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

TL;DR

This study tests whether PAHs in protoplanetary disks are frozen on icy pebbles and only detectable in the gas phase when these pebbles drift into warm dust traps. By comparing 3.3 μm PAH intensities with warm dust masses derived from ALMA continuum data inside the CO snowline across 26 disks, a strong correlation is found: $r=0.88\pm0.07$, supporting the scenario that PAHs sublimate into the gas phase when icy pebbles are transported to warmer disk layers. Weaker, sample-limited correlations are found with the FUV field and total dust mass, and longer-wavelength PAH features show similar but less robust trends. Spatially resolved PAH emission generally lies within or near dust traps, consistent with vertical transport and sublimation in warm regions, implying that pebble transport governs disk chemistry and may influence planet formation pathways.

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are commonly detected in protoplanetary disks, but it is unclear what causes the wide range of intensities across the samples. In this work, the measured PAH intensities of a range of disks are 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. A sample is constructed from 26 T Tauri and Herbig disks located within 300 pc, with constraints on the 3.3 $μ$m PAH intensity and with high-resolution ALMA continuum data. The midplane temperature is derived using a power-law or with radiative transfer modeling. The warm dust mass is computed by integrating the flux within the 30 K radius and convert to a dust mass. A strong correlation with a Pearson coefficient of 0.88+/-0.07 between the 3.3 micron 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 like FUV radiation field or total dust mass are much weaker. Correlations with PAH features at 6.2, 8.6 and 11.3 micron are potentially weaker, but this could be explained by the smaller sample for which these data were available. 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 towards warm dust traps inside the 30 K radius and vertically transported upwards 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.

Figures (9)

• Figure 1: Graphic of the proposed scenario of sublimation in a disk with a warm dust trap, viewed from the side, based on vanderMarel2021-irs48. If the bulk of the dust content is located in a dust trap inside the CO snowline at 30 K, this implies that fragments of its icy pebbles can be transported vertically to the higher disk layers above the H$_2$O snow surface at 150 K, and sublimate their icy content into the gas-phase, where it becomes observable. If the bulk of the pebbles is located outside the CO snowline, its icy layers may remain frozen out and its contents are not revealed in the gas phase. Such a scenario may also apply to PAHs, which is the hypothesis of this work.
• Figure 2: Azimuthally averaged profiles of the ALMA continuum images (see Figure \ref{['fig:gallery']}) of the targets in this study. The profiles are normalized to the peak and shown in logarithmic scale. The beam size is indicated with a horizontal blue bar in the top right of each profile. The pink shaded region shows the CO snowline in each disk, as defined by the 22-30 K temperature regime, based on the derived temperature profile (see text). For IRS48, the snowline is beyond the shown radial range. The part of the profile to the left of the pink region is considered the 'warm dust region' where at least part of the ice is sublimated, potentially releasing PAHs.
• Figure 3: The 3.3 $\mu$m PAH intensity as function of various stellar and disk parameters: the parameter $\chi$ (the FUV radiation field at 150 au from the star), the FUV radiation field at the inner disk edge $R_{in}$ and at the disk outer edge $R_{size}$. The fourth plot shows the 3.3 $\mu$m PAH intensity as function of the total disk dust mass. Upper limits are indicated as empty circles with arrows. The Pearson correlation coefficient is indicated in the corner.
• Figure 4: The 3.3 $\mu$m PAH intensity as function of the warm dust mass, computed from the millimeter flux inside the 30 K radius (CO snowline). The different colors represent the transition disks (red, TDs), compact disks (navy, CDs) and ring disks (purple, RDs). Upper limits are indicated as empty circles with arrows. The dashed line shows the best linear fit for the data points with the grey lines showing the spread between the fits. The fit has a Pearson coefficient of $r=0.88\pm0.07$ and the data are strongly correlated.
• Figure 5: PAH intensity as function of warm dust mass for the long wavelength features at 6.2, 8.6 and 11.3 $\mu$m, as well as the 3.3 $\mu$m for the subsample for which the other PAH features were available (Table \ref{['tab:pahacke']}).