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

Abstract

We present high resolution near-infrared spectra of $\beta$ 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 $n_{\mathrm{H}}=(2.5_{-1.2}^{+7.1})\times 10^5$ 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 $(0.17_{-0.08}^{+0.47})$ M$_\earth$. We compare this mass to the gas mass necessary to brake the metals in the disk through ion-neutral reactions.

Accepted to ApJ