Reference study to characterise plasma and magnetic properties of ultra-cool atmospheres

Abstract

Radio and X-ray emission from brown dwarfs suggest that an ionised gas and a magnetic field with a sufficient flux density must be present. We perform a reference study for late M-dwarfs, brown dwarfs and giant gas planet to identify which ultra-cool objects are most susceptible to plasma and magnetic processes. Only thermal ionisation is considered. We utilise the {\sc Drift-Phoenix} model grid where the local atmospheric structure is determined by the global parameters T${}_{\mathrm{eff}}$, $\log (g)$ and [M/H]. Our results show that it is not unreasonable to expect H${}_{\alpha }$ or radio emission to origin from Brown Dwarf atmospheres as in particular the rarefied upper parts of the atmospheres can be magnetically coupled despite having low degrees of thermal gas ionisation. Such ultra-cool atmospheres could therefore drive auroral emission without the need for a companion's wind or an outgassing moon. The minimum threshold for the magnetic flux density required for electrons and ions to be magnetised is well above typical values of the global magnetic field of a brown dwarf and a giant gas planet. Na${}^{+}$, K${}^{+}$ and Ca${}^{+}$ are the dominating electron donors in low-density atmospheres (low log(g), solar metallicity) independent of T${}_{\mathrm{eff}}$. Mg${}^{+}$ and Fe${}^{+}$ dominate the thermal ionisation in the inner parts of M-dwarf atmospheres. Molecules remain unimportant for thermal ionisation. Chemical processes (e.g. cloud formation) affecting the most abundant electron donors, Mg and Fe, will have a direct impact on the state of ionisation in ultra-cool atmospheres.

22 pages, accepted for publication in MNRAS

References in corpus (17)

• Detection of atmospheric haze on an extrasolar planet: The 0.55 - 1.05 micron transmission spectrum of HD189733b with the Hubble Space Telescope
• Large-scale magnetic topologies of early M dwarfs
• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
• HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering
• Atmospheric Circulation of Hot Jupiters: A Shallow Three-Dimensional Model
• Atmospheric Dynamics of Hot Exoplanets
• Dust in Brown Dwarfs and Extra-solar Planets I. Chemical composition and spectral appearance of quasi-static cloud layers
• Consistent simulations of substellar atmospheres and non-equilibrium dust-cloud formation
• Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513-46546: Broadband Coherent Emission at the Substellar Boundary?
• BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
• The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf
• Ohmic Dissipation in the Interiors of Hot Jupiters
• Stellar Activity and Coronal Heating: an overview of recent results
• Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS J13142039+1320011)
• Wideband Dynamic Radio Spectra of Two Ultra-cool dwarfs
• Quasi-Quiescent Radio Emission from the First Radio-Emitting T Dwarf
• The Deepest Constraints on Radio and X-ray Magnetic Activity in Ultracool Dwarfs from WISE J104915.57-531906.1

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