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Brown dwarf disks with Herschel: Linking far-infrared and (sub)-mm fluxes

Brown dwarf disks are excellent laboratories to test our understanding of disk physics in an extreme parameter regime. In this paper we investigate a sample of 29 well-characterized brown dwarfs and very low mass stars, for which Herschel far-infrared fluxes as well as (sub)-mm fluxes are available. We have measured new Herschel PACS fluxes for 11 objects and complement these with (sub)-mm data and Herschel fluxes from the literature. We analyze their spectral energy distributions in comparison with results from radiative transfer modeling. Fluxes in the far-infrared are strongly affected by the shape and temperature of the disk (and hence stellar luminosity), whereas the (sub)-mm fluxes mostly depend on disk mass. Nevertheless, there is a clear correlation between far-infrared and (sub)-mm fluxes. We argue that the link results from the combination of the stellar mass-luminosity relation and a scaling between disk mass and stellar mass. We find strong evidence of dust settling to the disk midplane. The spectral slopes between near- and far-infrared are mostly between $-0.5$ and $-1.2$ in our sample, comparable to more massive T Tauri stars, which may imply that the disk shapes are similar as well, though highly-flared disks are rare among brown dwarfs. We find that dust temperatures in the range of 7-15 K, calculated with $T\approx25\,(L/L_\odot)^{0.25}$ K, are appropriate for deriving disk masses from (sub)-mm fluxes for these low luminosity objects. About half of our sample hosts disks with at least one Jupiter mass, confirming that many brown dwarfs harbour sufficient material for the formation of Earth-mass planets in their midst.