A Deep Radio Limit for the TRAPPIST-1 System

Abstract

The first nearby very-low mass star planet-host discovered, TRAPPIST-1, presents not only a unique opportunity for studying a system of multiple terrestrial planets, but a means to probe magnetospheric interactions between a star at the end of the main sequence and its close-in satellites. This encompasses both the possibility of persistent coronal solar-like activity, despite cool atmospheric temperatures, and the presence of large-scale magnetospheric currents, similar to what is seen in the Jovian system. Significantly, the current systems include a crucial role for close-in planetary satellites analogous to the role played by the Galilean satellites around Jupiter. We present the first radio observations of the seven-planet TRAPPIST-1 system using the Karl G. Jansky Very Large Array, looking for both highly circularly polarized radio emission and/or persistent quiescent emissions. We measure a broadband upper flux density limit of <8.1 µJy across 4-8 GHz, and place these observations both in the context of expectations for stellar radio emission, and the possible electrodynamic engines driving strong radio emissions in very-low mass stars and brown dwarfs, with implications for future radio surveys of TRAPPIST-1 like planet-hosts. We conclude that magnetic activity of TRAPPIST-1 is predominantly coronal and does not behave like the strong radio emitters at the stellar/sub-stellar boundary. We further discuss the potential importance of magnetic field topology and rotation rates, demonstrating that a TRAPPIST-1 like planetary system around a rapidly rotating very-low mass star can generate emission consistent with the observed radio luminosities of very-low mass stars and brown dwarfs.

Magnetism in the Brown Dwarf Regime

“…Surface-averaged field strengths were initially thought to saturate at fast rotation rates (Reiners et al 2009), but recent Zeeman broadening innovations demonstrate that dipolar field strengths ( 4 kG) can exceed saturation levels in the strongest mulipolar objects (Shulyak et al 2017). These stronger fields are in accord with observed kG fields in rapidly rotating brown dwarfs (Kao et al 2018) and imply that dipolar magnetic topologies may be important for strong ECM radio activity (Pineda et al 2017;Pineda & Hallinan 2018).…”

“…The subsequent identification of the flaring component of ultracool dwarf radio emission as coherent electron cyclotron maser emission (ECM) confirmed that ultracool dwarfs can host kilogauss (kG) magnetic fields (Hallinan et al 2006(Hallinan et al , 2008. Soon thereafter, the discovery of the first T dwarf (∼890 K) at 4-8 GHz (Route & Wolszczan 2012) demonstrated that magnetic behavior persists well beyond the observed drop-off in chromospheric emission/white-light flaring seen across early-to-mid L spectral types (Schmidt et al 2015;Pineda et al 2016;Gizis et al 2017).…”

“…Together, these detections are inspiring a wide breadth of investigations into the effects of magnetism on substellar objects, their observable properties, and the underlying physics governing these effects from stars to planets. We are studying the impact of auroral electron beams on brown dwarf atmospheres (Pineda et al 2017) and the nature of the electrodynamic engine driving ECM emission (Schrijver 2009;Nichols et al 2012;Turnpenney et al 2017), testing substellar magnetic dynamo mechanisms (Kao et al 2016(Kao et al , 2018, modeling the magnetic topology in ECM source regions (Yu et al 2011;Lynch et al 2015;Leto et al 2016), and conducting multiwavelength searches for magnetic star-planet interactions (Hallinan et al 2013;Turnpenney et al 2018;Pineda & Hallinan 2018). With these recent advances, the next decade is poised to address many outstanding questions in brown dwarf magnetism and enter into an era of characterizing the full magnetic ecosystems of brown dwarfs and their close cousins, exoplanets.…”

“…To these ends we need new radio monitoring programs to expand the current sample of radio brown dwarfs (<30; Pineda et al 2017;Kao et al 2018). Such programs will need to identify or rule out periodic pulsations of ∼10 µJy on minute timescales, and extend to the slower rotators (P 10 hr) to test the dependence of the observed radio emission strength on rotation period in comparison to model predictions (e.g., Nichols et al 2012).…”

Constraining the Radio Emission of TRAPPIST-1

“…A few models exist to explain UCD magnetic field generation and corresponding radio emission. For example, Hallinan et al (2007) and Pineda (2017) argue that radio emission due to ECMI requires large, dipolar magnetic fields. In the case of gyrosynchrotron radiation, Williams et al (2014) propose that the magnetic field topology, rather than strength, is responsible for the presence of radio emission.…”