The James Webb Space Telescope (JWST) has made a groundbreaking discovery by detecting ultraviolet-fluorescent carbon monoxide in a protoplanetary debris disc for the first time. This significant finding was detailed in a pre-print publication on arXiv by Cicero Lu from the Gemini Observatory and his co-authors. The discovery revolves around the star HD 131488, located approximately 500 light-years away in the Centaurus constellation.
HD 131488 is a relatively young star, estimated to be around 15 million years old, and is classified as an “Early A-type” star, meaning it is hotter and more massive than our Sun. Previous research utilizing the Atacama Large Millimeter/submillimeter Array (ALMA) had already identified substantial amounts of “cold” carbon monoxide gas and dust situated about 30-100 astronomical units (AU) from the star. Additional preliminary data from the Gemini Observatory and the NASA Infrared Telescope Facility (IRTF) indicated the presence of hot dust and solid-state features closer to the star, while optical studies suggested the existence of hot atomic gas, including calcium and potassium.
When JWST focused on HD 131488 in February 2023, it detected a small quantity of “warm” carbon monoxide gas, approximately equivalent to one-hundred-thousandths of the mass of the cold gas in the outer disc. This warm gas was spread between 0.5 AU and 10 AU from the star. Notably, the study revealed a significant discrepancy between the vibrational and rotational temperatures of the carbon monoxide. The vibrational temperature, which gauges how quickly molecules vibrate, was around 8800K, while the rotational temperature peaked at about 450K, dropping to 150K further from the star. This stark difference indicates that the molecules are not in thermal equilibrium, which contributes to their fluorescence.
In analyzing the carbon isotopes, the researchers found an unusually high ratio of Carbon-12 to Carbon-13, suggesting that dust grains may be obstructing the light within the sparse warm gas cloud. To produce the specific light patterns observed by JWST, carbon monoxide requires “collisional partners,” other molecules that interact with it. The study examined two potential partners: hydrogen and water vapor from exocomets being disrupted by the star. The findings leaned toward the latter hypothesis, marking a significant development in understanding the dynamics of protoplanetary discs.
The research contributes to a long-standing debate among scientists regarding the origins of carbon-rich debris discs like that of HD 131488. Two primary hypotheses exist: either these discs are remnants from the star’s formation, or the gas is continuously replenished by the destruction of comets. The results from this study strongly support the latter theory, suggesting that the disc’s gaseous components are indeed replenished through cometary collisions.
The implications of these discoveries extend beyond simply understanding the disc’s composition. The presence of significant amounts of carbon and oxygen in the “terrestrial zone” of the disc, coupled with a lack of hydrogen, implies that any planets forming in this region would exhibit high metallicity. This characteristic distinguishes them from planets that arise in hydrogen-rich primordial nebulae.
Ultimately, these pioneering findings exemplify the capabilities of the JWST. Since its launch, the telescope has consistently produced extraordinary insights into the universe. As research continues, it is anticipated that more star systems similar to HD 131488 will emerge, further illuminating the composition and formation of carbon-rich debris discs.
