Physical Conditions of Gas Components in Debris Disks of 49 Ceti and HD 21997

Abstract

Abstract 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) × 1017 cm−2 and 8–11 K for 49 Ceti and (2.6–15) × 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.