Abstract
We present a JWST MIRI/MRS spectrum of the inner disk of WISE J044634.16 - 262756.1B (hereafter J0446B), an old ( ∼ 34 Myr) M4.5 star but with hints of ongoing accretion. The spectrum is molecule-rich and dominated by hydrocarbons. We detect 14 molecular species (H ₂ , CH ₃ , CH ₄ , C ₂ H ₂ ,¹³ CCH ₂ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₄ , C ₄ H ₂ , C ₆ H ₆ , HCN, HC ₃ N, CO ₂and¹³ CO ₂ ) and 2 atomic lines ([Ne II] and [Ar II]), all observed for the first time in a disk at this age. The detection of spatially unresolved H ₂and Ne gas strongly supports that J0446B hosts a long-lived primordial disk, rather than a debris disk. The marginal H ₂ O detection and the high C ₂ H ₂ /CO ₂column density ratio indicate that the inner disk of J0446B has a very carbon-rich chemistry, with a gas-phase C/O ratio≳ 2, consistent with what have been found in most primordial disks around similarly low-mass stars. In the absence of significant outer disk dust substructures, inner disks are expected to first become water-rich due to the rapid inward drift of icy pebbles, and evolve into carbon-rich as outer disk gas flows inward on longer timescales. The faint millimeter emission in such low-mass star disks implies that they may have depleted their outer icy pebble reservoir early and already passed the water-rich phase. Models with pebble drift and volatile transport suggest that maintaining a carbon-rich chemistry for tens of Myr likely requires a slowly evolving disk withα- viscosity≲10⁻⁴ . This study represents the first detailed characterization of disk gas at∼ 30 Myr, strongly motivating further studies into the final stages of disk evolution.