substantially lower optical depth than the main isotopolgue, offers the best chance to reveal structure associated with the HR 8799 system. Figure \[fig:13CO\] shows a set of $^{13}$CO channel maps obtained with natural weighting and a $4\arcsec$ FWHM Gaussian taper, resulting in a $\sim6\farcs5$ beam. Among the positive and negative corrugations indicative of missing flux, the most significant emission is located away from the star, and there is no clear evidence for any systematic motions that might be associated with rotation of gas in the debris disk. The linewidth is consistent with the typical turbulent broadening found in small molecular clouds. The apparent association in space and velocity between HR 8799 and this extended molecular emission is intriguing but remains mysterious.

![image](fig4.eps)

\[fig:13CO\]

SMA/ALMA Disk Model Fits {#sec:model}
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To provide a quantitative characterization of the 1.3 millimeter emission from the HR 8799 disk, we make the simple assumption that the structure can be represented by an axisymmetric and geometrically thin belt, and we model the observations using the visibility fitting procedure described in @macgregor13. The surface brightness of the belt is assumed to be a radial power law, $I_\nu \propto r^x$, between an inner and outer radius, $R_\text{in}$ and 