Modelling the Spatial Distribution and Origin of CO Gas in Debris Disks
  The detection of gas in debris disks raises the question of whether this gas
is a remnant from the primordial protoplanetary phase, or released by the
collision of secondary bodies. In this paper we analyze ALMA observations at
1-1.5" resolution of three debris disks where the $^{12}$CO(2-1) rotational
line was detected: HD131835, HD138813, and HD156623. We apply the iterative
Lucy-Richardson deconvolution technique to the problem of circumstellar disks
to derive disk geometries and surface brightness distributions of the gas. The
derived disk parameters are used as input for thermochemical models to test
both primordial and cometary scenarios for the origin of the gas. We favor a
secondary origin for the gas in these disks and find that the CO gas masses
($\sim 3\times10^{-3}$ M$_{\oplus}$) require production rates ($\sim 5\times
10^{-7}$ M$_{\oplus}$~yr$^{-1}$) similar to those estimated for the bona-fide
gas rich debris disk $\beta$ Pic.
