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Massive young stellar objects (YSOs), like low-mass YSOs, are thought to besurrounded by optically thick envelopes and/or discs and are observed to haveassociated regions that produce polarized light at near-infrared wavelengths.These polarized regions are thought to be lower-density outflows along thepolar axes of the YSO envelopes. Using the 0.2 arcsec spatial resolution of theNear-Infrared Camera and Multi-Object Spectrometer on the Hubble SpaceTelescope we are examining the structure of the envelopes and outflow regionsof massive YSOs in star-forming regions within a few kpc of the Sun. Here wereport on 2 micron polarimetry of Mon R2-IRS3, S140-IRS1, and AFGL 2591.  All three sources contain YSOs with highly-polarized monopolar outflows, withMon R2-IRS3 containing at least two YSOs in a small cluster. The central starsof all four YSOs are also polarized, with position angles perpendicular to thedirections of the outflows. We infer that this polarization is due toscattering and absorption by aligned grains. We have modelled our observationsof S140-IRS1 and AFGL 2591 as light scattered and absorbed both by sphericalgrains and by elongated grains that are aligned by magnetic fields. Models thatbest reproduce the observations have a substantial toroidal component to themagnetic field in the equatorial plane. Moreover, the toroidal magnetic fieldin the model that best fits AFGL 2591 extends a large fraction of the height ofthe model cavity, which is 10^5 au. We conclude that the massive YSOs in thisstudy all show evidence of the presence of a substantial toroidal magneticfield.

Aligned grains and inferred toroidal magnetic fields in the envelopes of massive young stellar objects