System 77
System 77 map
System 77 is a solitary large brown dwarf of spectral type L. Its mass of 0.061 puts it close to the lower limit for stardom. It has long
passes its lithium-fusion phase and is slowly cooling and contracting. Its photosphere is still fully convective, similar to the
convection
found in many main sequence stars. An elliptical disc of icy debris surrounds this brown dwarf at 454 million km. Small planetoids have
begun to form in this disc but no sizeable planets appear to have formed, presumably due to insufficient disc mass.
System 77 brown dwarf
Brown dwarfs generally form in a fully convective state, since they are cool enough for the formation of molecular hydrogen which
absorbs photons more readily than atomic hydrogen by the process of collision induced absorption (collisions between the hydrogen
molecules stimulate the absorption of photons). This makes the dwarf more opaque which makes it harder for heat to escape. This
causes thermal instability or convection as pockets of warm gas rise and cool and then sink again. Small brown dwarfs may cool to
such an extent, that after one million to one billion years, convection ceases at their visible surface or photosphere. Larger brown
dwarfs retain convective photospheres for longer. This is a good opportunity to study a large brown dwarf and refine our models of
convection and cooling in brown dwarfs.
brown dwarf convection
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The brown dwarf's convective photosphere.