JWST/NIRSpec Detects Warm CO Emission in the Terrestrial-Planet Zone of HD 131488

TL;DR

This paper reports the first detection of warm CO gas in a debris disk around HD 131488 using JWST, revealing UV fluorescence excitation and suggesting the presence of other molecules in the inner disk region.

Contribution

The study presents the first observation of warm, fluorescent CO gas in a debris disk, demonstrating JWST's capability to detect tenuous gas at very low masses.

Findings

Warm CO gas detected within 10 AU of HD 131488.

CO is excited by UV fluorescence, not thermal equilibrium.

Warm CO mass estimated at 1.25×10⁻⁷ Earth masses.

Abstract

We have obtained a high-resolution, JWST NIRSpec $2.87$ -- $5.14$ $\mu$m spectrum of the debris disk around HD 131488. We discover CO fundamental emission indicating the presence of warm fluorescent gas within $\sim10$ AU of the star. The large discrepancy in CO's vibrational and rotational temperature indicates that CO is out of thermal equilibrium and is excited with UV fluorescence. Our UV fluorescence model gives a best fit of $1150\,$K with an effective temperature of $450$, $332$, and $125\,$K for the warm CO gas kinetic temperature within $0.5$, $1$, and $10\,$AU to the star and a gas vibrational temperature of $8800\,$K. The newly discovered warm CO gas population likely resides between sub-AU scales and $\sim\,10\,$AU, interior to the cold CO reservoir detected beyond $35\,$AU with HST STIS and ALMA. The discovery of warm, fluorescent gas in a debris disk is the first such detection ever made. The detection of warm CO raises the possibility of unseen molecules (H$_2$O, H$_2$, etc) as collisional partners to excite the warm gas. We estimated a lower mass limit for CO of $1.25\times 10^{-7}\text{M}_{\oplus}$, which is $10^{-5}$ of the cold CO mass detected with ALMA and HST. We demonstrate that UV fluorescence emerges as a promising avenue for detecting tenuous gas at $10^{-7}$ Earth-mass level in debris disks with JWST.