2020ApJ...905..122H

Characterization of the gas component in debris disks is of fundamental importance for understanding their origin. To address this goal, we have conducted non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral lines of CO including those of rare isotopologues (¹³CO and C¹⁸O) that have been observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Compact Array (ACA). The analyses have been carried out for a wide range of H₂ density, and the observed line intensities are found to be reproduced as long as the H₂ density is higher than 10³ cm^–3. The CO column density and the gas temperature are evaluated to be (1.8-5.9) x 10¹⁷ cm^–2 and 8-11 K for 49 Ceti and (2.6-15) x 10¹⁷ cm^–2 and 8-12 K for HD 21997, respectively, where the H₂ collision is assumed for the rotational excitation of CO. The results do not change significantly even if electron collision is considered. Thus, CO molecules can be excited under environments containing no H₂ or a small number of H₂ molecules, even where collisions with CO, C, O, and C⁺ would make an important contribution to the CO excitation in addition to H₂. Meanwhile, our result does not rule out the case of abundant H₂ molecules. The low gas temperature observed in the debris disks is discussed in terms of inefficient heating by interstellar and stellar UV radiation.