The star cluster Westerlund 2
Westerlund 2 — Hubble’s 25th anniversary image
Wide-field image of Westerlund 2 (ground-based image)
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Pan across Westerlund 2
Flight through star cluster Westerlund 2 - slow
The NASA/ESA Hubble Space Telescope was
used to conduct a three-year study of the crowded, massive and young
star cluster Westerlund 2. The research found that the material
encircling stars near the cluster’s centre is mysteriously devoid of the
large, dense clouds of dust that would be expected to become planets in
a few million years. Their absence is caused by the cluster’s most
massive and brightest stars that erode and disperse the discs of gas and
dust of neighbouring stars. This is the first time that astronomers
have analysed an extremely dense star cluster to study which
environments are favourable to planet formation.
This time-domain study from 2016 to 2019 sought to
investigate the properties of stars during their early evolutionary
phases and to trace the evolution of their circumstellar environments [1].
Such studies had previously been confined to the nearest, low-density,
star-forming regions. Astronomers have now used the Hubble Space
Telescope to extend this research to the centre of one of the few young
massive clusters in the Milky Way, Westerlund 2, for the first time.
Astronomers have now found that planets have a tough time
forming in this central region of the cluster. The observations also
reveal that stars on the cluster’s periphery do have immense
planet-forming dust clouds embedded in their discs. To explain why some
stars in Westerlund 2 have a difficult time forming planets while others
do not, researchers suggest this is largely due to location. The most
massive and brightest stars in the cluster congregate in the core.
Westerlund 2 contains at least 37 extremely massive stars, some weighing
up to 100 solar masses. Their blistering ultraviolet radiation and
hurricane-like stellar winds act like blowtorches and erode the discs
around neighbouring stars, dispersing the giant dust clouds.
“Basically, if you have monster stars, their energy is going to alter the properties of the discs,” explained lead researcher Elena Sabbi, of the Space Telescope Science Institute in Baltimore, USA. “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.”
Westerlund 2 is a unique laboratory in which to study
stellar evolutionary processes because it’s relatively nearby, is quite
young, and contains a rich stellar population. The cluster resides in a
stellar breeding ground known as Gum 29, located roughly 14 000 light-years away in the constellation of Carina (The Ship’s Keel). The stellar nursery is difficult to observe because it is surrounded by dust, but Hubble’s Wide Field Camera 3
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
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,” 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.”
Sabbi and her team found that of the nearly 5000 stars in
Westerlund 2 with masses between 0.1 and 5 times the Sun’s mass, 1500 of
them 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. They did not detect these dips in brightness in stars
residing within four light-years of the centre.
“We think they are planetesimals or structures in formation,” Sabbi explained. “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.”
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 discs and the importance of stellar
feedback in modifying the properties of these systems,” said Sabbi. “We can use all of this information to inform models of planet formation and stellar evolution.”
This cluster will also be an excellent target for follow-up
observations with the upcoming NASA/ESA/CSA James Webb Space Telescope,
an infrared observatory. Hubble has helped astronomers identify the
stars that have possible planetary structures. With the Webb telescope,
researchers will be able to study which discs around stars are not
accreting material and which discs still have material that could build
up into planets. Webb will also study the chemistry of the discs in
different evolutionary phases and watch how they change, to help
astronomers determine what role the environment plays in their
evolution.
“A major conclusion of this work is that the powerful
ultraviolet radiation of massive stars alters the discs around
neighbouring stars,” said 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.”
Notes
[1] These observations were made under Hubble observing programs #14087, #15362, and #15514.
More Information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study
consists of E. Sabbi, M. Gennaro, J. Anderson, V. Bajaj, N. Bastian, J.
S. Gallagher, III, M. Gieles, D. J. Lennon, A. Nota, K. C. Sahu, and P.
Zeidler.
Image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team
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