Westerlund 2 is a star cluster about 20,000 light years away. It’s young—only about one or two million years old—and its core contains some of the brightest and hottest stars we know of. Also some of the most massive ones.
There’s something unusual going on around the massive hot stars at the heart of Westerlund 2. There should be huge, churning clouds of gas and dust around those stars, and their neighbours, in the form of circumstellar disks.
But in Westerlund 2’s case, some of the stars have no disks.
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When stars form from huge molecular clouds, there’s material left over. That material forms the star’s circumstellar disk. That disk rotates around the young star, and gradually, planet’s form out of it. We can see exoplanets forming like this in distant solar systems, and astronomers are pretty sure this is what happened in our own Solar System.
Astronomers used the Hubble space telescope to conduct a three-year study of large young stars near the center of Westerlund 2. They found that while stars on the periphery of the cluster do have disks, the brightest, most massive stars that dominate the core of the cluster don’t have any.
So they’ll likely never have any planets.
In star clusters like Westerlund 2, the most massive and hottest stars reside in the core. It looks like those stars are so energetic they’re changing the properties of their disks, preventing planets from forming in the normal way. Not only that, but the 30 most energetic stars in the center are blasting out so much ultraviolet radiation that they’re shredding the disks around neighbouring, lower-mass stars.
The study outlining these results is titled “Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope.” Lead author of the study is Elena Sabbi of Space Telescope Science Institute. The study is published in The Astrophysical Journal.
“With an age of less than about two million years, Westerlund 2 harbours some of the most massive, and hottest, young stars in the Milky Way,” said team member Danny Lennon of the Instituto de Astrofísica de Canarias and the Universidad de La Laguna. “The ambient environment of this cluster is therefore constantly bombarded by strong stellar winds and ultraviolet radiation from these giants that have masses of up to 100 times that of the Sun.”
“Basically, if you have monster stars, their energy is going to alter the properties of the disks around nearby, less massive stars,” explained lead author Sabbi in a press release. “You may still have a disk, but the stars change the composition of the dust in the disks, so it’s harder to create stable structures that will eventually lead to planets. We think the dust either evaporates away in 1 million years, or it changes in composition and size so dramatically that planets don’t have the building blocks to form.”
Westerlund 2 is an extremely dense star cluster. This study marks the first time that astronomers have studied such a cluster so thoroughly, or for so long. It’s part of an effort to understand the conditions that allow or prevent planets from forming.
The stars at Westerlund 2’s heart touch on another issue in astronomy. The cluster is young, at only one or two million years old. Yet it contains massive stars in its core. Astronomers wonder if these huge stars formed in place, or if they migrated there.
The types of stars confirm the cluster’s age. Westerlund 2 contains a lot of stars that are pre-main sequence. That means the cluster can’t be much older than 2 million years, at the oldest. There are also several sub-types of Wolfe-Rayet stars—called WNh stars—that are part of the cluster, though some appear to have been ejected as runaways. The presence of these young, hot stars also constrains the age of the Westerlund 2 cluster.
The three-year study was done with Hubble’s Wide Field Camera 3 (WFC3). Using that camera, the researchers identified almost 5,000 stars with masses ranging between 0.1 to 5 solar masses. 1500 of those stars show light fluctuations as clumps of material in their rotating circumstellar disks block out some of their light intermittently. Astronomers think those clumps are nascent planets, forming out of the circumstellar disks.
The thing is, Hubble saw most of those fluctuations at stars that were outside the core. Inside the core, where the most massive and hottest stars reside, they didn’t detect these dips. In fact, they didn’t find any dips in brightness that indicate planet-forming clumps, around stars within four light years of the center.
“We think they are planetesimals or structures in formation,” explained lead author Sabbi. “These could be the seeds that eventually lead to planets in more evolved systems. These are the systems we don’t see close to very massive stars. We see them only in systems outside the center.”
Westerlund 2 is kind of like a natural star-forming laboratory for astronomers. Astronomers can not only watch how stars evolve, but they can watch as stars with different masses interact with each other.
“Hubble’s observations of Westerlund 2 give us a much better sense of how stars of different masses change over time, and how powerful winds and radiation from very massive stars affect nearby lower-mass stars and their disks,” Sabbi said. “We see, for example, that lower-mass stars, like our Sun, that are near extremely massive stars in the cluster still have disks and still can accrete material as they grow. But the structure of their disks (and thus their planet-forming capability) seems to be very different from that of disks around stars forming in a calmer environment farther away from the cluster core. This information is important for building models of planet formation and stellar evolution.”
In their paper, the authors call Westerlund 2 a “gold mine” when it comes to studying pre-Main Sequence (PMS) stars and their disks.
“The analysis of the Galactic YMC <young massive cluster> Wd2 with the WFC3 on board HST showed that these systems are gold mines for studying the properties of variable PMS stars and investigate the evolution of their circumstellar disks as a function of mass and age,” the authors write. They also point out that about one third of the stars in Wd2 are variable stars, and those stars constitute an important part of this study, because variability can be observed from a great distance.
They write: “The comparison among the spatial distributions of the five populations of variables highlights how local conditions can affect the evolution of circumstellar disks.”
There are an enormous number of different types of variable stars. As part of their work, the researchers divided the PMS variable stars in Wd2 into five populations:
- WTTS or weak line T-Tauri stars: their light varies due to magnetic spots on their surface that rotate in and out of view.
- CTTS or classic T-Tauri stars: variable because they’re still accreting material from the circumstellar disk
- dippers: their light dips due to features of their circumstellar disks
- bursters: emit bursts of light that peak in the xray part of the spectrum
- EBs: eclipsing binaries: the light is variable due to binary eclipsing
In their paper they say that “The spatial distribution of the different populations of variable PMS stars suggests that stellar feedback and UV radiation from massive stars play an important role in the evolution of circumstellar and planetary disks.”
The researchers were able to tease out some of that detail in this study. As the image shows, Weak-line T-Tauri stars are more concentrated around the two densest clumps of stars in WD2, while Classic T-Tauri stars and bursters are spread wider. Also, dippers are not present in the center of the two density clumps, where the most massive stars dwell.
Since dippers display dips in brightness due to features of their circumstellar disks, it sems that the huge bright stars in the center are disrupting the dippers’ disks.
“A major conclusion of this work is that the powerful ultraviolet radiation of massive stars alters the discs around neighbouring stars,” said team member Danny Lennon of the Instituto de Astrofísica de Canarias .
In their paper the authors write “If the dramatic drop in luminosity experienced by the dippers is, as commonly accepted, due to the presence of large dusty structures and planetesimals, the absence of dippers in the two higher density clumps of Wd2 could explain why planetary systems appear to be extremely rare in … younger dense clusters” like Wd2.
It’ll take more observation, with better instruments, to discover more of the deail around these enormously powerful young stars, and how they affect the disks of their neighbouring stars. Astronomers say that a lot, and these days that often means the James Webb Space Telescope.
“High spatial resolution follow-up observations in the near- and mid-infrared using, for example, NIRSpec and MIRI on the James Webb Space Telescope will be needed to definitely characterize the properties of the disks in Wd2 and other YMCs <Young Massive Clusters> to determine how local conditions affect the evolution of these systems and the formation of planetary systems.”
Sounds good. Now launch the darn thing!
- Press Release: Hubble Finds that “Distance” From the Brightest Stars is Key to Preserving Primordial Discs
- Press Release: In Planet Formation, It’s Location, Location, Location
- Research Paper: Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope.
- Universe Today: Astronomers See Adorable Baby Planets Forming Around a Young Star