James Webb Space Telescope Discovers Unique Gas in Protoplanetary Disc
The James Webb Space Telescope (JWST) has achieved a significant milestone by detecting ultraviolet-fluorescent carbon monoxide in a protoplanetary debris disc surrounding the star HD 131488. This groundbreaking discovery, detailed in a pre-print study by Cicero Lu and his team at the Gemini Observatory, marks the first time such findings have been made in this type of astronomical environment.
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. It is classified as an “Early A-type” star, which indicates that it is hotter and more massive than our Sun. Previous research conducted by the Atacama Large Millimeter/submillimeter Array (ALMA) has revealed substantial amounts of “cold” carbon monoxide gas and dust situated about 30 to 100 astronomical units (AU) from the star.
Recent preliminary data from the NASA Infrared Telescope Facility (IRTF) and the Gemini Observatory suggested the presence of hot dust and solid-state features closer to the star, while additional optical studies indicated the existence of hot atomic gases such as calcium and potassium in the inner disc. However, it was the infrared spectrum that provided the key insights into the inner workings of this disc, a domain in which the JWST excels.
In February 2023, the JWST observed HD 131488 for approximately one hour. The telescope detected a small quantity of “warm” carbon monoxide gas, roughly equivalent to one hundred thousandths of the mass of the cold gas found in the outer disc. This warm gas was distributed between 0.5 AU and 10 AU from the star and exhibited intriguing characteristics. Notably, there was a significant difference between the vibrational and rotational temperatures of the gas.
The vibrational temperature, which indicates the speed of atomic vibrations within the molecule, reached a maximum of around 4,500 Kelvin, while the rotational temperature soared to an astonishing 8,800 Kelvin. This discrepancy suggests that the molecules are not in thermal equilibrium, explaining the observed fluorescence of the carbon monoxide.
The study also identified a high ratio of Carbon-12 to Carbon-13, indicating potential dust grains trapped within the warm gas cloud. To create the light patterns detected by the JWST, carbon monoxide requires “collisional partners”—other molecules that interact with it to dissipate energy. The research examined two potential partners, concluding that water vapor from disintegrating comets is a more likely candidate than hydrogen.
This finding supports the “exocometary” hypothesis, which suggests that comets play a crucial role in sustaining the gas in such discs. The researchers propose two main theories regarding the formation of carbon-rich debris discs like HD 131488. The first theory suggests these discs are remnants from the star’s formation, while the second posits that the gas is continuously replenished by the destruction of comets. The results from this study provide strong evidence in favor of the latter explanation.
Understanding the composition of the inner disc raises significant implications for planetary formation. The presence of substantial amounts of carbon and oxygen, coupled with a lack of hydrogen in this “terrestrial zone,” indicates that any planets forming there would likely have high metallicity, distinguishing them from planets that originate in hydrogen-rich primordial nebulae.
These pioneering discoveries exemplify the capabilities of the JWST, which has consistently delivered groundbreaking results since its launch. The findings from HD 131488 contribute valuable insights into the dynamics of carbon-rich discs and their role in planetary formation, potentially paving the way for further studies of similar star systems in the universe.
