Environmental effects on the ionization of brown dwarf atmospheres

Abstract

Brown dwarfs emit bursts of Halpha, 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. 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. We consider the effect of photoionisation by Lyman continuum radiation in three different environments: the InterStellar Radiation Field (ISRF), O and B stars in star forming regions, and also for 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. 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.