Massive stellar winds made of hundreds of thousands of tiny pieces
Washington, February 6: With the completion of the most detailed study ever of the fierce wind from a giant star, ESA's XMM-Newton space observatory has revealed for the first time that the stellar wind is not a uniform breeze but is fragmented into hundreds of thousands of pieces.
The winds from massive stars are at least a hundred million times stronger than the solar wind emitted by our own Sun and can significantly shape their surrounding environment.
They might trigger the collapse of surrounding clouds of gas and dust to form new stars or, conversely, blast the clouds away before they have the chance to get started.
Despite their important role, however, the detailed structure of the winds from massive stars remains poorly understood. Are they steady and uniform, or broken up and gusty?
Astronomers have now gained a detailed glimpse into this wind structure by taking observations with XMM-Newton spread over a decade to study variability in the X-ray emission from Zeta Puppis. One of the nearest massive stars to Earth, it is bright enough to be seen with the naked eye in the constellation of Puppis, in the southern hemisphere.
The X-rays arise from collisions between slow- and fast-moving clumps in the wind, which heats them to a few million degrees. As individual colliding clumps in the wind are heated and cooled, the strength and energy of the emitted X-rays vary.
If only a small number of large fragments are present, variations in the combined emission could be large. Conversely, as the number of fragments grows, a change in the X-ray emission from any given fragment becomes less important, and the overall variability decreases.
In Zeta Puppis, the X-ray emission was found to be remarkably stable over short timescales of just a few hours, pointing to a very large number of fragments. There must still be clumps in the wind to make X-rays in the first place, but there must be many of them to yield such low variability.
However, unexpected variation in the emission was seen on the order of several days, implying the presence of a few very large structures in the wind, possibly spiral-arm-like features superimposed on the highly fragmented wind co-rotating with the star.
"Studies at other wavelengths had already hinted that the winds from massive stars are not simply a uniform breeze, and the new XMM-Newton data confirm this, but also reveal hundreds of thousands of individual hot and cool pieces," said Yael Naze, Universite de Liege, Belgium, who led the study's analysis.