Reference study to characterize plasma and magnetic properties of ultracool atmospheres

M. I. Rodriguez-Barrera, Ch. Helling, C. R. Stark, A. M. Rice

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Abstract

Radio and X-ray emission from brown dwarfs (BDs) suggest that an ionized gas and a magnetic field with a sufficient flux density must be present. We perform a reference study for late M-dwarfs (MD), BDs and giant gas planet to identify which ultracool objects are most susceptible to plasma and magnetic processes. Only thermal ionization is considered. We utilize the drift-phoenix model grid where the local atmospheric structure is determined by the global parameters Teff, log (g) and [M/H]. Our results show that it is not unreasonable to expect Hα or radio emission to origin from BD atmospheres as in particular the rarefied upper parts of the atmospheres can be magnetically coupled despite having low degrees of thermal gas ionization. Such ultracool 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 magnetized is well above typical values of the global magnetic field of a BD 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 Teff. Mg+ and Fe+ dominate the thermal ionization in the inner parts of MD atmospheres. Molecules remain unimportant for thermal ionization. Chemical processes (e.g. cloud formation) affecting the most abundant electron donors, Mg and Fe, will have a direct impact on the state of ionization in ultracool atmospheres.
Original languageEnglish
Pages (from-to)3977-3995
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume454
Issue number4
Early online date27 Oct 2015
DOIs
Publication statusPublished - 21 Dec 2015

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magnetic property
magnetic properties
ionization
plasma
atmospheres
atmosphere
gas giant planets
gas
electron
flux density
gas ionization
Phoenix (AZ)
planet
lower atmosphere
electrons
radio
outgassing
magnetic field
atmospheric structure
ionized gases

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Rodriguez-Barrera, M. I. ; Helling, Ch. ; Stark, C. R. ; Rice, A. M. / Reference study to characterize plasma and magnetic properties of ultracool atmospheres. In: Monthly Notices of the Royal Astronomical Society. 2015 ; Vol. 454, No. 4. pp. 3977-3995.
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Reference study to characterize plasma and magnetic properties of ultracool atmospheres. / Rodriguez-Barrera, M. I.; Helling, Ch.; Stark, C. R.; Rice, A. M.

In: Monthly Notices of the Royal Astronomical Society, Vol. 454, No. 4, 21.12.2015, p. 3977-3995.

Research output: Contribution to journalArticle

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AU - Rodriguez-Barrera, M. I.

AU - Helling, Ch.

AU - Stark, C. R.

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AB - Radio and X-ray emission from brown dwarfs (BDs) suggest that an ionized gas and a magnetic field with a sufficient flux density must be present. We perform a reference study for late M-dwarfs (MD), BDs and giant gas planet to identify which ultracool objects are most susceptible to plasma and magnetic processes. Only thermal ionization is considered. We utilize the drift-phoenix model grid where the local atmospheric structure is determined by the global parameters Teff, log (g) and [M/H]. Our results show that it is not unreasonable to expect Hα or radio emission to origin from BD atmospheres as in particular the rarefied upper parts of the atmospheres can be magnetically coupled despite having low degrees of thermal gas ionization. Such ultracool 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 magnetized is well above typical values of the global magnetic field of a BD 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 Teff. Mg+ and Fe+ dominate the thermal ionization in the inner parts of MD atmospheres. Molecules remain unimportant for thermal ionization. Chemical processes (e.g. cloud formation) affecting the most abundant electron donors, Mg and Fe, will have a direct impact on the state of ionization in ultracool atmospheres.

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