exoALMA. XXIV. Formaldehyde Emission in Protoplanetary Disks of exoALMA Compared with Their Properties and Dynamical State

Abstract

The presence of asymmetries and substructures in protoplanetary disks, revealed by both dust and gas emission, highlights the potential interplay and the broader connection between chemistry and dynamics in disk evolution. We explore multiple relationships using the nonparametric Kendall-$τ$ correlation to examine formaldehyde (H$_2$CO) emission with relation to stellar and disk properties for a subset of disks from the exoALMA sample. We also retrieve the H$_2$CO column density and excitation temperature using four transitions, measured in radial bins of 100 au, and quantify the level of asymmetry in the resolved peak intensity of the H$_2$CO emission. From our correlation analysis, we find no correlations with sufficient statistical significance. However, we identify tentative relationships that can be tested with larger samples. In particular, we report a proposed correlation ($2.1σ$) between stellar effective temperature and the formaldehyde excitation conditions, suggesting that, to first order, the central star dominates the nature of the H$_2$CO emission over possible dynamical asymmetries traced by dust. Although a correlation with the stellar luminosity was also expected, a larger sample is required to confirm or refute this trend. A possible correlation with spectral type, together with the broad range of H$_2$CO excitation temperatures within the inner 100 au of the studied disks, hint at possible multiple chemical formation pathways for H$_2$CO, including both gas-phase reactions and ice-surface chemistry on dust grains.

Figures (6)

• Figure 1: Left: integrated emission maps of the H$_2$CO 4(0,4)-3(0,3) line for the observed sources. Right: peak intensity emission for the same line. Continuum emission at 0.8 mm contours is overlaid to compare with the formaldehyde emission in the disks. Beams are illustrated on the bottom-right with solid white for continuum images and dashed lines for line emission. The bottom-left bar represents a respective 100 au physical scale for each source. For most disks, there are no clear correspondences between the dust continuum contours and the integrated maps.
• Figure 2: Rotational diagrams of the 11 targeted disks for the inner 100 au (left) and 100-200 au region (right). Blue triangles represent upper limits for undetected transitions, and the dotted lines show the best MCMC fit. In AA Tau, HD 34282, MWC 758, and RX J1615 it is possible that there is more than one emitting component.
• Figure 3: Top: comparison between formaldehyde column density in the two 100 au radial bins, with the dashed line indicating the identity. Bottom: excitation temperature of H$_2$CO between the inner 100 au and the successive 100-200 au radial bin. No significant difference between the two temperatures can be stated given the uncertainties due to the lower SNR of the emission in the outer 100 au.
• Figure 4: Top: weighted Kendall-$\tau$ correlation values between compared parameters/metrics, with redder more positively correlated and bluer more anticorrelated pairs. For correlation purposes, the 1.3 mm continuum flux and the line emission flux were normalized to a 100 pc distance for every disk. Bottom:$Z$-scores of the compared parameters and metrics representing the level of statistical significance of the weighted Kendall-$\tau$ correlation values.
• Figure 5: Subset of correlations explored with $z$-scores $\geq$2.00 between the NAI$_\mathrm{H_2CO}$, the total H$_2$CO$_{\mathrm{4(0,4)-3(0,3)}}$ line flux, and the retrieved excitation temperature of formaldehyde in the inner 100 au of the observed disks.