Observations of the Brackett decrement in the Class I source HH100 IR
  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. 
Brunella Nisini, Simone Antoniucci, Teresa Giannini