Volume 618, October 2018
|Number of page(s)||10|
|Published online||23 October 2018|
Environmental effects on the ionisation of brown dwarf atmospheres
Centre for Exoplanet Science, University of St Andrews, UK
2 School of Physics & Astronomy, University of St. Andrews, St. Andrews KY16 9SS, UK
3 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
Accepted: 24 April 2018
Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient.
Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets.
Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects.
Results. The external radiation environment plays an important role for the atmospheric ionisation of very low-mass, ultra-cool objects. Lyman-continuum irradiation greatly increases the level of ionisation in the uppermost atmospheric regions. Our results suggest that a shell of an almost fully ionised atmospheric gas emerges for brown dwarfs in star-forming regions and brown dwarfs in white dwarf binary systems. As a consequence, brown dwarf atmospheres can be magnetically coupled, which is the presumption for chromospheric heating to occur and for aurorae to emerge. First tests for assumed chromosphere-like temperature values suggest that the resulting free-free X-ray luminosities are comparable with those observed from non-accreting brown dwarfs in star-forming regions.
Key words: radiation mechanisms: non-thermal / planet-star interactions / stars: atmospheres / brown dwarfs
© ESO 2018
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.