The Search for Volatile Carbon Transport in Protoplanetary Disks

Understanding the chemistry of protoplanetary disks is essential to understanding planetary formation and composition. Many protoplanetary disks are depleted in CO compared to the interstellar medium from which they form. A possible explanation of this phenomena is CO gas condensing onto grains as CO₂ ice in the cold outer disk. As these grains move into the hotter inner disk via radial drift, the CO₂ ice would sublimate into gas. This should result in an anti-correlation between CO emission in the outer disk and CO₂ emission in the inner disk. To test this hypothesis we reimage archival Atacama Large Millimeter/Submillimeter Array observations of ¹³CO and C¹⁸O 3-2 emission towards several protoplanetary disks and compare the resulting fluxes to archival CO₂ data from Spitzer. Only the relationship between ¹³CO and CO₂ is analyzed, as C¹⁸O emission was detected towards too few sources to create a sizable C¹⁸O data set. When first analyzing the relationship between ¹³CO and CO₂, no significant trend is discovered. However, when disks around M-type stars were removed from analysis, a positive (albeit somewhat weak) correlation is observed between ¹³CO and CO₂ fluxes. The lack of an inverse trend contradicts the possibility of CO being converted to CO₂. The positive trend may simply be a result of disks with higher carbon content having higher abundances of multiple carbon based species. More observations are needed to confirm this trend.