Any planets forming around the young star SZ Cha had better get a move on.
 |
An artist’s impression of a swirling disk of gas and dust
forming planets around a young star. (Image credit: NASA/ESA/CSA/Ralf Crawford
(STScI)) |
New observations from the James Webb Space Telescope suggest
the amount of ionized neon gas present in dusty, planet-forming disks can tell
us how quickly planets must form before the disk itself disappears.
Planets are believed to be born in disks of gas and dust that
swirl around newborn stars. Astronomers have observed these disks before, but
the entire planetary formation process takes hundreds of thousands — and even
millions — of years to complete. That means we don't usually get to see the
disks change on small timescales. Rather, the features just appear as snapshots
frozen in time.
Now, however, the James Webb Space Telescope has observed one planet-forming disk changing quite substantially.
In 2008, a team led by Catherine Espaillat, who was then at the University of Michigan but is now at Boston University, used NASA’s Spitzer Space Telescope to detect an infrared emission line associated with doubly ionized neon ([Ne III]). The signal was coming from a planet-forming disk ringing the young star SZ Chamaeleontis (SZ Cha). An atom is "ionized" when one of its outer electrons is struck by a high-energy photon and knocked out of position; "doubly ionized" atoms involve the loss of two electrons from the impact of two photons.
When originally detected, the presence of [Ne III] in SZ
Cha’s disk was considered a rarity among disks typically being bombarded by
X-ray radiation from their young parent stars. Its existence actually implied
that, instead of X-rays, lower-energy radiation in the form of extreme
ultraviolet (EUV) must be the dominant type of radiation field in the SZ Cha
system. This sort of radiation is thought to eat away at all the gas and dust
in the planet-forming disk, thereby breaking molecules apart, but it's not
thought to disintegrate disks as fast as X-rays do. X-rays can erode a
planet-forming disk 100 times faster than ultraviolet light can.
Thus, the rate at which a disk "evaporates," and
therefore the amount of time planets have to form within that disk before it
disappears, depends upon the energy of radiation present.
However, when Espaillat and her team followed up on SZ Cha
in 2023 with the JWST’s Mid-Infrared Instrument (MIRI), they found the doubly
ionized neon had more or less vanished compared to the amount of singly ionized
neon [Ne II]. This lack of doubly ionized neon implies that X-ray radiation,
rather than ultraviolet, has indeed asserted itself as the dominant radiation
field in the SZ Cha system.
Ultimately, this finding could have big repercussions in our
understanding of how quickly planets must form before their disk dissipates.
"In computer models of developing systems, extreme
ultraviolet radiation allows for one million more years of planet formation
than if evaporation is predominantly caused by X-rays," said Boston
University’s Thanawuth Thanathibodee in a statement.
As such, the abundance of doubly ionized neon could be
considered a proxy for the amount of ultraviolet versus X-ray radiation
impacting a planet-forming disk in a given moment. By measuring its abundance,
astronomers can better constrain the timescale in which planets must form
within a system before their birthing disk dissipates.
Two spectra of SZ Cha, one taken by Spitzer in 2008 and the
other by the JWST in 2023. The Spitzer data clearly shows both singly and
doubly ionized neon, but the JWST observations find that the doubly ionized
neon has all but vanished. (Image
credit: NASA/ESA/CSA/Ralf Crawford (STScI)) |
Complementary ground-based observations also found a piece
of the puzzle: The CHIRON spectrometer on the SMARTS 1.5-meter telescope at the
Cerro Tololo Inter-American Observatory measured "blueshifted"
hydrogen-alpha emissions associated with the star in the SZ Cha system.
Blueshift is a Doppler-like shift that indicates something in the distant
universe is moving toward us — in this case, the hydrogen.
Scientists interpret the situation as a "stellar
wind" of particles emanating from the star, headed closer to our corner of
the cosmos.
The wind is thought to be dense enough to absorb ultraviolet
light, but still allow X-rays through, permitting X-ray radiation to assert
itself over the evolution of the star system. The fact that the doubly ionized
neon was seen in 2008 but not fifteen years later, in 2023, suggests the wind
might be variable and perhaps related to what kind of radiation permeates the
star system's planet-forming disk.
Further studies are now being planned, not only with the
JWST but also other observatories, covering different parts of the
electromagnetic spectrum to try to get to the bottom of what is happening in
the SZ Cha system. Hopefully, this will help scientists finally catch the
stellar wind fluctuations. "We need to rethink, re-observe and gather more
information," said Espaillat.
"We’ll be following the neon signs."
Reference: Research Paper