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Ro-vibrational CO Detected in the $β$~Pictoris Circumstellar Disk

We present high resolution near-infrared spectra of $β$ Pictoris - a nearby young star with a debris disk. Fundamental low-J CO absorption lines are detected and strict upper limits are placed on the flux of v=2-1 low-J CO emission lines. The limit on the UV fluorescent emission flux in the v=2-1 lines is used to place a tight constraint on the inner extent of the CO gas. Assuming \ion{H}{1} is the primary collision partner, the subthermal population of the low-J v=0 rotational levels constrains the density of the gas in the disk to $n_{\rm H} = (2.5^{+7.1}_{-1.2})\times10^5$ cm$^{-3}$. If the distribution of hydrogen follows that of the other metals in the disk, we find that the mass of the gas in the disk is $(0.17^{+0.47}_{-0.08})$ M$_\earth$. We compare this mass to the gas mass necessary to brake the metals in the disk through ion-neutral reactions.