The James Webb Space Telescope (JWST) was involved in another first discovery, recently available in pre-printed form on arXiv by Cicero Lu of the Gemini Observatory and his co-authors. This time, humanity’s most advanced space telescope has found UV-fluorescent carbon monoxide in a protoplanetary debris disk for the first time. Some features of this disk have also been discovered that have significant implications for the theory of planet formation.
HD 131488 is a relatively young (about 15 million years old) star in the Centaurus Lupus supergroup, located (no surprise) in the constellation Centaurus, about 500 light-years away. It is classified as an “early A-type” star, meaning it is both hotter and more massive than our Sun. It’s also not the first time it’s been the subject of an article about his CD.
Previous ALMA studies using radio frequencies found a huge amount of “cold” CO gas and dust about 30-100 AU from the star. Additional preliminary infrared data from the Gemini Observatory and NASA’s Infrared Telescope Facility (IRTF) showed that there was likely hot dust and some solid-state structures in the star’s inner zone. Additional optical studies even suggested that the inner disk contained “hot atomic gas” such as calcium and potassium, which is not the same as CO since it is by definition a molecule.
Video showing the formation of planets in a protoplanetary disk. Photo credit: NASA video
But the key to truly understanding what was going on inside the disk lay in the infrared spectrum, and this is where JWST shines. Or more specifically, where it collects data about things that shine on it. When it turned its attention to HD 131488, which was probably only the case for about an hour in February 2023, it found a small amount of “warm” CO gas, equivalent to about hundreds of thousands of the mass of the cold gas in the outer disk.
This gas was distributed between 0.5 and 10 AU and had some interesting properties. First, there was a difference between “vibration temperature” and “rotation temperature.” The vibrational temperature of a gas indicates how quickly the atoms within the molecule swing back and forth, while the rotational temperature indicates how quickly the molecules are rotating – something that corresponds to kinetic energy. In a normal gas state, such as would be found in a typical space, these two temperatures would be equal because the collisions between the particles would equalize them to something called local thermal equilibrium.
However, with HD 131488 the difference is enormous. The CO molecule’s rotation temperature is only about 450 K maximum (and drops to 150 K further from the star), while its rotation temperature is a blistering 8800 K, which corresponds to the UV glare of its parent star. This shows that they are not in thermal equilibrium and also explains why the molecules fluoresce (appear warm).
*Comet collisions occur in a protoplanetary disk. Photo credit: NASA / JPL-Caltech*
The carbon-12 to C-13 ratio was also found to be high for this type of environment, suggesting that there are likely some dust grains trapped in the sparse warm gas cloud that are blocking the light. To emit the light pattern found by JWST, CO also needs “collision partners” – other molecules that bounce off of them and use up some of their energy. Two potential partners have been examined, with hydrogen appearing less likely, while water vapor from comets destroyed by the star appears more likely.
This “exocometary” hypothesis is a central result of the work. Scientists have long debated what creates this relatively rare class of CO-rich debris disks like HD 131488 and how they trap their gas. To explain this, two hypotheses have been put forward: first, that CO-rich disks are simply left over from the star’s birth, and second, that the gas is constantly replenished by the destruction of comets.
The results of this study clearly support the second explanation. But they also have an impact on planet formation. Because there was a significant amount of carbon and oxygen in this “terrestrial zone” of the disk, as well as a lack of hydrogen, any planet that formed there would have high “metallicity” (i.e., elements that are not hydrogen). This would distinguish them from hydrogen-rich primordial nebulae.
Ultimately, these unique discoveries are exactly what JWST was designed for, and the company has produced a steady stream of them since its launch. Undoubtedly there are other star systems like HD 131488 that can provide further evidence for the CO-rich disk debate, but for now this paper provides ample evidence for how these relatively rare systems form.
Learn more:
CX Lu et al – JWST/NIRSpec detects warm CO emission in the terrestrial-planetary zone of HD 131488
UT – Why rocky planets form early: ALMA survey shows planet-forming disks lose gas faster than dust
UT – Astronomers see carbon-rich nebulae where planets are forming
UT – A protoplanetary disk that refuses to grow up
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