Everyone knows that the James Webb Space Telescope is a ground-breaking infrared space telescope that’s helping us better understand the cosmos. The JWST’s discerning infrared eyes are deepening our understanding of everything from exoplanets to primitive galaxies to the birth of stars.
But it’s not the first ground-breaking infrared space telescope we’ve launched. There was IRAS, then ISO, then the Spitzer Space Telescope. The Spitzer is the JWST’s most recent infrared predecessor, and the JWST is observing one of the same targets that the Spitzer did, taking note of some puzzling changes.
In 2008, the Spitzer observed SZ Chamaeleontis (SZ Cha), a T-Tauri star that’s only a few million years old and still growing. It’s typical of young stars and is surrounded by a protoplanetary disk, a thick disk of rotating gas and dust from which planets form. Our own Sun was similar to this five billion years ago before the Solar System took shape.
When the Spitzer observed SZ Cha, it noticed a specific type of the chemical element neon in the disk, called Neon iii. Neon can only ionize under extreme energy, so its presence is evidence of the star’s extreme UV (EUV) light. It’s also scarce in disks being bombarded by X-rays, indicating that SZ Cha wasn’t very energetic in X-ray emissions.
Young stars can be very energetic, and that can power the photoevaporation of their protoplanetary disks. This puts planets in a race against time. They must form before the disk becomes too diffuse. Though Spitzer’s observations of Neon III indicate powerful stellar radiation, it’s EUV radiation. EUV radiation is powerful enough to ionize stubborn neon, but it’s not as effective at photoevaporating the disk surrounding SZ Cha.
But now, the JWST has observed SZ Cha and found something quite different. It found Neon iii, but it also found Neon ii. More importantly, the two exist in a ratio that’s at a typical level. What does it mean? “One way to distinguish between EUV and X-ray creation of neon fine-structure emission is by measuring the [Ne iii]-to-[Ne ii] line flux ratio,” the authors explain.
Contrasting data from NASA’s James Webb and Spitzer space telescopes show a change in the disk surrounding the star SZ Chamaeleontis (SZ Cha) in just 15 years. In 2008, Spitzer’s detection of significant neon III made SZ Cha an outlier among similar young protoplanetary disks. However, when Webb followed up on SZ Cha in 2023, the ratio of neon II to III was within typical levels. What happened? Image Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
This means that the young star is radiating different energy. “This points to a switch from EUV-dominated to X-ray-dominated photoevaporation of the disk,” the authors point out.
“Once again, the universe is showing us that none of its methods are as simple as we might like to make them.”
Catherine Espaillat, Boston University.
Instead of EUV, the star is bombarding its protoplanetary disk with X-rays. The problem is that X-rays are much more efficient at blowing away the disk, and this means that the clock is ticking for any planets forming in the disk.
These results are in a new paper published in The Astrophysical Journal Letters. The paper is “JWST Detects Neon Line Variability in a Protoplanetary Disk.” The lead author is Catherine Espaillat of Boston University.
“Planets are essentially in a race against time to form up in the disk before it evaporates,” explained Thanawuth Thanathibodee of Boston
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