and Si IV lines after the planet passed the approaching quadrature, followed by a flux decrease in the Si IV doublet. In the second epoch these variations are contemporaneous with flux decreases in the Si II and C II doublets. All epochs show flux decreases in the N V doublet as well, albeit at different orbital phases. These flux decreases are consistent with absorption from optically thin clouds of gas, are mostly localized at low and redshifted radial velocities in the star rest frame, and occur preferentially before and during the planet transit. These three points make it unlikely that the variations are purely stellar in origin, yet we show that the occulting material is also unlikely to originate from the planet. We thus tentatively propose that the motion of 55 Cnc e at the fringes of the stellar corona leads to the formation of a cool coronal rain. The inhomogeneity and temporal evolution of the stellar corona would be responsible for the differences between the three visits. Additional variations are detected in the C II doublet in the first epoch and in the O I triplet in all epochs with a different behavior that points toward intrinsic stellar variability. Further observations at FUV wavelengths are required to disentangle definitively between star-planet interactions in the 55 Cnc system and the activity of the star.
Highlights
The population of close-in planets is shaped by interactions with their host star
While Jupiter-mass planets are too massive to be significantly affected by evaporation, losing a few percent of their mass over their lifetime (e.g., Lecavelier des Etangs 2007; Hubbard et al 2007; Ehrenreich & Désert 2011), lower mass planets could be stripped of most of their atmosphere (e.g., The airglow templates are available at the CDS via anonymous ftp to cdsarc.u- strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/A117
Flux uncertainties and spectral calibration Similar to Wilson et al (2017), we found that the uncertainties produced by the CALCOS pipeline on the flux values were largely overestimated compared to the dispersion of the measurements, which was found to be consistent with a noise budget dominated by photon noise
Summary
The deposition of stellar X-ray and extreme ultraviolet radiation (XUV) into an exoplanet upper atmosphere can lead to its hydrodynamic expansion and the escape of large amounts of gas from the gravitational well of the planet (e.g., Vidal-Madjar et al 2003; Lecavelier des Etangs et al 2004; Johnstone et al 2015). This evaporation can sustain extended exospheres that have been detected around Jupiter-mass planets (Vidal-Madjar et al 2003, 2004; Lecavelier des Etangs et al 2010, 2012; Bourrier et al 2013; Ehrenreich et al 2012; Fossati et al 2010; Haswell et al 2012) and a Neptune- mass planet (Kulow et al 2014; Ehrenreich et al 2015; Lavie et al 2017).
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