  The Galactic blue supergiant SBW1 with its circumstellar ring nebula
represents the best known analog of the progenitor of SN 1987A. High-resolution
imaging has shown H-alpha and IR structures arising in an ionized flow that
partly fills the ring's interior. To constrain the influence of the stellar
wind on this structure, we obtained an ultraviolet (UV) spectrum of the central
star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum
shows none of the typical wind signatures, indicating a very low mass-loss
rate. Radiative transfer models suggest an extremely low rate below 10$^{-10}$
Msun/yr, although we find that cooling timescales probably become comparable to
or longer than the flow time below 10$^{-8}$ Msun/yr. We therefore adopt this
latter value as a conservative upper limit. For the central star, the model
yields $T_{\rm eff}$=21,000$\pm$1000 K, $L\simeq$5$\times$10$^4$ $L_{\odot}$,
and roughly Solar composition except for enhanced N abundance. SBW1's very low
mass-loss rate may hinder the wind's ability to shape the surrounding nebula.
The very low mass-loss rate also impairs the wind's ability to shed angular
momentum; the spin-down timescale for magnetic breaking is more than 500 times
longer than the age of the ring. This, combined with the star's slow rotation
rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at
least 10 times lower than expected from mass-loss recipes, without any account
of clumping. The physical explanation for why SBW1's wind is so weak presents
an interesting mystery.
