A Panchromatic View of Brown Dwarf Aurorae

Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multi-wavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as the consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestation of auroral phenomena in brown dwarf atmospheres, like H$\alpha$, and define their distinguishing characteristics. We conclude that large amplitude photometric variability in the near infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral H$\alpha$ emission and quiescent radio emission in ECMI pulsing brown dwarfs, suggesting a potential underlying physical connection between the quiescent and auroral emissions. We also discuss the electrodynamic engines powering brown dwarf aurorae and the possible role of satellites around these systems to both power the aurorae and seed the magnetosphere with plasma.Comment: 26 pages, 17 figures, and 2 tables; accepted to Ap

A Deep Radio Limit for the TRAPPIST-1 System

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 $\mu$Jy 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.Comment: 9 pages, accepted by AAS Journal

Using High-resolution Optical Spectra to Measure Intrinsic Properties of Low-mass Stars: New Properties for KOI-314 and GJ 3470

We construct high signal-to-noise "template" spectra by co-adding hundreds of spectra of nearby dwarfs spanning K7 to M4, taken with Keck/HIRES as part of the California Planet Search. We identify several spectral regions in the visible (370-800 nm) that are sensitive to the stellar luminosity and metallicity. We use these regions to develop a spectral calibration method to measure the mass, metallicity, and distance of low-mass stars, without the requirement of geometric parallaxes. Testing our method on a sample of nearby M dwarfs, we show that we can reproduce stellar masses to about 8%-10%, metallicity to ~0.15 dex, and distance to 11%. We were able to make use of HIRES spectra obtained as part of the radial velocity monitoring of the star KOI-314 to derive a new mass estimate of 0.57 ± 0.05 M_☉, a radius of 0.54 ± 0.05 R_☉, a metallicity, [Fe/H], of –0.28 ± 0.10, and a distance of 66.5 ± 7.3 pc. Using HARPS archival data and combining our spectral method with constraints from transit observations, we are also able to derive the stellar properties of GJ 3470, a transiting planet hosting M dwarf. We estimate a mass of 0.53 ± 0.05 M_☉, a radius of 0.50 ± 0.05 R_☉, a metallicity, [Fe/H], of 0.12 ± 0.12, and a distance of 29.9±_(3.4)^(3.7)pc

The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog III: The Spatial Dependence of Magnetic Activity in the Galaxy

We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data Release 7. This analysis explores the spatial distribution of M dwarf activity as a function of both vertical distance from the Galactic plane (Z) and planar distance from the Galactic center (R). We confirm the established trends of decreasing magnetic activity (as measured by Hα emission) with increasing distance from the mid-plane of the disk and find evidence for a trend in Galactocentric radius. We measure a non-zero radial gradient in the activity fraction in our analysis of stars with spectral types dM3 and dM4. The activity fraction increases with R and can be explained by a decreasing mean stellar age with increasing distance from the Galactic center

UV Spectral Characterization of Low-Mass Stars With AstroSat UVIT for Exoplanet Applications: The Case Study of HIP 23309

Characterizing rocky exoplanet atmospheres is a key goal of exoplanet science, but interpreting such observations will require understanding the stellar UV irradiation incident on the planet from its host star. Stellar UV mediates atmospheric escape, photochemistry, and planetary habitability, and observations of rocky exoplanets can only be understood in the context of the UV SED of their host stars. Particularly important are SEDs from observationally favorable but poorly understood low-mass M-dwarf stars, which are the only plausible targets for rocky planet atmospheric characterization for the next 1-2 decades. In this work, we explore the utility of AstroSat UVIT for the characterization of the UV SEDs of low-mass stars. We present observations of the nearby M0 star HIP 23309 in the FUV and NUV gratings of UVIT. Our FUV spectra are consistent with contemporaneous HST data and our NUV spectra are stable between orbits, suggesting UVIT is a viable tool for the characterization of the SEDs of low-mass stars. We apply our measured spectra to simulations of photochemistry and habitability for a hypothetical rocky planet orbiting HIP 23309 and elucidate the utility and limitations of UVIT in deriving UV SEDs of M-dwarf exoplanet hosts. Our work validates UVIT as a tool to complement HST in the characterization of exoplanet host stars and carries implications for its successor missions like INSIST.Comment: Accepted to A