The James Webb Space Telescope (JWST) has made a groundbreaking discovery, identifying ultraviolet-fluorescent carbon monoxide in a protoplanetary debris disc for the first time. This research, led by Cicero Lu at the Gemini Observatory and his colleagues, highlights crucial features of the debris disc surrounding the young star HD 131488, located approximately 500 light years away in the Upper Centaurus Lupus subgroup of the Centaurus constellation.
The star, estimated to be around 15 million years old, is categorized as an “Early A-type” star, which characterizes it as both hotter and more massive than our Sun. Previous investigations by the Atacama Large Millimeter/submillimeter Array (ALMA) detected significant amounts of “cold” carbon monoxide gas and dust between 30-100 AU from HD 131488. Additional infrared data from the Gemini Observatory and the NASA Infrared Telescope Facility (IRTF) suggested the presence of hot dust and solid-state features in the inner regions of the disc.
In February 2023, JWST focused on HD 131488 for approximately one hour and uncovered a small quantity of “warm” carbon monoxide gas, amounting to about one hundred thousandths of the mass of the cold gas in the outer disc. This gas was distributed between 0.5 AU and 10 AU from the star, revealing intriguing characteristics.
Understanding the Thermal Dynamics of the Disc
A key finding from JWST’s observations was the disparity between the vibrational and rotational temperatures of the carbon monoxide molecules. The vibrational temperature, indicating the motion of atoms within the molecule, reached a maximum of around 8,800 K, aligning with the ultraviolet radiation from the star. In contrast, the rotational temperature peaked at only 450 K, decreasing to 150 K further from the star. The significant difference between these two temperatures indicates that the gas is not in thermal equilibrium, contributing to the observed fluorescence of the carbon monoxide.
Furthermore, the study revealed a high ratio of Carbon-12 to Carbon-13, suggesting that dust grains may be obstructing light within the warm gas cloud. To emit the light patterns observed by JWST, the carbon monoxide requires “collisional partners”—other molecules that interact with them and absorb some of their energy. Among the candidates studied, water vapor from disintegrating comets emerged as the more probable partner, while hydrogen appeared less likely.
Implications for Planetary Formation Theories
The findings support a long-debated hypothesis regarding the formation of carbon-rich debris discs like that of HD 131488. Two primary theories have circulated: one posits that these discs are remnants from the star’s formation, while the other suggests that they are continually replenished by the destruction of comets. The results of this study lend weight to the latter explanation, indicating that the gas within the disc is actively sustained by the ongoing collisions of comets.
Additionally, the presence of substantial amounts of carbon and oxygen in this “terrestrial zone” of the disc, along with a relative scarcity of hydrogen, suggests that any planets forming in this region would possess high “metallicity.” This contrasts with planets formed in hydrogen-rich primordial nebulae, potentially leading to a distinct chemical makeup.
Ultimately, these pioneering discoveries underscore the capabilities of JWST, which has been consistently delivering significant findings since its launch. With the potential for further exploration of star systems similar to HD 131488, the research contributes valuable evidence to the ongoing discourse about the formation and evolution of carbon-rich debris discs in our universe.
