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

We have obtained a high-resolution, JWST NIRSpec $2.87$ – $5.14$ $μ$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.

💡 Research Summary

This research presents a groundbreaking discovery made using the James Webb Space Telescope (JWST), specifically utilizing the NIRSpec instrument to observe the debris disk surrounding the star HD 131488. The study reports the first-ever detection of warm CO (carbon monoxide) fundamental emission within the terrestrial-planet zone, approximately within 10 AU of the host star. This finding is significant because it reveals a previously unknown population of gas in the inner regions of the disk, distinct from the cold CO reservoir located beyond 35 AU, which had been previously identified by the Hubble Space Telescope (HST) and ALMA.

A key technical highlight of the paper is the identification of a significant thermal non-equilibrium state within the CO gas. The researchers observed a massive discrepancy between the gas’s vibrational temperature and its rotational temperature. While the vibrational temperature was measured at an extremely high 8800 K, the kinetic (rotational) temperatures were much lower, ranging from approximately 1150 K near the star to 332 K at a distance of 1 AU. This disparity provides definitive evidence that the CO molecules are being excited through a process known as UV fluorescence, where the absorption of stellar ultraviolet radiation drives the molecules into higher energy states, rather than through simple thermal collisions.

The mass of the detected CO is incredibly minute, with an estimated lower limit of $1.25\times 10^{-7}$ Earth masses ($\text{M}_{\oplus}$), representing only about $10^{-5}$ of the mass found in the outer, colder reservoir. Despite this extremely low density, the high-resolution spectroscopy of JWST/NIRSpec allowed for a successful detection, demonstrating the instrument’s unprecedented sensitivity. This discovery opens a new window for exoplanetary science; the authors suggest that the mechanism of UV fluorescence can be leveraged to search for other elusive molecules, such as water vapor (H$_2$O) and molecular hydrogen (H$_2$), in the tenuous atmospheres of debris disks. Ultimately, this study establishes UV fluorescence as a powerful diagnostic tool for probing the chemical composition and physical conditions of the inner regions of planetary systems, paving the way for future discoveries in the search for habitable environments.