On the properties of discs around accreting brown dwarfs

TL;DR

This study models accreting brown dwarf systems with discs, revealing a natural accretion rate dichotomy, highlighting limitations in age/mass derivation from photometry, and suggesting biases in current accretion rate observations.

Contribution

It provides a self-consistent grid of brown dwarf disc models, analyzes their observational signatures, and proposes improved selection criteria and insights into accretion rate biases.

Findings

Identifies a natural dichotomy in accretion rates at a0log 0mdot > -9 and 0mdot 0a0 -9.

Shows that age and mass estimates from photometry are unreliable even for typical accretors.

Suggests higher accretion rate systems are less likely to be correctly identified, indicating a potential bias in observed accretion relationships.

Abstract

We present a grid of models of accreting brown dwarf systems with circumstellar discs. The calculations involve a self-consistent solution of both vertical hydrostatic and radiative equilibrium along with a sophisticated treatment of dust sublimation. We have simulated observations of the spectral energy distributions and several broadband photometric systems. Analysis of the disc structures and simulated observations reveal a natural dichotomy in accretion rates, with \logmdot $>-$9 and $\leq -$9 classed as extreme and typical accretors respectively. Derivation of ages and masses from our simulated photometry using isochrones is demonstrated to be unreliable even for typical accretors. Although current brown dwarf disc candidate selection criteria have been shown to be largely reliable when applied to our model grid we suggest improved selection criteria in several colour indices. We show that as accretion rates increase brown dwarf disc systems are less likely to be correctly identified. This suggests that, within our grid, systems with higher accretion rates would be preferentially lost during brown dwarf target selection. We suggest that observations used to assert a $\dot{M}\propto M_*^2$ relationship may contain an intrinsic selection bias.