A homogeneous analysis of disks around brown dwarfs

Abstract

We re-analyzed the Herschel/PACS data of a sample of 55 brown dwarfs (BDs) and very low mass stars with spectral types ranging from M5.5 to L0. We investigated the dependence of disk structure on the mass of the central object in the substellar regime based on a homogeneous analysis of Herschel data from flux density measurements to spectral energy distribution (SED) modeling. A systematic comparison between the derived disk properties and those of sun-like stars shows that the disk flaring of BDs and very low mass stars is generally smaller than that of their higher mass counterparts, the disk mass is orders of magnitude lower than the typical value found in T Tauri stars, and the disk scale heights are comparable in both sun-like stars and BDs. We further divided our sample into an early-type brown dwarf (ETBD) group and a late-type brown dwarf (LTBD) group by using spectral type (=M8) as the border criterion. We systematically compared the modeling results from Bayesian analysis between these two groups, and found the trends of flaring index as a function of spectral type also present in the substellar regime. The spectral type independence of the scale height is also seen between high-mass and very low-mass BDs. However, both the ETBD and LTBD groups feature a similar median disk mass of 10^{-5}Msun and no clear trend is visible in the distribution, probably due to the uncertainty in translating the far-IR photometry into disk mass, the detection bias and the age difference among the sample. Unlike previous studies, our analysis is completely homogeneous in Herschel/PACS data reduction and modeling with a statistically significant sample. Therefore, we present evidence of stellar-mass-dependent disk structure down to the substellar mass regime, which is important for planet formation models. (Abridged Version)