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We have carried out two sets of observations to quantify the properties ofSiO gas in the unusual HD 172555 debris disk: (1) a search for the J=8-7rotational transition from the vibrational ground state, carried out with theAPEX sub-millimeter telescope and heterodyne receiver at 863 microns, and (2) asearch at 8.3 microns for the P(17) ro-vibrational transition of gas phase SiO,carried out with VLT/VISIR with a resolution, $\lambda/\Delta\lambda$, of30000. The APEX measurement resulted in a 3 $\sigma$ non-detection of aninterstellar feature, but only an upper limit to emission at the radialvelocity and linewidth expected from HD 172555. The VLT/VISIR result was alsoan upper limit. These were used to provide limits for the abundance of gasphase SiO, for a range of temperatures. The upper limit from our APEXdetection, assuming an 8000 K primary star photospheric excitation, falls morethan an order of magnitude below the self-shielding stability threshold derivedby Johnson et al. (2012). Our results thus favor a solid-state origin for the8.3 micron feature seen in the Spitzer IRS spectrum of the circumstellar excessemission, and the production of circumstellar O$^+$ and Si$^+$ by SiO UVphotolysis. The implications of these estimates are explored in the frameworkof models of the HD 172555 circumstellar disk.

Constraints on the presence of SiO gas in the debris disk of HD 172555