  We have carried out two sets of observations to quantify the properties of
SiO gas in the unusual HD 172555 debris disk: (1) a search for the J=8-7
rotational transition from the vibrational ground state, carried out with the
APEX sub-millimeter telescope and heterodyne receiver at 863 microns, and (2) a
search at 8.3 microns for the P(17) ro-vibrational transition of gas phase SiO,
carried out with VLT/VISIR with a resolution, $\lambda/\Delta\lambda$, of
30000. The APEX measurement resulted in a 3 $\sigma$ non-detection of an
interstellar feature, but only an upper limit to emission at the radial
velocity and linewidth expected from HD 172555. The VLT/VISIR result was also
an upper limit. These were used to provide limits for the abundance of gas
phase SiO, for a range of temperatures. The upper limit from our APEX
detection, assuming an 8000 K primary star photospheric excitation, falls more
than an order of magnitude below the self-shielding stability threshold derived
by Johnson et al. (2012). Our results thus favor a solid-state origin for the
8.3 micron feature seen in the Spitzer IRS spectrum of the circumstellar excess
emission, and the production of circumstellar O$^+$ and Si$^+$ by SiO UV
photolysis. The implications of these estimates are explored in the framework
of models of the HD 172555 circumstellar disk.
