SOFIA Enables First Clear Look into Star-Forming Region Westerlund 2

Researchers obtained the first clear picture of a step in star formation using the Stratospheric Observatory for Infrared Astronomy, or SOFIA, long-term program called FEEDBACK.

Using the German REceiver for Astronomy at Terahertz Frequencies, or GREAT, in one of its advanced configurations called upGREAT, SOFIA’s FEEDBACK program enabled high-resolution insights into the star-forming region called Westerlund 2 — one of the brightest and most massive star formation regions in the Milky Way. When massive stars begin to form, they emit large quantities of protons, electrons, and heavy atoms, which together are called stellar winds. In extreme cases, the stellar winds can create bubbles of hot plasma and gas in their surrounding clouds.

A color image of the emissions in RCW 49 showing the shell, ridge, inner dust ring, and transition boundary.
A color image of the emissions in RCW 49, the star-forming region of Westerlund 2. Credit: Tiwari et al.

Maitraiyee Tiwari, a postdoctoral researcher, as well as her team at the University of Maryland, analyzed the star cluster Westerlund 2, and found the cluster is surrounded by one of these expanding bubbles of warm gas. They also identified the origin of this bubble, its size, and the energy that drives its expansion. The results were recently published in The Astrophysical Journal.

Tiwari and her team created the detailed picture of Westerlund 2 by measuring the radiation emitted by the star cluster across the entire electromagnetic spectrum, from high-energy X-rays to low-energy radio waves. Previous data in the radio and sub-millimeter wavelengths showed neither the bubble nor how it expanded into the surrounding gas.

The most important measurements of the current study include the far-infrared data on ionized carbon at a wavelength of 157 µm, which can currently only be observed with SOFIA. Due to the expansion movement of the bubble, the wavelength of this line is slightly stretched or compressed, which leads to a so-called red or blue shift — depending on whether the bubble is moving away from the earth or toward it. Based on this wavelength shift, Tiwari and her colleagues were able to determine how the bubble is expanding. Combined with the measurements from the rest of the electromagnetic spectrum, it results in a 3D view of the stellar wind around Westerlund 2.

The researchers also found evidence of the formation of new stars in the envelope region of this bubble. According to their findings, the bubble ruptured about one million years ago, releasing hot plasma and slowing the expansion of its envelope. After another two hundred thousand to three hundred thousand years, however, another particularly bright star — called a Wolf-Rayet star — developed in Westerlund 2, and its strong winds restimulated the bubble, leading the process of expansion and star formation to begin once again.

Stars — albeit less massive ones — will continue to be born in this shell for a very long time.

Researchers aim to understand the primary processes that drive and regulate star formation, and how these processes differ between different star formation regions. So far, it appears that the expansion is always there but can differ in different star formation regions.