With a uniform VLT SINFONI data set of nine targets , we have developed an empirical grid of J , H , K spectra of the atmospheres of objects estimated to have very low substellar masses of \sim 5-20 M _ { Jup } and young ages ranging from \sim 1-50 Myr .
Most of the targets are companions , objects which are especially valuable for comparison with atmosphere and evolutionary models , as they present rare cases in which the age is accurately known from the primary .
Based on the youth of the sample , all objects are expected to have low surface gravity , and this study investigates the critical early phases of the evolution of substellar objects .
The spectra are compared with grids of five different theoretical atmosphere models .
This analysis represents the first systematic model comparison with infrared spectra of young brown dwarfs .
The fits to the full JHK spectra of each object result in a range of best fit effective temperatures of \pm 150-300K whether or not the full model grid or a subset restricted to lower log ( g ) values is used .
This effective temperature range is significantly larger than the uncertainty typically assigned when using a single model grid .
Fits to a single wavelength band can vary by up to 1000K using the different model grids .
Since the overall shape of these spectra is governed more by the temperature than surface gravity , unconstrained model fits did not find matches with low surface gravity or a trend in log ( g ) with age .
This suggests that empirical comparison with spectra of unambiguously young objects targets ( such as those presented here ) may be the most reliable method to search for indications of low surface gravity and youth .
Based on comparison with previous observations , the SINFONI spectra represent a second epoch for the targets 2M0141 and DH Tau B , and the combined data show no variations in the spectral morphology over time .
The analysis of two other targets , AB Pic B and CT Cha B , suggests that these objects may have lower temperatures , and consequently lower masses , than previously estimated .