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

TL;DR

The paper uses JWST/MIRI-MRS spectra of the T Cha protoplanetary disk to extract PAH properties by decomposing the aromatic infrared bands and performing self-consistent radiative-transfer modelling with a two-disk structure. PAHs are modeled as stochastically heated grains in the outer disk, with opacities customized to reproduce the observed class C AIBs; the outer disk geometry (wall and outer region) is constrained to fit the 14–25 μm continuum and mm fluxes. The analysis finds a population of small PAHs with $N_C \,\simeq\,10$–$26$ and an ionization fraction $\phi \simeq 0.15$, giving $M_ ext{PAH}/M_ ext{dust} \,=\,(6.9\pm0.3)\times10^{-3}$ (≈$17\%$ of the ISM value); about 85% of these PAHs are neutral. The outer disk wall requires sub-micron grains ($0.01-4$ μm) to reproduce the mid-IR continuum, and a sub-micron dust population in the gap helps explain a ~10 μm plateau, suggesting possible dust replenishment and active FUV photoevaporation in the system.

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 $μ$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.

Figures (18)

• Figure 1: A collection of photometric fluxes and spectra of T Cha are shown. The blue vertical lines represent the variability in optical fluxes Alcala+1993. IR photometric fluxes are from 2MASS, WISE, IRAC, IRAS, and Herschel. The millimetre band fluxes correspond to ALMA at 0.85 and 3 mm, SEST at 1.3 mm, and ATCA at 3.3 mm. The mid-IR spectra from Spitzer (grey) and JWST (orange) shows a varying continuum. Inset figure: The AIBs at 6.2, 8.1, and 11.3 $\mu$m, prominently appearing in the JWST spectrum, are marked. The two strong lines in the JWST spectrum correspond to the fine-structure [Ar ii] 6.98 $\mu$m and [Ne ii] 12.81 $\mu$m transitions.
• Figure 2: Spectral decomposition of the 6.2 (top), 8.1 (middle), and 11.3 (bottom) $\mu$m AIBs observed in the JWST spectrum of T Cha. The band profiles are fitted with Drude profile components (blue solid lines). The sum of the underlying stellar and dust continuum is shown in red solid line. The black dashed line shows the total fit to the AIB profiles. See more details in Sec. \ref{['sec:pahprofiles']} and Appendix \ref{['app:nautilus']}.
• Figure 3: Model spectrum (black line), generated in varying resolution, is overplotted with the observed mid-IR JWST spectrum (orange line), optical photometric fluxes (blue vertical lines) and millimetre band fluxes (red circles). The mid-IR Spitzer spectrum (grey line) is shown as well for comparison. Inset plot: Fitting of the mid-IR continuum, along with the observed AIBs at 6.2, 8.1, and 11.3 $\mu$m, and the model generated continuum with the respective PAH bands, is shown in more detail.
• Figure 4: Radial profiles of the dust surface density ($\Sigma_\mathrm{d}$) and the aspect ratio (scale height to radius ratio, $H(r)/r$) of the modelled disk is shown in orange and green solid lines, respectively.
• Figure 5: Comparison of the T Cha disk model computed in this paper that includes small dust of $0.01-1$$\mu$m within the gap region and the same model without any dust included in the gap.