  The Brackett decrement in the Class I source HH100 IR has been observed and
analyzed to set constraints on the origin of the IR HI emission in this young
object. We have used both low resolution (R ~ 800) observations of the Brackett
lines from Brg, to Br24, and medium resolution (R~9000) spectra of the Brg,
Br12 and Br13 lines. The dereddened fluxes indicates that the lines remain
moderately thick up to high quantum numbers. Moreover, the profiles of the
three lines observed in medium resolution are all broad and nearly symmetric,
with a trend for the lines at high n-number to be narrower than the Brg, line.
With the assumption that the three lines have different optical depths and
consequently trace zones at different physical depths, we interprete the
observed profiles as evidence that the ionized gas velocity in the HI emitting
region is increasing as we move outwards, as expected in an accelerating wind
more than in an infalling gas. We have modelled the observed line ratios and
velocities with a simplified model for the HI excitation in an expanding
circumstellar gas. Such a comparison indicates that the observations are
consistent with the emission coming from a very compact region of 4-6 solar
radii, where the gas has been already accelerated to velocities of the order of
200 \kms, with an associated mass flow rate of the ionized component of the
order of 10^{-7} solar masses per year. This implies that the observed lines
should originate either from a stellar wind or from the inner part of a disk
wind, providing that the disk inner truncation radius is close to the stellar
surface.
