Polycyclic aromatic hydrocarbon (PAH) abundances in the disk around T Chamaeleontis

PAH sizes, ionisation fraction, and mass from JWST observations

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
³ Data Observatory Foundation, Eliodoro Yáñez 2990, Providencia, Santiago, Chile

^★ Corresponding author: rbandyo@das.uchile.cl

Received: 22 May 2025
Accepted: 22 September 2025

Abstract

Context. The T Tauri star T Chamaeleontis (T Cha) is known to have a protoplanetary disk (PPD) with a dust gap separating the inner and outer disk regions. The mid-infrared (MIR) JWST spectrum of T Cha shows multiple prominent aromatic IR bands (AIBs) around 6.2, 8.1, and 11.3 μm. AIBs are commonly accepted as the emission stemming from polycyclic aromatic hydrocarbon (PAH) molecules. Although PAHs may be instrumental for disk processes involving gas heating, thermochemistry, and photoevaporation, their abundances in PPDs are not well understood thus far.

Aims. We aim to characterise 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, ionisation fraction, and mass, in the PPD of T Cha.

Methods. We primarily used the archival MIRI-MRS JWST spectrum of T Cha for this work. We performed a spectral fitting of the observed AIBs to identify the possible underlying PAH emission components. We transferred the stellar radiation through a parametric disk model to reproduce the observed MIR continuum and the AIB features, as well as the optical photometric and millimetre band fluxes of T Cha. We included PAH dust grains, which were stochastically heated in our model calculations to simulate the AIBs. Thus, we were able to estimate the PAH abundances from our modelling. We used the results from previous observations and modelling efforts to reduce our model degeneracies.

Results. We estimated the PAH abundances in T Cha, along with other important disk parameters, from our modelling. The overall disk morphology derived in this work, made up of an inner and an outer disk separated by a dust gap, 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 estimated a population of small PAHs of ≲26 C atoms, with an ionised PAH fraction of ~0.15. We also obtained a PAH-to-dust mass ratio of ~7 × 10⁻³, which amounts to ~17% of the PAH-to-dust mass ratio observed in the ISM. We predicted that the outer disk would have a frontal wall with smaller dust grains (of sizes limited up to μm-order) to properly fit the continuum slope within 14–25 μm. In our model, we propose the possibility of sub-micron dust grains within the gap, to justify an observed plateau around ~10 μm in the JWST spectrum.

Conclusions. While our models do exhibit degeneracies, we have been able to predict a population of smaller and mostly neutral PAHs in the disk of T Cha. We also suggest that the disk might be undergoing FUV photoevaporation based on the high PAH-to-dust mass ratio estimated from our modelling.