Flares, Rotation, Activity Cycles and a Magnetic Star-Planet Interaction Hypothesis for the Far Ultraviolet Emission of GJ 436

Abstract

Variability in the far ultraviolet (FUV) emission produced by stellar activity affects photochemistry and heating in orbiting planetary atmospheres. We present a comprehensive analysis of the FUV variability of GJ 436, a field-age, M2.5V star ($P_{\mathrm{rot}}\approx 44$ d) orbited by a warm, Neptune-size planet ($M\approx 25M_{\oplus }$, $R\approx 4.1R_{\oplus }$, $P_{\mathrm{orb}}\approx 2.6$ d). Observations at three epochs from 2012 to 2018 span nearly a full activity cycle, sample two rotations of the star and two orbital periods of the planet, and reveal a multitude of brief flares. Over 2012-2018, the star's $7.75\pm 0.10$ yr activity cycle produced the largest observed variations, $38\pm 3$% in the summed flux of major FUV emission lines. In 2018, variability due to rotation was $8\pm 2$%. An additional $11\pm 1$% scatter at 10 min cadence, treated as white noise in fits, likely has both instrumental and astrophysical origins. Flares increased time-averaged emission by 15% over the 0.88 d of cumulative exposure, peaking as high as 25$\times$ quiescence. We interpret these flare values as lower limits given that flares too weak or too infrequent to have been observed likely exist. GJ 436's flare frequency distribution (FFD) at FUV wavelengths is unusual compared to other field-age M dwarfs, exhibiting a statistically-significant dearth of high energy ($>4\times 10^{28}$ erg) events that we hypothesize to be the result of a magnetic star-planet interaction (SPI) triggering premature flares. If an SPI is present, GJ 436 b's magnetic field strength must be $\lesssim$100 G to explain the statistically insignificant increase in orbit-phased FUV emission. Erratum: Due to an arithmetic error, the published limit on the magnetic field strength is incorrect. The correct limit is $\lesssim$10 G.

18 pages, 9 figures, published in the Astronomical Journal

References in corpus (28)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
• Improved parameters for extrasolar transiting planets
• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• Stellar Activity Masquerading as Planets in the Habitable Zone of the M dwarf Gliese 581
• The On/Off Nature of Star-Planet Interactions
• Lyman-$\alpha$ Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b
• Magnetic Cycles and Rotation Periods of Late Type Stars from photometric time series
• Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star
• A global analysis of Spitzer and new HARPS data confirms the loneliness and metal-richness of GJ 436 b
• The long egress of GJ~436b's giant exosphere
• Magnetic field strengths of hot Jupiters from signals of star-planet interactions
• Indications for an influence of Hot Jupiters on the rotation and activity of their host stars
• MOST detects variability on tau Bootis possibly induced by its planetary companion
• A Jupiter-like Planet Orbiting the Nearby M Dwarf GJ832
• Discovery of an Extremely Short Duration Flare from Proxima Centauri Using Millimeter through FUV Observations
• Assessing magnetic torques and energy fluxes in close-in star-planet systems
• An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b
• CoRoT-2a magnetic activity: hints for possible star-planet interaction
• No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter
• Ultraviolet C II and Si III Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b
• The CARMENES search for exoplanets around M dwarfs: Stable radial-velocity variations at the rotation period of AD~Leonis -- A test case study of current limitations to treating stellar activity
• SALT observations of the Chromospheric Activity of Transiting Planet Hosts: Mass Loss and Star Planet Interactions
• HAZMAT. VII. The Evolution of Ultraviolet Emission with Age and Rotation for Early M Dwarf Stars
• Detailed stellar activity analysis and modelling of GJ 832: Reassessment of the putative habitable zone planet GJ 832c
• A Study of the Magnetic Activity and Variability of GJ 436
• Spectroscopic study of solar transition region oscillations in the quiet-Sun observed by IRIS using Si IV spectral line

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• Tighter constraints on the atmosphere of GJ 436 b from combined high-resolution CARMENES and CRIRES${}^{+}$ observations
• On stellar hotspots due to star-planet magnetic interactions: How much power can actually be transmitted to the chromosphere?
• Constraining an exoplanets magnetic field using star-planet interactions
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