Westerlund 2’s Core is No Place to Form Planets, Astronomers Say

Astronomers using the NASA/ESA Hubble Space Telescope have observed young stars in a crowded stellar cluster called Westerlund 2. They’ve found that lower-mass stars in the cluster’s core face a rough-and-tumble neighborhood that suppresses planet formation; these stars do not have the large, dense disks of dust and gas that eventually could become planets.

Westerlund 2, also known as ESO 127-18, is located roughly 20,000 light-years away in the constellation of Carina.

Discovered by the Swedish astronomer Bengt Westerlund in the 1960s, the cluster resides in a giant stellar nursery known as Gum 29.

Westerlund 2, which is about 10 light-years across and is only 2 million years old, is a unique laboratory to study stellar evolutionary processes

It contains at least 37 extremely massive stars, some weighing up to 100 solar masses. Their ultraviolet radiation and stellar winds act like blowtorches and erode the planet-forming disks around neighboring stars, dispersing the giant dust clouds.

“Basically, if you have monster stars, their energy is going to alter the properties of the disks,” said lead author Dr. Elena Sabbi, an astronomer in Space Telescope Science Institute.

“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 difficult to observe because it is surrounded by dust, but Hubble’s Wide Field Camera 3 (WFC3) can peer through the dusty veil in near-infrared light, giving astronomers a clear view of the cluster.

Hubble’s sharp vision was used to resolve and study the dense concentration of stars in the central region of the cluster.

“With an age of less than about two million years, Westerlund 2 harbors some of the most massive, and hottest, young stars in the Milky Way,” said co-author Dr. Danny Lennon, an astronomer in 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.”

The researchers found that of the nearly 5,000 stars in Westerlund 2 with masses between 0.1 and 5 times the Sun’s mass, 1,500 show dramatic fluctuations in their luminosity, which is commonly accepted as being due to the presence of large dusty structures and planetesimals.

Orbiting material would temporarily block some of the starlight, causing fluctuations in brightness. However, Hubble only detected the signature of dust particles around stars outside the central region.

The scientists did not detect these dips in brightness in stars residing within 4 light-years of the center.

“We think they are planetesimals or structures in formation. 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,” Dr. Sabbi said.

Thanks to Hubble, astronomers can now see how stars are accreting in environments that are like the early Universe, where clusters were dominated by monster stars.

So far, the best known nearby stellar environment that contains massive stars is the starbirth region in the Orion Nebula.

However, Westerlund 2 is a richer target because of its larger stellar population.

“Westerlund 2 gives us much better statistics on how mass affects the evolution of stars, how rapidly they evolve, and we see the evolution of stellar disks and the importance of stellar feedback in modifying the properties of these systems,” Dr. Sabbi said.

“We can use all of this information to inform models of planet formation and stellar evolution.”

The results were published in the Astrophysical Journal.