When the Wall Fell: Study of Polycyclic Aromatic Hydrocarbons in T Chamaeleontis using JWST

TL;DR

This study uses JWST and Spitzer observations to analyze PAH emissions in T Chamaeleontis, revealing how disk wall destruction influences PAH features and providing insights into disk evolution and PAH processing in young stellar objects.

Contribution

First detection of PAH features in T Cha's disk with JWST, showing how disk changes affect PAH emission and charge state over nearly two decades.

Findings

PAH emission increased after disk wall destruction in 2022.

PAH population remains largely neutral across multiple epochs.

PAH features exhibit a 'class C' spectral profile with low hydrogen mode abundance.

Abstract

We investigate the polycyclic aromatic hydrocarbon (PAH) emission features of T Cha, a G8-type T Tauri star that has exhibited "seesaw"-type mid-infrared continuum variability over nearly two decades due to the destruction of the disk's inner wall, using JWST/MIRI and Spitzer observations. We report the first detection of weak PAH emission at 6.2, 7.7, and 8.6 microns in the Spitzer/IRS spectrum from 2005. The inner wall destruction in the 2022 JWST epoch allowed more ultraviolet photons to reach the outer disk, increasing the flux levels of PAH bands and enabling their detection well above the continuum. The 11.2 micron PAH flux increases by a factor of three, yet its profile shape remains remarkably stable. The 6.2/11.2 micron flux ratio has increased, but the charge state of the PAH population remains 75% neutral. The PAH features exhibit a "class C" spectral profile, with redshifted peaks and broadened wings consistent with emission from low-mass T Tauri disks. A weak 12.7/11.2 micron ratio points to a lower abundance of duo- and trio-hydrogen modes, implying a predominantly zigzag carbon structure. A faint "class A" sub-component in the 6.2 and 7.7 micron bands may indicate additional PAH processing by ultraviolet radiation from accretion hotspots. Placement on PAH charge-size grids locates T Cha in the low-ionisation, small-size regime (NC <= 30), signifying a largely neutral PAH population across multiple epochs spanning 18 years. Through multi-epoch, high-resolution data from JWST and Spitzer, we identify T Cha as a benchmark source for probing disk evolution and PAH processing, emphasizing the potential of temporal monitoring with JWST.