Study Investigates Properties of Stars During Early Evolutionary Phases

A three-year study was performed on Westerlund 2—a huge, crowded, and young star cluster—by using the NASA/ESA Hubble Space Telescope.

This image shows the sparkling centerpiece of Hubble’s 25th anniversary tribute. Westerlund 2 is a giant cluster of about 3000 stars located 20,000 light-years away in the constellation Carina. Hubble’s near-infrared imaging camera pierces through the dusty veil enshrouding the stellar nursery, giving astronomers a clear view of the dense concentration of stars in the central cluster. Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team.

The study discovered that the material that surrounds the stars close to the center of the cluster is strangely devoid of the large, dense dust clouds that would be predicted to become planets in a few million years.

This absence is due to the most brightest and massive stars in the cluster that erode and scatter the disks of dust and gas of neighboring stars. For the first time, astronomers have investigated a highly dense star cluster to analyze the environments favorable for planet formation.

Spanning from 2016 to 2019, the aim of this time-domain study was to analyze the properties of stars in their early evolutionary phases and to outline the evolution of their circumstellar environments.

Earlier, studies such as these were restricted to the nearest, low-density, star-forming regions. At present, astronomers have used the Hubble Space Telescope to extend this study, for the first time, to the center of Westerlund 2, one of the few young massive clusters in the Milky Way.

They have now discovered that it is very difficult for planets to form in this central region of the cluster. Moreover, the observations show that stars lined on the periphery of the cluster exhibit immense planet-forming dust clouds integral to their disks. According to the researchers, location is the main reason why certain stars in Westerlund 2 finding it difficult to form planets while others do not.

The brightest and most massive stars in the cluster gather in the core. Westerlund 2 includes nearly 37 extremely massive stars, with a few weighing up to 100 solar masses. Their hurricane-like stellar winds and blistering ultraviolet radiation behave like blowtorches that erode the disks surrounding the neighboring stars, thereby scattering the giant dust clouds.

Basically, if you have monster stars, their energy is going to alter the properties of the discs. You may still have a disc, but the stars change the composition of the dust in the discs, 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.

Elena Sabbi, Lead Researcher, Space Telescope Science Institute, Baltimore, USA

Westerlund 2 is an exclusive laboratory where researchers can analyze stellar evolutionary processes since it is comparatively closeby, includes a rich stellar population, and is relatively young. The cluster is located in a stellar breeding ground called Gum 29, situated about 14,000 light-years away in the constellation of Carina (The Ship’s Keel).

It is hard to observe this stellar nursery since it is surrounded by dust. However, Hubble’s Wide Field Camera 3 has the ability to see through the dusty veil in near-infrared light, thus offering astronomers a clear view of the cluster. Using Hubble’s sharp vision, the astronomers resolved and analyzed the dense concentration of stars in the central cluster.

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. 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.

Danny Lennon, Team Member, Instituto de Astrofísica de Canarias and Universidad de La Laguna

Sabbi and her colleagues discovered that among the approximately 5000 stars in Westerlund 2 with masses ranging from 0.1 to 5 times the mass of Sun, 1500 of them exhibit drastic luminosity fluctuations, which is widely regarded as being due to the existence of huge dusty structures and planetesimals.

Some of the starlight would be temporarily blocked by the orbiting material, leading to fluctuations in brightness. But Hubble could detect only the signature of dust particles surrounding the stars residing outside the central region. These dips were not detected in the brightness of stars located within four light-years from 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 centre.

Elena Sabbi, Lead Researcher, Space Telescope Science Institute, Baltimore, USA

With Hubble, astronomers were able to observe how stars accrete in environments that similar to the early Universe, where monster stars dominated the clusters. To date, the starbirth region in the Orion Nebula is the best known adjacent stellar environment including massive stars. But Westerlund 2 is a richer target as it has a higher 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 discs and the importance of stellar feedback in modifying the properties of these systems,” added Sabbi. “We can use all of this information to inform models of planet formation and stellar evolution.”

Moreover, this cluster will be a perfect target for follow-up observations using the upcoming NASA/ESA/CSA James Webb Space Telescope, an infrared observatory. Using Hubble, astronomers have been able to find the stars with possible planetary structures.

The Webb telescope will enable astronomers to investigate the disks surrounding stars that do not accrete material and those that still have material that could become planets. Webb will also analyze the chemistry of the disks in various stages of evolution and observe how they transform, which will be helpful for astronomers to identify the role played by the environment in their evolution.

A major conclusion of this work is that the powerful ultraviolet radiation of massive stars alters the discs around neighbouring stars,” added Lennon. “If this is confirmed with measurements by the James Webb Space Telescope, this result may also explain why planetary systems are rare in old massive globular clusters.”

Journal Reference:

Sabbi, E., et al. (2020) Time-domain study of the young massive cluster Westerlund 2 with the Hubble Space Telescope. I. The Astrophysical Journal.


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