  Mass accretion rate determinations are fundamental for an understanding of
the evolution of pre-main sequence star circumstellar disks. Magnetospheric
accretion models are used to derive values of the mass accretion rates in
objects of very different properties, from brown dwarfs to intermediate-mass
stars; we test the validity of these models in the brown dwarf regime, where
the stellar mass and luminosity, as well as the mass accretion rate, are much
lower than in T Tauri stars. We have measured simultaneously two infrared
hydrogen lines, Pab and Brg, in a sample of 16 objects in the star-forming
region rho-Oph. The sample inc ludes 7 very low mass objects and brown dwarfs
and 9 T Tauri stars. Brown dwarfs where both lines are detected have a ratio
Pab/Brg of ~2. Larger values, >=3.5, are only found among the T Tauri stars.
The low line ratios in brown dwarfs indicate that the lines cannot originate in
the column of gas accreting from the disk onto the star along the magnetic
field lines, and we suggest that they form instead in the shocked photosphere,
heated to temperatures of ~3500 K. If so, in analogy to veiling estimates in T
Tauri stars, the hydrogen infrared line fluxes may provide a reliable measure
of the accretion rate in brown dwarfs.
