The James Webb Space Telescope (JWST) has made a significant discovery by detecting ultraviolet-fluorescent carbon monoxide in a protoplanetary debris disc for the first time. This finding, detailed in a pre-print study led by Cicero Lu at the Gemini Observatory, provides new insights into the formation of planets around the star known as 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.
Previously, observations from the Atacama Large Millimeter/submillimeter Array (ALMA) revealed a substantial amount of cold carbon monoxide gas and dust situated 30-100 astronomical units (AU) from the star. In addition, preliminary infrared data from both the Gemini Observatory and the NASA Infrared Telescope Facility (IRTF) indicated the presence of hot dust and solid-state features closer to the star. Optical studies suggested the existence of hot atomic gases, including calcium and potassium, in the inner disc.
The JWST’s capabilities were pivotal in elucidating the dynamics of the inner portion of the disc around HD 131488. During a focused observation in February 2023, the telescope identified a small quantity of warm carbon monoxide gas, approximately equivalent to one hundred thousand times the mass of the cold gas in the outer disc. This warm gas was detected between 0.5 AU and 10 AU from the star, revealing intriguing characteristics.
A key finding was the disparity between the vibrational and rotational temperatures of the carbon monoxide molecules. The vibrational temperature, reflecting the speed of atomic vibrations within the molecules, reached a maximum of around 8800 Kelvin, closely aligned with the ultraviolet radiation emitted by the star. In contrast, the rotational temperature peaked at about 450 Kelvin, decreasing to 150 Kelvin further from the star. This significant difference indicates that the molecules are not in thermal equilibrium, explaining their fluorescent appearance.
The research also uncovered a high ratio of Carbon-12 to Carbon-13 in the environment, suggesting the presence of dust grains within the warm gas cloud that obstructs light. For the observed fluorescence pattern to occur, the carbon monoxide requires “collisional partners”—other molecules that interact with it and absorb some of its energy. The study proposed that water vapor from disintegrating comets is a more likely candidate for these partners than hydrogen.
The findings lend support to a long-standing debate regarding the formation of carbon-rich debris discs like that of HD 131488. Two primary hypotheses have emerged: one posits that such disks are remnants from the star’s formation, while the other suggests that they are continually replenished by the destruction of comets. The results from this study strongly favor the latter explanation, suggesting a dynamic process at play.
Additionally, the presence of substantial carbon and oxygen in the terrestrial zone of the disc, coupled with a relative scarcity of hydrogen, implies that any planets forming in this region would exhibit high metallicity. This could set them apart from hydrogen-rich primordial nebulae, influencing their development and characteristics.
These groundbreaking discoveries exemplify the capabilities of the JWST, which has consistently delivered valuable insights since its launch. As researchers continue to analyze similar star systems like HD 131488, further evidence may emerge to enhance our understanding of the formation and evolution of such rare carbon-rich environments.
For more detailed insights, refer to the study by C. X. Lu et al. on the JWST’s detection of warm carbon monoxide emission in the protoplanetary disc of HD 131488.
