2004A&A...421..187N

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 Brγ to Br24, and medium resolution (R∼9000) spectra of the Brγ, 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 Brγ 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 from a circumstellar gas with a velocity law V=V₀+(V_max-V₀)(1-(r_i/r)^α). Such a comparison indicates that the observations are consistent with the emission coming from a very compact region of 4-6R_☉, where the gas has been already accelerated to velocities of the order of 200km/s, with an associated mass flow rate of the ionized component of the order of 10^–7M_☉/yr. 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. It is also expected that the gas ionization fraction is relatively high as testified by the high rate of ionized mass loss derived. Our analysis, however, does not resolve the problem of how to reproduce the observed symmetrical line profiles, which at present are apparently difficult to model by both wind and accretion models. This probably points to the fact that the real situation is more complicated than described in the simple model presented here.