  Direct detection of gravitational waves is opening a new window onto our
universe. Here, we study the sensitivity to continuous-wave strain fields of a
kg-scale optomechanical system formed by the acoustic motion of superfluid
helium-4 parametrically coupled to a superconducting microwave cavity. This
narrowband detection scheme can operate at very high $Q$-factors, while the
resonant frequency is tunable through pressurization of the helium in the
0.1-1.5 kHz range. The detector can therefore be tuned to a variety of
astrophysical sources and can remain sensitive to a particular source over a
long period of time. For reasonable experimental parameters, we find that
strain fields on the order of $h\sim 10^{-23} /\sqrt{\rm Hz}$ are detectable.
We show that the proposed system can significantly improve the limits on
gravitational wave strain from nearby pulsars within a few months of
integration time.
