Questing for Minute Traces of Molecular Gas in Circumstellar Debris Disks: AU Microscopii and 49 Ceti Edition

<p>The presence of molecular gas in debris disks around main sequence stars provides an opportunity to study both stellar and planetary evolution. As molecular gas is not expected to survive beyond the pre-main sequence phase, it is not yet clear whether the gas is the remnant of a primordial protoplanetary disk or if it is created in second-generation via collisions from planetesimals, exocomets, or the icy mantles of dust grains. Here we present two related efforts to characterize the prevalence and properties of gas in debris disks. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we place the most sensitive upper limit to-date on CO(2-1) emission from an M star debris disk. We calculate a 3σ integrated flux value of 0:07 Jy km s^-1, corresponding to a maximum CO mass of 1:79 x 10^-7 M⊕, assuming the gas is in LTE. We also present the results of an ALMA search for HCN(4-3) around 49 Ceti. Despite hosting one of the brightest CO-rich debris disks yet discovered, our observations of 49 Ceti yield an HCN(4-3) 3 σ upper limit of 0:09 Jy km s^-1, corresponding to an HCN(4-3)/CO(3-2) line ux ratio of ≤ 0:015. We employ several methods of detecting faint line emission from debris disks, including a model based on Keplerian kinematics as well as a spectral-spatial shifting method previously used to detect faint CO emission from the Fomalhaut debris disk, and compare our results.</p>