High-energy environment of super-Earth 55 Cnc e I: Far-UV chromospheric variability as a possible tracer of planet-induced coronal rain

Abstract

The irradiation of close-in planets by their star influences their evolution and might be responsible for 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 UV transit observations. We used the Hubble Space Telescope to observe the transit in the FUV over 3 epochs in Apr. 2016, Jan. 2017, and Feb. 2017. These observations reveal significant short- and long-term variability in 55 Cnc chromospheric emission lines. In the last 2 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 doublet. 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 transit. These 3 points make it unlikely that the variations are purely stellar, yet we show that the occulting material is also unlikely to originate from the planet. We 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 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 between star-planet interactions and the activity of the star

22 pages, 20 figures, accepted for publication in A&A

References in corpus (22)

• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e
• Five Planets Orbiting 55 Cancri
• Lyman-$\alpha$ Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b
• Near-UV Absorption, Chromospheric Activity, and Star-Planet Interactions in the WASP-12 system
• Atmospheric escape from HD 189733b observed in HI Lyman-alpha: detailed analysis of HST/STIS September 2011 observations
• Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability
• Classification of magnetized star--planet interactions: bow shocks, tails, and inspiraling flows
• The long egress of GJ~436b's giant exosphere
• Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-$\alpha$ line
• Hubble Space Telescope detection of oxygen in the atmosphere of exoplanet HD189733b
• The effects of stellar winds on the magnetospheres and potential habitability of exoplanets
• MOST detects variability on tau Bootis possibly induced by its planetary companion
• Search for water in a super-Earth atmosphere: High-resolution optical spectroscopy of 55 Cancri e
• A Case for an Atmosphere on Super-Earth 55 Cancri e
• Transit of Exomoon Plasma Tori: New Diagnosis
• Solar LImb Prominence CAtcher and Tracker (SLIPCAT): An Automated System and Its Preliminary Statistical Results
• Ground-Based Transit Observations of the Super-Earth 55 Cnc e
• Rossiter-McLaughlin Observations of 55 Cnc e
• On the correlation between stellar chromospheric flux and the surface gravity of close-in planets

Cited by in corpus (9)

• HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Star
• CHEOPS Precision Phase Curve of the Super-Earth 55 Cnc e
• Lava Worlds: From Early Earth to Exoplanets
• MOVES III. Simultaneous X-ray and ultraviolet observations unveiling the variable environment of the hot Jupiter HD 189733b
• Hydrodynamic Escape of Mineral Atmosphere from Hot Rocky Exoplanet. I. Model Description
• The Exosphere as a Boundary: Origin and Evolution of Airless Bodies in the Inner Solar System and Beyond Including Planets with Silicate Atmospheres
• Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence
• Observability of hydrogen-rich exospheres in Earth-like exoplanets
• The Hubble PanCET program: Long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470