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With a uniform VLT SINFONI data set of nine targets, we have developed anempirical grid of J,H,K spectra of the atmospheres of objects estimated to havevery low substellar masses of \sim5-20 MJup and young ages of \sim1-50 Myr.Most of the targets are companions, objects which are especially valuable forcomparison with atmosphere and evolutionary models, as they present rare casesin which the age is accurately known from the primary. Based on the sampleyouth, all objects are expected to have low surface gravity, and this studyinvestigates the critical early phases of the evolution of substellar objects.The spectra are compared with grids of five different theoretical atmospheremodels. This analysis represents the first systematic model comparison withinfrared spectra of young brown dwarfs. The fits to the full JHK spectra ofeach object result in a range of best fit effective temperatures of +/-150-300Kwhether or not the full model grid or a subset restricted to lower log(g)values is used. This effective temperature range is significantly larger thanthe uncertainty typically assigned when using a single model grid. Fits to asingle wavelength band can vary by up to 1000K using the different models.Since the overall shape of these spectra is governed more by the temperaturethan surface gravity, unconstrained model fits did not find matches with lowsurface gravity or a trend in log(g) with age. This suggests that empiricalcomparison with spectra of unambiguously young objects targets (such as theseSINFONI data) may be the most reliable method to search for indications of lowsurface gravity and youth. For two targets, the SINFONI data are a second epochand the data show no variations in morphology over time. The analysis of twoother targets, AB Pic B and CT Cha B, suggests that these objects may havelower temperatures, and consequently lower masses, than previously estimated.

Spectroscopy across the brown dwarf/planetary mass boundary - I. Near-infrared JHK spectra