By blocking the light from the star, Webb reveals the dusty disk around it

AU Microscopii is a small red dwarf star about 32 light-years away. They are very faint to the naked human eye, but this does not diminish their attractiveness. The star contains at least two exoplanets and hosts a stellar debris disk.

It’s also young, only about 23 million years old, and is the second closest main sequence star to Earth. JWST recently imaged the star and its surroundings and found something surprising.

These observations were part of JWST’s Guaranteed Time Observations (GTO). Part of the GTO aims to find exoplanets around dark red dwarfs. Planets around faint stars are easy to spot because starlight does not blind instruments to faint dips in light from planets passing in front of the stars. This is especially important in this case because observers have been looking for planets in very wide orbits, which our exoplanet-hunting efforts have struggled to locate.

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The debris disk surrounding AU Microscopii is composed of debris from the collision of planetesimals. It’s dense and hard to see, but JWST’s powerful infrared instruments can peer inside all that dust. This is uncharted territory and once again shows how valuable JWST is.

These observations are part of new research presented at the 241st meeting of the American Astronomical Society in Seattle, Washington. The lead author is Kellen Lawson of NASA’s Goddard Space Flight Center. Lawson is part of a team developing a new program to find planets in wide orbits around low-mass stars, which has been challenging.

This image is from the Lawson Show at the American Astronomical Society. The yellow box identifies the target of the team’s monitoring software. It shows how difficult it is to detect planets on wide orbits with indirect methods such as the transit method. Using JWST and focusing on low-mass stars, the team expects to find more low-mass planets in this intractable region. Image credit: Kellen Lawson.

“The debris disk is constantly being replenished by collisions of young planets. By studying them, we get a unique window into the recent dynamical history of this system,” said Lawson.

It is not just the disk but the system as a whole that makes it such a desirable target for notes.

“This system is one of the very few examples of a young, well-known exoplanet and debris disk close enough and bright enough to be studied comprehensively with Webb’s uniquely powerful instruments,” said Josh Schleider of NASA’s Goddard Space Flight Center. Monitoring program researcher and study co-author.

JWST’s NIRCam (Near Infrared Camera) played the leading role in this work. NIRCam contains many coronal vertebrae, which are instruments that mask the star’s light so that NIRCam can better image the region around the star. Using the ECG instrument, NIRCam can spot the debris disk within five astronomical units of the star, or roughly the distance of Jupiter from the sun in our solar system. The sides of the disc helped to tip over the notes.

In what has become a familiar refrain, JWST exceeded expectations.

The team used a near infrared webcam (NIRCam) to study the AU microphone. NIRCam’s crown, which blocked out the intense light of the central star, allowed the team to study the region very close to the star. The location of the star, which is hidden, is marked with a white graphic representation in the center of each image. A dashed circle shows the area that is blocked by the coronal vertebra. Credits: Science: NASA, ESA, CSA, Kellen Lawson (NASA-GSFC), Joshua E. Schlieder (NASA-GSFC)
Image processing: Alyssa Pagan (STScI)

The top blue image is a shorter wavelength than the bottom red image, and it’s also much brighter. This means that the disk contains a lot of fine dust that can scatter shorter wavelengths of light more efficiently. This is consistent with other studies of the star that showed insufficient radiation pressure to blow the dust away.

“Our first look at the data far exceeded expectations. It was more detailed than we expected. It was brighter than we expected. We discovered the disk sooner than we expected. We’re hopeful that as we dig deeper, there will be more surprises that we didn’t expect,” Schleider stated.

The team also found that the disk contains fast-moving clumps of material. In an exchange with Universe Today, lead author Lawson spoke about the importance of blocks. “The ‘fast-moving lumps’ were first reported in 2015 and are likely made of the same material as the rest of the disk,” said Lawson. “It appears that these clouds of dust are being accelerated to a high speed by a hitherto unknown driver. One hypothesis is favored in Boccaletti et al What works is that something like a planet inside the disk can interact with the material of the disk to produce the clumps.”

Lawson also said that he is not aware of any other disk with clumps like this and that their study could help test this hypothesis.

This image from Lawson's presentation at the AAS meeting explains another aspect of their surveillance program.  The dotted red box shows where they hope to find more exoplanets.  If planets are responsible for generating the masses of fast-moving material in the disk, they must be in this region.  Image credit: Kellen Lawson.
This image from Lawson’s presentation at the AAS meeting explains another aspect of their surveillance program. The dotted red box shows where they hope to find more exoplanets. If planets are responsible for generating the masses of fast-moving material in the disk, they must be in this region. Image credit: Kellen Lawson.

Observing the dusty AU Microscopii disk is an achievement, but not the primary purpose of the job. The main objective is to search for giant planets in wide orbits like the gas giants and ice giants in our solar system. Most of the exoplanets we’ve found are much closer to their stars. This is not necessarily because our solar system is rare, and some do not have planets in wide orbits. This is likely due to detection bias in exoplanet search methods. But the power of JWST can transform this dynamic.

“This is the first time that we really have the sensitivity to directly observe planets with wide orbits that are significantly less massive than Jupiter and Saturn. This is really new uncharted territory in terms of directly imaging low-mass stars,” Lawson explained.

Lawson and colleagues’ paper is still under review. But once it is published, it will be another example of JWST’s ability to address relevant questions in astronomy.


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