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

TL;DR

This study used sub-millimeter and mid-infrared observations to set upper limits on SiO gas in the HD 172555 debris disk, suggesting a solid-state origin for observed features and implications for disk chemistry.

Contribution

First to provide observational upper limits on gas phase SiO in HD 172555's debris disk using APEX and VLT/VISIR, informing disk composition models.

Findings

Upper limits on SiO gas abundance were established.

Results favor a solid-state origin for observed spectral features.

Implications for UV photolysis and disk chemistry were discussed.

Abstract

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.