ESA - Massive stellar winds are made of tiny pieces

 
 

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 Artist’s impression comparing a smooth stellar wind (left) with a highly fragmented stellar wind (right) of a massive star like zeta Puppis. A decade’s-worth of observations with ESA’s XMM-Newton have revealed that the wind of zeta Puppis is fragmented into hundreds of thousands of individual hot (red) and cool (blue) clumps. Studying stellar winds is vital not only to understand mass loss from the star itself and thus its expected lifetime, but also how the winds inject material and energy into the surrounding environment and influence the birth and death of other stars.

 
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This portrait of a star-forming cloud, called NGC 346, is a combination of multiwavelength data from ESA’s XMM-Newton space-borne X-ray observatory, NASA's Spitzer Space Telescope and the European Southern Observatory's New Technology Telescope

The infrared observations highlight cold dust in red, visible data show glowing gas in green, and X-rays show very warm gas in blue. Ordinary stars appear as blue spots with white centres, while young stars enshrouded in dust appear as red spots with white centres.

The colourful picture demonstrates that stars in this region are being created by two different types of triggered star formation - one involving wind, and the other, radiation. Triggered star formation occurs when massive stars spur new, smaller stars into existence. The first radiation-based mechanism is demonstrated around the centre of the cloud. There, radiation from the massive stars is eating away at the surrounding dust cloud, creating shock waves that compress gas and dust into new stars. This compressed material appears as an arc-shaped orange-red filament, while the new stars within this filament are still blanketed with dust and cannot be seen.

The second wind-based mechanism is at play higher up in the cloud. The isolated, pinkish blob of stars at the upper left was triggered by winds from a massive star located to the left of it. This massive star blew up in a supernova explosion 50 000 years ago, but before it died, its winds pushed gas and dust together into new stars. While this massive star cannot be seen in the image, a bubble created when it exploded can be seen near the large, white spot with a blue halo at the upper left (this white spot is actually a collection of three stars).

NGC 346 is the brightest star-forming region in the Small Magellanic Cloud, an irregular dwarf galaxy that orbits our Milky Way galaxy, 210 000 light-years away.

 
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Nearly 12.5 million light-years away, in the dwarf galaxy NGC 4449, stellar fireworks on display have been captured by the Hubble Space Telescope’s Advances Camera for Surveys.

It is likely that the current widespread starburst in the galaxy
was triggered by interaction or merger with a smaller companion.

Observed in the visible (blue and green), infrared, and hydrogen-alpha regions of the spectrum, hundreds of thousands of vibrant blue and red stars are visible in this new image. Hot bluish white clusters of massive stars are scattered throughout the galaxy, interspersed with numerous dustier reddish regions where star formation is taking place. Massive, dark clouds of gas and dust are silhouetted against starlight.

5 February 2013 ESA’s XMM-Newton space observatory has completed the most detailed study ever of the fierce wind from a giant star, showing for the first time that it is not a uniform breeze but is fragmented into hundreds of thousands of pieces.
Massive stars are relatively rare, but play a very important role in recycling materials in the Universe. They burn their nuclear fuel much more rapidly than stars like the Sun, living only for millions of years before exploding as a supernova and returning most of their matter to space.
But even during their brief lives, they lose a significant fraction of their mass through fierce winds of gas driven off their surfaces by the intense light emitted from the star.
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,” says Yaël Nazé, Université de Liège, Belgium, who led the study’s analysis.
“This is the first time constraints have been placed on the number of fragments in a stellar wind of an adult massive star, a number which far exceeds theoretical predictions.”
To fully understand these observations, improved models of stellar winds will be needed, taking into account both the large-scale emission structures and the highly fragmented wind, in order to understand how they affect mass-loss in stellar giants. 
“Zeta Puppis also goes by the name Naos, which in antiquity was the name given to the innermost sanctuary of a temple, accessible to only a few people; thanks to XMM-Newton, scientists have been able to unlock the secrets of this mysterious stellar object,” adds Dr Nazé.
“This long-term XMM-Newton study of zeta Puppis has provided the first constraints on the number of fragments in a stellar wind from a massive star – there is no dataset with comparable sensitivity or time and or spectral coverage currently available for any other massive star,” says Norbert Schartel, ESA’s XMM-Newton project scientist.

 ESA

Guillermo Gonzalo Sánchez Achutegui

ayabaca@gmail.com

ayabaca@hotmail.com

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