ON THE ORIGIN OF THE HI LINE EMISSION ASSOCIATED WITH MASSIVE YOUNG STELLAR OBJECTS

We address the problem of the interpretation of the H I line emission associated with massive young stellar objects by comparing results of LTE calculations of H I Br-alpha and Pf-gamma profiles with observations of the exciting source of Sh2-106 obtained by Garden & Geballe. The theoretical results are derived by formal solution of the transfer equation as applied to a spherically symmetric stellar wind. Free-free and bound-free opacity are taken into account. It is found that the observed Br-alpha profile and flux can be reproduced by LTE stellar wind models that also satisfy the radio flux constraint upon M/upsilon(infinity) obtained by Felli et al. The best-fitting models are characterized by terminal velocities of around 100 km s-1 and relatively gradual velocity laws similar to the one derived for P Cygni. However the observed F(Pf-gamma)/F(Br-alpha) flux ratio cannot be reproduced. The effect of departures from LTE upon the line fluxes and flux ratio are discussed, and it is proposed that the observed asymmetry in the Pf-gamma profile is due to the combined effect of free-free opacity and a line source function that exceeds its LTE value. The failure to reproduce the observed F(Pf-gamma)/F(Br-alpha) ratio implies the simple spherically symmetric wind model is in difficulties and that it may have to be replaced by one locating the IR line emission in an outflow further removed from the stellar source.