Observational evidence for a possible link between PAH emission and dust trap locations in protoplanetary disks

Abstract

Context. Polycyclic aromatic hydrocarbons (PAHs) are commonly detected in protoplanetary disks, but it is unclear what causes the wide range of intensities across the samples. Aims. For this work, the measured PAH intensities of a range of disks were compared with ALMA dust continuum images in order to test whether there is evidence that PAHs are frozen out on pebbles in dust traps and only sublimate under certain conditions. Methods. A sample was constructed from 26 T Tauri and Herbig disks located within 300 pc, with constraints on the 3.3 μm PAH intensity and with high-resolution ALMA continuum data. The midplane temperature was derived using a power law or via radiative transfer modeling. The warm dust mass was computed by integrating the flux within the 30 K radius and converted to a dust mass. Results. A strong correlation with a Pearson coefficient of 0.88±0.07 between the 3.3 μm PAH intensity and the warm dust mass was found. The correlation is driven by the combination of deep upper limits and strong detections corresponding to a range of warm dust masses. Possible correlations with other disk properties, for example a far-UV radiation field or total dust mass, are much weaker. Correlations with PAH features at 6.2, 8.6, and 11.3 μm are potentially weaker, but this could be explained by the smaller sample for which these data were available. Conclusions. The correlation is consistent with the hypothesis that PAHs are generally frozen out on pebbles in disks, and are only revealed in the gas phase if those pebbles have drifted toward warm dust traps inside the 30 K radius and vertically transported upward to the disk atmosphere with sufficiently high temperature to sublimate PAHs into the gas phase. This is similar to previous findings on complex organic molecules in protoplanetary disks, and provides further evidence that the chemical composition of the disk is governed by pebble transport.