Abstract
Abstract We consider the magnetic interaction of exoplanets orbiting M dwarfs, calculating the expected Poynting flux carried upstream along Alfvén wings to the central star. A region of emission analogous to the Io footprint observed in Jupiter’s aurora is produced, and we calculate the radio flux density generated near the surface of the star via the electron-cyclotron maser instability. We apply the model to produce individual case studies for the TRAPPIST-1, Proxima Centauri, and dwarf NGTS-1 systems. We predict steady-state flux densities of up to ∼10 μJy and sporadic bursts of emission of up to ∼1 mJy from each case study, suggesting these systems may be detectable with the Very Large Array and the Giant Metrewave Radio Telescope, and perhaps the Square Kilometre Array in the future. Finally, we present a survey of 85 exoplanets orbiting M dwarfs, identifying 11 such objects capable of generating radio emission above 10 μJy.
Highlights
Seven terrestrial planets have been discovered orbiting the nearby ultracool dwarf star TRAPPIST-1 (Gillon et al 2016, 2017), and an Earth-sized planet, Proxima b, has been observed orbiting our nearest stellar neighbor, Proxima Centauri (Anglada-Escudé et al 2016)
The “Radiometric Bode’s Law” (RBL) is an empirical scaling relation based on observations of radio emission from magnetized solar system planets that is often extrapolated to estimate the radio power expected from exoplanets (e.g., Lazio et al 2004; Zarka 2007)
The prediction from our results, that exoplanetinduced radio emission should be detectable from M dwarfs with the Very Large Array (VLA), Giant Metrewave Radio Telescope (GMRT), and Square Kilometre Array (SKA), must take into account whether a signal can be extracted from any background noise in radio observations
Summary
Seven terrestrial planets have been discovered orbiting the nearby ultracool dwarf star TRAPPIST-1 (Gillon et al 2016, 2017), and an Earth-sized planet, Proxima b, has been observed orbiting our nearest stellar neighbor, Proxima Centauri (Anglada-Escudé et al 2016). We apply Saur et al.ʼs (2013) analytic formulation for the magnetic energy communicated from an exoplanet to the central star via Alfvén wings to three case studies, each providing a distinct motivation for examination We apply this formulation to TRAPPIST-1, a star hosting a system of multiple terrestrial exoplanets planets; second, we apply it to Proxima Centauri, the closest star to the solar system, and host to a terrestrial exoplanet; and we apply it to NGTS-1, a more distant M dwarf (224 pc) hosting a recently discovered hot Jupiter. Where MA is the Alfvén Mach number, the interaction generates two standing Alfvén waves, making up Alfvén wings (Neubauer 1980), which can propagate upstream of the flow, transporting energy and momentum in that direction (see Figure 1) The condition for this case of the Alfvén mode is satisfied in the solar system in the interaction of satellites with magnetized planets, and the sub-Alfvénic interaction between. Equations (1) and (2) define the necessary conditions that must be met in order for the Poynting flux to communicate energy from the star–planet interaction back to the central star
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