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Coherent Radio Emission from a Quiescent Red Dwarf Indicative of Star–Planet Interaction

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Coherent Radio Emission from a Quiescent Red Dwarf Indicative of Star–Planet Interaction LETTERS https://doi.org/10.1038/s41550-020-1011-9 Coherent radio emission from a quiescent red dwarf indicative of star–planet interaction H. K. Vedantham 1,2 ✉ , J. R. Callingham 1, T. W. Shimwell 1,3, C. Tasse4, B. J. S. Pope 5, M. Bedell6, I. Snellen3, P. Best7, M. J. Hardcastle 8, M. Haverkorn9, A. Mechev3, S. P. O’Sullivan 10,11, H. J. A. Röttgering 3 and G. J. White12,13 Low-frequency (ν ≲ 150 MHz) stellar radio emission is high circularly polarized fraction of 64 ± 6% (Fig. 1). The transient expected to originate in the outer corona at heights compa- nature and high polarization fraction are inconsistent with known rable to and larger than the stellar radius. Such emission properties of extragalactic radio sources, but consistent with that from the Sun has been used to study coronal structure, mass of stellar and planetary emissions11. On the basis of the positional ejections and space-weather conditions around the planets1. co-incidence, transient nature and high circularly polarized frac- Searches for low-frequency emission from other stars have tion, we conclusively associate the radio source with GJ 1151. The detected only a single active flare star2 that is not representa- astrometric uncertainty of ~0.2″ in LoTSS data is insufficient to tive of the wider stellar population. Here we report the detec- astrometrically differentiate between the stellar corona and a hypo- tion of low-frequency radio emission from a quiescent star, GJ thetical planetary magnetosphere as the site of emission. 1151—a member of the most common stellar type (red dwarf To determine the spectro-temporal characteristics of the radio or spectral class M) in the Galaxy. The characteristics of the emission, we extracted its time-averaged spectrum and frequency- emission are similar to those of planetary auroral emissions3 averaged light curve (Methods). We found that despite temporal (for example, Jupiter’s decametric emission), suggesting a variability, the emission persisted for the entire 8 h observation. coronal structure dominated by a global magnetosphere with The emission is also detected over the entire available bandwidth, low plasma density. Our results show that large-scale cur- 120 < ν < 167 MHz (where ν is the observed frequency), and has rents that power radio aurorae operate over a vast range of an approximately flat spectral shape (Fig. 2). The in-band radio 21 1 mass and atmospheric composition, ranging from terrestrial power for an isotropic emitter is PR 2 ´ 10 ergs sÀ . The peak 12 2 planets to main-sequence stars. The Poynting flux required radiation brightness temperature Iis Tb 3:7 ´ 10 xÀ K where à to produce the observed radio emission cannot be generated x* is the radius of the emitter in unitsI of GJ 1151’s stellar radius by GJ 1151’s slow rotation, but can originate in a sub-Alfvénic R 1:3 ´ 1010 cm. à interaction of its magnetospheric plasma with a short-period I A unique aspect of this detected radio source is that it is associ- exoplanet. The emission properties are consistent with theo- ated to a star with a quiescent chromosphere. Stellar radio emission retical expectations4–7 for interaction with an Earth-size planet at gigahertz frequencies is predominately non-thermal in origin in an approximately one- to five-day-long orbit. and is powered by chromospheric magnetic activity. The majority We discovered radio emission in the direction of the quies- of stellar radio detections are of a small class of magnetically active cent red dwarf star GJ 1151 by cross-matching catalogued radio stars such as flare stars12,13 (for example, AD Leo), rapid rotators14 sources in the Low Frequency Array (LOFAR) Two-Metre Sky (for example, FK Com) and close binaries15 (for example, Algol). Survey (LoTSS) data release I8, with nearby stars within a distance GJ 1151, in contrast, is a canonical ‘quiescent’ star, such as the Sun, of d < 20 pc from the Gaia data release 2 database9. The distance cut based on all available chromospheric activity indicators (Table 1). was imposed to maximize our chances of finding inherently faint For comparison, relatively intense broadband noise storms on stellar and planetary radio emission while maintaining a low false the Sun are arcmin-scale sources with brightness temperatures of 10 9 16 association rate . We found one match at high significance: GJ Tb 10 K (ref. ). Such an emitter will be three orders of magni- 1151, which is the closest catalogued star within the radio survey tudeI fainter than the radio source in GJ 1151 if observed from the footprint. The radio source lies at a distance of 0.17(55)″ in right same distance. ascension and 0.63(45)″ in declination from the proper-motion- In addition to the quiescent nature of GJ 1151, the properties of corrected optical position of GJ 1151 (1σ errors in parentheses here- the observed radio emission are distinct from prototypical stellar after; see Extended Fig. 1). bursts at centimetre wavelengths. Stellar radio emission falls into GJ 1151 was observed by four partially overlapping LoTSS point- two broad phenomenological categories11. (1) Incoherent gyrosyn- ings conducted within a span of about one month. The LoTSS radio chrotron emission, similar to solar noise storms16, characterized by 10 source ILT J115055.50+482225.2 is detected in only one, and has a a low degree of polarization, brightness temperatures of Tb≲10 K, I 1ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands. 2Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands. 3Leiden Observatory, Leiden University, Leiden, The Netherlands. 4GEPI, Observatoire de Paris, Université PSL, CNRS, Meudon, France. 5NASA Sagan Fellow, Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY, USA. 6Flatiron Institute, Simons Foundation, New York, NY, USA. 7Institute for Astronomy, Royal Observatory, Edinburgh, UK. 8Centre for Astrophysics Research, University of Hertfordshire, Hatfield, UK. 9Radboud University Nijmegen, Nijmegen, The Netherlands. 10Hamburger Sternwarte, Universität Hamburg, Hamburg, Germany. 11School of Physical Sciences and Centre for Astrophysics and Relativity, Dublin City University, Glasnevin, Ireland. 12Department of Physics and Astronomy, The Open University, Milton Keynes, UK. 13RAL Space, STFC Rutherford Appleton Laboratory, Didcot, UK. ✉e-mail: [email protected] NatURE AstronomY | VOL 4 | JUNE 2020 | 577–583 | www.nature.com/natureastronomy 577 LETTERS NATURE ASTRONOMY 0.5 +48° 23′ 30″ 0.4 Flux density (mJy beam +48° 23′ 00″ 0.3 +48° 22′ 30″ 0.2 +48° 22′ 00″ Declination (J2000) –1 0.1 ) +48° 21′ 30″ 0 +48° 21′ 00″ 1′ 1′ 11 h 51 min 0.0 s 11 h 51 min 0.0 s 11 h 51 min 04.0 s 11 h 50 min 56.011 s h 50 min 52.011 s h 50 min 48.011 h s 51 min 04.0 s 11 h 50 min 56.011 s h 50 min 52.011 s h 50 min 48.0 s Right ascension (J2000) Right ascension (J2000) Fig. 1 | Total intensity deconvolved images of the region around GJ 1151 for two different epochs. Left panel: 16 June 2014. Right: 28 May 2014. The cross- hairs point to the location of GJ 1151 (see Extended Fig. 1 for astrometric details). The inset in both panels displays the Stokes V (circular polarization) image for the respective epoch. The time–frequency-averaged Stokes I and V flux densities are 0.89(8) mJy and 0.57(4) mJy, respectively. The grey circle in top-left corner indicates the width of the point spread function. a 1.50 b 1.50 1.25 1.25 1.00 1.00 0.75 0.75 (mJy) (mJy) ty ty 0.50 0.50 densi densi 0.25 0.25 Flux Flux 0 0 –0.25 –0.25 StokesI StokesV StokesI StokesV –0.50 –0.50 0 1 2 3 4 5 6 7 8 120 130 140 150 160 Time since MJD 56823.6251 (h) Frequency (MHz) Fig. 2 | The variability of the flux density. a,b, The temporal (a) and spectral (b) variability of the total flux density (Stokes I; black circles) and circular polarized flux density (Stokes V; magenta squares) of the radio source in GJ 1151. The spectrum is measured over the entire 8 h exposure and the time series is measured over the entire bandwidth. The error bars span ±1σ. MJD is the modified Julian date. bandwidths of Δν=ν 1 and a duration of many hours. (2) Coherent relativistic plasma are plasma and cyclotron emission, which lead to emission (plasmaI or cyclotron emission), similar to solar radio emission at harmonics of the plasma frequency νp and the cyclotron bursts, characterized by a high degree of circular polarization (up to frequency νc, respectively. 100%), narrow instantaneous bandwidths (Δν=ν 1) and a dura- Stellar busts at centimetre wavelengths have previously been suc- tion ranging from seconds to minutes. The observedI emission does cessfully modelled as fundamental plasma emission from coronal not fit into either of these phenomenological classes. It is broad- loops19. However, the emissivity of the fundamental emission drops band, has a duration of >8 h and is highly circularly polarized. The nonlinearly with decreasing frequency. For typical coronal scale closest analogue of such emission is auroral radio emission from heights of quiescent red dwarfs, the height-integrated fundamen- substellar objects such as planets and ultracool dwarfs3,17,18. While tal emission is restricted to brightness temperatures of <1011 K at canonical stellar radio bursts are powered by impulsive heating of 150 MHz (Methods), which cannot account for the observed emis- 11,19 12 plasma trapped in compact coronal loops of size much smaller sion with Tb 10 K. Second harmonic plasma emission has than the stellar radius, radio aurorae in substellar objects are driven a higher emissivityI at low frequencies but cannot attain the high by global current systems in a large-scale dipolar magnetic field.
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