Using data from NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope, astronomers analyzed the movement of carbon atoms in an expanding bubble of gas surrounding the star cluster Westerlund 2. The researchers found that a single bubble surrounds Westerlund 2 and disproved earlier studies suggesting there may be two bubbles surrounding the star cluster.

This image of the open star cluster Westerlund 2 and its surroundings has been released to celebrate Hubble’s 25th year in orbit. The image’s central region, containing the star cluster, blends visible-light data taken by the Advanced Camera for Surveys and near-infrared exposures taken by the Wide Field Camera 3. The surrounding region is composed of visible-light observations taken by the Advanced Camera for Surveys. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / A. Nota / Westerlund 2 Science Team.

Discovered by the Swedish astronomer Bengt Westerlund in the 1960s, Westerlund 2 is a giant cluster of about 3,000 stars.

Also known as ESO 127-18, the cluster is located approximately 20,000 light-years away in the constellation of Carina.

Westerlund 2 is about 10 light-years across and resides in a giant stellar nursery called Gum 29 (RCW 49).

It is only 2 million years old, but contains some of the brightest, hottest and most massive stars ever discovered.

“When massive stars form, they blow off much stronger ejections of protons, electrons and atoms of heavy metal, compared to our Sun,” said Dr. Maitraiyee Tiwari, a postdoctoral researcher in the Department of Astronomy at the University of Maryland and the Max Planck Institute for Radioastronomy.

“These ejections are called stellar winds, and extreme stellar winds are capable of blowing and shaping bubbles in the surrounding clouds of cold, dense gas.”

“We observed just such a bubble centered around the brightest cluster of stars in this region of the Galaxy, and we were able to measure its radius, mass and the speed at which it is expanding.”

The surfaces of these expanding bubbles are made of a dense gas of ionized carbon, and they form a kind of outer shell around the bubbles. New stars are believed to form within these shells.

But like soup in a boiling cauldron, the bubbles enclosing these star clusters overlap and intermingle with clouds of surrounding gas, making it hard to distinguish the surfaces of individual bubbles.

Dr. Tiwari and her colleagues created a clearer picture of the bubble surrounding Westerlund 2 by measuring the radiation emitted from the cluster across the entire electromagnetic spectrum, from high-energy X-rays to low-energy radio waves.

Previous studies, which only radio and submillimeter wavelength data, had produced low-resolution images and did not show the bubble.

Among the most important measurements was a far-infrared wavelength emitted by a specific ion of carbon in the shell.

“We can use spectroscopy to actually tell how fast this carbon is moving either towards or away from us,” said Ramsey Karim, a Ph.D. student in the Department of Astronomy at the University of Maryland.

“This technique uses the Doppler effect, the same effect that causes a train’s horn to change pitch as it passes you. In our case, the color changes slightly depending on the velocity of the carbon ions.”

By determining whether the carbon ions were moving toward or away from Earth and combining that information with measurements from the rest of the electromagnetic spectrum, the astronomers were able to create a 3D view of the expanding stellar-wind bubble surrounding Westerlund 2.

In addition to finding a single, stellar wind-driven bubble around the star cluster, they found evidence of new stars forming in the shell region of this bubble.

Their analysis also suggests that as the bubble expanded, it broke open on one side, releasing hot plasma and slowing expansion of the shell roughly a million years ago.

But then, about 200,000 or 300,000 years ago, a Wolf-Rayet star called WR 20 evolved in Westerlund 2, and its energy re-invigorated the expansion of the cluster’s shell.

“We saw that the expansion of the bubble surrounding Westerlund 2 was reaccelerated by winds from another very massive star, and that started the process of expansion and star formation all over again,” Dr. Tiwari said.

“This suggests stars will continue to be born in this shell for a long time, but as this process goes on, the new stars will become less and less massive.”

The findings were published in the Astrophysical Journal.


M. Tiwari et al. 2021. SOFIA FEEDBACK Survey: Exploring the Dynamics of the Stellar Wind-Driven Shell of RCW 49. ApJ 914, 117; doi: 10.3847/1538-4357/abf6ce

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