Stars

JWST Follows Neon Signs Toward New Thinking on Planet Formation

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 University, another astronomer on the research team. “In computer models of developing systems, extreme ultraviolet radiation allows for 1 million more years of planet formation than if the evaporation is predominately caused by X-rays.”

So, what’s up with SZ Cha? It couldn’t have switched from EUV emissions to X-ray emissions like this. 15 years is a mere inconsequential blip in the lifetime of a star. Why the unusual Spitzer findings 15 years ago versus the more normal current findings by the JWST?

“Both the Spitzer and Webb data are excellent, so we knew this had to be something new we were observing in the SZ Cha system – a significant change in conditions in just 15 years,” added co-author Ardjan Sturm of Leiden University, Leiden, Netherlands.

Wind. It comes down to the stellar wind.

“To explain the variability in the [Ne iii]-to-[Ne ii] line flux ratio of SZ Cha between 2008 and 2023, we propose a variable wind,” the researchers write.

All stars emit stellar winds, though their strength varies. In this image, powerful stellar winds from an aged central star form the so-called Butterfly Nebula (NGC 6302). The stellar winds from young T Tauri stars like SZ Cha don’t form a nebula, but they do interact with the protoplanetary disk. Image Credit: HUBBLE/NASA AND ESA

All stars emit stellar winds. They’re fast-flowing streams of charged particles that interact with the stellar environment. Though they appear to be stable most of the time, that’s not quite true. “However, there is abundant evidence that essentially all hot-star winds contain time-dependent structure on a variety of spatial scales,” one paper explained. So stellar winds from young T-Tauri stars like SZ Cha are actually very hot and very dynamic.

The research team behind the new paper thinks that when the Spitzer looked at SZ Cha in 2008, it happened to catch the star during a quieter, relatively wind-free period. When present, the wind actually absorbs UV light and leaves X-rays to hammer away at the protoplanetary disk. But Spitzer caught the star when the wind was absent, or at least quiet. That means that the EUV was able to ionize the Neon, leading to the detection of Neon III.

This is a rare finding and one of only five times astronomers have detected Neon III in a disk.

“Once again, the universe is showing us that none of its methods are as simple as we might like to make them. We need to rethink, re-observe, and gather more information. We’ll be following the neon signs,” said Espaillat.

What might this mean for any planets forming in SZ Cha’s disk?

Research shows that X-ray Photoevaporation (XPE) can drive the migration of giant planets like Jupiter. But for that to happen, the giant planets have to form first.

And when it comes to SZ Cha, there may not be enough time.

Evan Gough

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