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Characterization of gas component in debris disks is of fundamentalimportance for understanding its origin. Toward this goal, we have conductednon-LTE (local thermodynamic equilibrium) analyses of the rotational spectrallines of CO including those of rare isotopologues ($^{13}$CO and C$^{18}$O)observed toward the gaseous debris disks of 49 Ceti and HD 21997 with theAtacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array(ACA). The analyses have been carried out for a wide range of the H$_{2}$density, and the observed line intensities are found to be reproduced, as faras the H$_{2}$ density is higher than 10$^{3}$ cm$^{-3}$. The CO column densityand the gas temperature are evaluated to be (1.8-5.9)$\times$10$^{17}$cm$^{-2}$ and 8-11 K for 49 Ceti and (2.6-15)$\times$10$^{17}$ cm$^{-2}$ and8-12 K for HD 21997, respectively, where the H$_{2}$ collision is assumed forthe rotational excitation of CO. The results do not change significantly evenif electron collision is considered. Thus, CO molecules can be excited underenvironments of no H$_{2}$ or a small number of H$_{2}$ molecules, even wherethe collision with CO, C, O, and C$^{+}$ would make an important contributionfor the CO excitation in addition to H$_{2}$. Meanwhile, our result does notrule out the case of abundant H$_{2}$ molecules. The low gas temperatureobserved in the debris disks is discussed in terms of inefficient heating byinterstellar and stellar UV radiation.

Physical conditions of gas components in debris disks of 49 Ceti and HD 21997