When the wall fell: Study of polycyclic aromatic hydrocarbons in T chamaeleontis using JWST

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/Mid-Infrared Instrument and Spitzer observations. We report the first detection of weak PAH emission at 6.2, 7.7, and 8.6 μm in the Spitzer/Infrared Spectrograph spectrum from 2005. The destruction of the inner wall in the 2022 JWST epoch allowed more ultraviolet photons to reach the outer disk, increasing the flux levels of PAH bands and allowing their detection well above the continuum. The 11.2 μm PAH flux increases by a factor of 3, yet its profile shape remains remarkably stable, and the 6.2/11.2 μm 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 lowmass T Tauri disks, while the weak 12.7/11.2 ratio points to a lower abundance of duo and trio hydrogen modes, implying a predominantly zigzag carbon structure. A faint “class A” subcomponent in the 6.2 and 7.7 μm bands may indicate additional PAH processing by ultraviolet radiation from accretion hotspots. Placement on PAH charge–size grids locates T Cha in the low-ionization, small-size regime (NC ≤ 30), signifying a largely neutral PAH population in multiple epochs spanning 18 yr. Through multiepoch, 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