Polycyclic aromatic hydrocarbon (PAH) abundances in the disk around T Chamaeleontis (T Cha): PAH sizes, ionization fraction, and mass during JWST observations

Abstract

The T Tauri star T Cha is known to have a protoplanetary disk with a dust gap separating the inner and outer disk regions. The mid-infrared JWST spectrum of T Cha shows multiple prominent aromatic infrared bands (AIBs) around 6.2, 8.1, and 11.3 $\mu$m. AIBs are commonly accepted as the emission stemming from PAH molecules. We aim to characterize the PAHs giving rise to the AIBs observed in the JWST spectrum of T Cha. Our objective is to estimate the PAH abundances, in terms of their sizes, ionization fraction, and mass, in the disk of T Cha. We perform spectral fitting of the observed AIBs to identify the possible underlying PAH emission components. We transfer the stellar radiation through a parametric disk model in order to reproduce the mid-IR spectrum, optical photometric fluxes, and mm continuum band fluxes of T Cha. We include PAH dust grains, which are stochastically heated, in our model calculations to simulate the AIBs. Thus, we estimate the PAH abundances from our modelling. We use the results from previous observations and modelling efforts to reduce our model degeneracies. The overall disk morphology - an inner and an outer disk separated by a dust gap - derived in this work is consistent with the previous results from Spitzer, VLT, and ALMA observations. PAHs are located within the outer disk in our model. Given our best fiducial model, we estimate a population of small PAHs of <26 C atoms, with an ionized PAH fraction of $\sim0.15$. We also obtain a PAH-to-dust mass ratio of ~7$\times$10$^{-3}$, which amounts to ~17% of the PAH-to-dust mass ratio observed in the ISM. We predict the outer disk to have a frontal wall with smaller dust grains, sizes limited up to $\mu$m-order, in order to properly fit the continuum slope within 14-25 $\mu$m. We propose a possibility of sub-micron dust grains within the gap to justify an observed plateau around ~10 $\mu$m in the JWST spectrum.