Brown dwarf discs
Brown dwarf discs › Disc Structure
Accreting Brown Dwarf and Disc, Spectra and Photometry
Disc Structure (Vertical Hydrostatic Equilibrium, 300au)In Mayne and Harries (2010) we explored the disc structure of models with an outer radius of 300au and vertical structure produced by solving vertical hydrostatic equilibrium in the disc. This page presents a brief summary of these results.
The disc surface density is conserved, Σβ-α, with ρ∝ r-α and h∝ rβ (where r is the radial coordinate, h the vertical coordinate and ρ the density).
FlaringThe discs around brown dwarf stars are significantly larger in vertical structure than their higher mass counterparts, under vertical hydrostatic equilibrium. This increased flaring leads occulation of he central star at lower inclinations than would be expected in a CTTS (Classical T Tauri Star). Additionally, as the flux reaching the outer disc increases, the flaring will also increase. Therefore, models within our ("Ad-hoc") grid with higher accretion rates will have, in general, more highly flared discs. This effectively means that accreting brown dwarfs will be obscured by their discs at lower inclinations than non (or lower level) accretors.
The Figures above show the density structure of a typical (left panel) and extreme (right panel) accreting brown dwarf disc system. The vertical height increases, at 50au, by around more than 5 au.
Dust Sublimation and the Inner EdgeOur models also include a sophisticated treatment of dust sublimation. This results in changes to the radial (and vertical) structure of the disc at the inner edge. In Mayne and Harries (2010) we found that significant dust erosion at the inner edge (for typical inner edge locations based on reasonable rotation rates and a co-rotation radius) does not occur until the accretion rate of the system exceeds a value of logṀ=-9. We therefore (and for veiling reasons, see photometric spreads) separated our systems into typical and extreme accretors, with logṀ ≤ -9, and logṀ > -9, respectively.
For typical accretors dust sublimation is not significant and the inner disc edge remains a vertical wall as prescribed by our initial density distribution. As one moves into the extreme accretors the inner edge is eroded to greater degrees (with increasing accretion rate). The inner edge becomes convex in shape, due to the density dependence of the dust sublimation temperature, and larger (vertically), due to the scaleheight of the disc increasing with the radial position of the inner edge. This sequence is illustrated below.
The figures above show the initial (greyscale) and final (colour scale) density structures (left images), and the final temperature structure (right image), for accretion rates of logṀ=-12, -7 and -6 (top middle and bottom figures respectively). The systems shown have the same values for all other parameters and are typical of the ("Ad-hoc") grid as a whole. For the lowest, negligible, accretion rate (top figures) the initial and final density structures match (approximately) in shape and position, with no appreciable dust sublimation. The middle and bottom images show that for elevated, extreme, accretion rates the inner edge becomes convex and increases in scaleheight. This should lead to a lesser dependence on inclination of the infrared excess for higher accretion rate systems.