  Establishing the spatial extents and the nature of the outer stellar
populations of dwarf galaxies are necessary ingredients for the determination
of their total masses, current dynamical states and past evolution. We here
describe our investigation of the outer stellar content of the Bo\"otes I
ultra-faint dwarf galaxy, a satellite of the Milky Way Galaxy. We identify
candidate member blue horizontal branch and blue straggler stars of Bo\"otes I,
both tracers of the underlying ancient stellar population, using a combination
of multi-band Pan-STARRS photometry and Gaia astrometry. We find a total of
twenty-four candidate blue horizontal branch member stars with apparent
magnitudes and proper motions consistent with membership of Bo\"otes I, nine of
which reside at projected distances beyond the nominal King profile tidal
radius derived from earlier fits to photometry. We also identify four blue
straggler stars of appropriate apparent magnitude to be at the distance of
Bo\"otes I, but all four are too faint to have high-quality astrometry from
Gaia. The outer blue horizontal branch stars that we have identified confirm
that the spatial distribution of the stellar population of Bo\"otes I is quite
extended. The morphology on the sky of these outer envelope candidate member
stars is evocative of tidal interactions, a possibility that we explore further
with simple dynamical models.
