Kavli Affiliate: Gregory J. Herczeg
| First 5 Authors: Feng Long, Ilaria Pascucci, Adrien Houge, Andrea Banzatti, Klaus M. Pontoppidan
| Summary:
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 $gtrsim$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 $alpha-$viscosity $lesssim10^{-4}$. 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.
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