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

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 $μ$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 $μ$m-order, in order to properly fit the continuum slope within 14-25 $μ$m. We propose a possibility of sub-micron dust grains within the gap to justify an observed plateau around ~10 $μ$m in the JWST spectrum.

💡 Research Summary

This paper presents a comprehensive analysis of polycyclic aromatic hydrocarbon (PAH) abundances in the protoplanetary disk surrounding the T Tauri star T Chamaeleontis (T Cha), exploiting the unprecedented sensitivity and spectral resolution of the JWST Mid‑Infrared Instrument (MIRI) Medium‑Resolution Spectroscopy (MRS). The authors focus on three prominent aromatic infrared bands (AIBs) at 6.2 µm, a blended feature centered near 8.1 µm (encompassing the classic 7.7 µm and 8.6 µm bands), and 11.3 µm, which are clearly visible in the JWST spectrum but were only marginally detected in earlier Spitzer data.

The methodology proceeds in several stages. First, the JWST spectrum (obtained on 13 August 2022 as part of program 2260) is decomposed using multiple Drude profiles to capture the sub‑structure of each AIB. The fitting yields central wavelengths, fractional widths, and amplitudes for three to four sub‑components per band, providing a detailed empirical description of the PAH emission. Complementary data—optical photometry, near‑ and mid‑IR photometric points from 2MASS, WISE, IRAC, IRAS, Herschel, and (sub‑)mm fluxes from ALMA, SEST, and ATCA—are assembled to construct the full spectral energy distribution (SED) of the system.

Next, a parametric disk model is built that reproduces the SED, the JWST continuum, and the mm fluxes simultaneously. The model adopts the well‑established two‑zone geometry: an inner disk (≈ 0.1–10 au) and an outer disk (≈ 15–30 au) separated by a dust gap of roughly 10 au, consistent with previous Spitzer, VLT/AMBER, Herschel, SPHERE, and ALMA studies. The outer disk includes a “front wall” populated by sub‑micron to micron‑sized silicate and carbonaceous grains, which is required to match the observed 14–25 µm continuum slope. To account for a plateau around 10 µm in the JWST spectrum, the authors introduce a population of sub‑micron grains within the gap.

PAH physics is incorporated by adopting the optical constants from Draine & Li (2007) and treating the PAH population as stochastically heated grains. The size distribution is deliberately simplified to a single representative size of ≤ 26 carbon atoms (≈ 0.4–0.5 nm), reflecting a population of very small PAHs. The ionization fraction (ϕ_ion) is left as a free parameter; radiative transfer calculations propagate the stellar UV/FUV radiation through the disk surface, allowing PAHs to become partially ionized. By fitting the AIB strengths, the best‑fit model yields ϕ_ion ≈ 0.15, indicating that the majority of PAHs remain neutral. The PAH‑to‑dust mass ratio derived from the model is ≈ 7 × 10⁻³, which is about 17 % of the canonical interstellar medium (ISM) value (≈ 4 × 10⁻²). This reduced ratio suggests that a substantial fraction of PAHs have either been incorporated into larger grains, destroyed by FUV/X‑ray photons, or otherwise depleted during disk evolution.

The authors discuss the implications of these findings. The confinement of PAHs to the outer disk surface implies that PAH‑driven photoelectric heating can be efficient in the disk’s upper layers, potentially enhancing gas temperatures and influencing the chemistry of key molecules (e.g., CO, H₂O). Moreover, the relatively high PAH‑to‑dust ratio—though still below ISM levels—supports the notion that T Cha’s disk may be undergoing FUV‑driven photoevaporation, as PAHs can boost the photoevaporation efficiency by providing additional heating channels. The low ionization fraction aligns with expectations for a modest UV field typical of a G8V T Tauri star, yet the presence of a detectable PAH population demonstrates that even relatively cool pre‑main‑sequence stars can retain aromatic molecules in their disks.

The paper acknowledges remaining model degeneracies, particularly the interplay between PAH size, ionization state, and the amount of sub‑micron dust in the gap. Future high‑resolution JWST imaging, combined with ALMA gas tracers (e.g., CO, C II) and possibly JWST NIRSpec observations of PAH vibrational overtones, could break these degeneracies and map the spatial distribution of PAHs directly. Extending the analysis to include nitrogen‑bearing PAHs or aliphatic side‑groups would also refine the interpretation of the subtle shifts in AIB peak positions.

In summary, this study delivers the first quantitative assessment of PAH sizes, ionization fraction, and mass in the T Cha protoplanetary disk using JWST data. It finds a population of very small, predominantly neutral PAHs confined to the outer disk, with a PAH‑to‑dust mass ratio of ~7 × 10⁻³ (≈ 17 % of the ISM value). These results highlight the importance of PAHs for disk thermodynamics and suggest that PAH‑enhanced FUV photoevaporation may be active in this system, offering new insight into the chemical and physical evolution of transitional disks around T Tauri stars.