Constraints on the outflow in S106IR from He I 2.058- m absorption-line and H I emission-line profiles

High spectral resolution observations of the massive young stellar object S106IR powering the bipolar H II region Sh-2 106 are presented and discussed. The new data, obtained at the United Kingdom Infrared Telescope using Cooled Grating Spectrometer No. 4, include the first detection of the He I $${2}^{1} \text S-2^{1}\text P 2.058-\mu \text m$$ line on- source and show it to be a blueshifted absorption feature, indicative of out-flow beginning close to the surface of the young star. Weak emission in the He I $${4}^{3,1} \text D-{5}^{3,1}\text F 4.038-,4.041-\mu \text m$$ lines has also been detected and is used to remove the contaminating He I $${4}^{3,1} \text F-{5}^{3,1}\text G 4.049-\mu \text m$$ line from the wing of Br α. Low-contrast, high-velocity wings are seen in each of the H I emission lines Br α , Pfγ and Brγ . Their relative velocity widths, including asymmetries apparent in Br α and Pfγ, are consistent with formation in an optically thick, accelerating stellar wind. From the maximum blueshift observed in Br α, lower limits of $${\upsilon}_{\infty} \gt{} 340 \text {km}\enspace \text s^{-1}$$ and $$\dot{M} \gt{} 2.7 \times {10}^{-6} \text M_{\odot} \text {yr}^{-1}$$ are derived. It is proposed that the narrower core component in the H I emission lines is produced by a different body of gas that is optically thin to infrared line emission, but optically thick to continuum emission at radio wavelengths. The two regions of IR line formation can be tied together in a model wherein the high-Ṁ transonic wind is decelerated by extensive mass-loading far beyond the volume in which it is accelerated: on a still larger scale, this out-flow presumably merges with the spatially resolved bipolar flow. To reconcile the simultaneous observation of He I 2.058-µm absorption directly toward S106IR and emission in the same line off-source, significant equatorial enhancement of the mass-loss rate is demanded.