paper

The First JWST View of a 30-Myr-old Protoplanetary Disk Reveals a Late-stage Carbon-rich Phase

arXiv:2412.05535

Abstract

We present a JWST MIRI/MRS spectrum of the inner disk of WISE J044634.16 $-$ 262756.1B (hereafter J0446B), an old ( $\sim$ 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 $_{2}$ and $^{13}$ 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 $_{2}$ and 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 $_{2}$ column 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 with $I^\pm -$ viscosity $≲ 1 0^{- 4}$ . This study represents the first detailed characterization of disk gas at $\sim$ 30 Myr, strongly motivating further studies into the final stages of disk evolution.

Accepted by ApJL. 8 figures in the main text