  We present measurements of silica optomechanical resonators, known as bottle
resonators, passively cooled in a cryogenic environment. These devices possess
a suite of properties that make them advantageous for preparation and
measurement in the mechanical ground state, including high mechanical
frequency, high optical and mechanical quality factors, and optomechanical
sideband resolution. Performing thermometry of the mechanical motion, we find
that the optical and mechanical modes demonstrate quantitatively similar
laser-induced heating, limiting the lowest average phonon occupation observed
to just ~1500. Thermalization to the 9 mK thermal bath would facilitate quantum
measurements on these promising nanogram-scale mechanical resonators.
