1 Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
2 Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
3 Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4 Universität Heidelberg, Zentrum für Astronomie, Inst. für Theor. Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
5 Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, 5030 Casilla, Valparaíso, Chile
Received: 22 March 2015
Accepted: 13 July 2015
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. The broadband SEDs were compiled by adding previous photometry at shorter wavelengths and (sub-)millimeter data. We performed detailed SED analysis for the 46 targets that show infrared (IR) excess emission using radiative transfer models and evaluated the constraints on the disk parameters through Bayesian inference. 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 1 × 10-5 M⊙ 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.
Key words: stars: low-mass / circumstellar matter / brown dwarfs / protoplanetary disks
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2015