Theory, observation, and the strange truth about colliding stellar winds

Theory, observation, and the s...
Colliding stellar winds release enormous amounts of energy
Colliding stellar winds release enormous amounts of energy
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Colliding stellar winds release enormous amounts of energy
Colliding stellar winds release enormous amounts of energy
HD 5980's locations in the star-forming region NGC 346
HD 5980's locations in the star-forming region NGC 346

Stellar winds thrown out by massive stars behave in surprising ways, according to observations made with the European Space Agency's XMM-Newton observatory. The colliding winds of large stars actually become brighter, and emit more X-rays, as the matter emitted after a stellar event subsides – contrary to expectations. In its search for a reason, the team found answers in a 2014 study, theoretical at the time of writing. Chalk one up for science, then.

What phenomenon are we talking about?

In some respects, this is regular old activity caused by the nuclear fusion happening within a star, causing gas to be emitted. As the wind travels out into space, it can collide with winds from other stars, releasing energy in enormous quantities.

Just how windy is this?

Very. As the ESA puts it, stellar winds travel at millions of kilometers per hour and can put out the mass of the Earth in the space of just one month.

HD 5980's locations in the star-forming region NGC 346
HD 5980's locations in the star-forming region NGC 346

And we're talking about two big stars here. The pair studied goes by the name of HD 5980 in the in the star-forming region NGC 346 within the Small Magellanic Cloud dwarf galaxy. Both are 60 times the mass of the Sun while only 100 million km apart – a good deal less than the distance of the Earth to the Sun.

And their size complicates things?

Apparently so. Following a major eruption of one of the stars in 1994, the team from the University of Liège studied the pairing between 2000 and 2005 using two space telescopes: ESA's XMM-Newton and NASA's Chandra X-ray Observatory.

But things only got weird in 2016 when they took another look with the XMM-Newton. "We expected HD 5980 to fade gently over the years as the erupting star settled back to normal – but to our surprise it did just the opposite," says astrophysicist Yaël Nazé. "We had never seen anything like that in a wind–wind collision."

Apparently the star pairing was two and half times brighter in 2016 than when first studied by the team, and even more vigorous with its X-ray emissions. This despite the stars putting out less matter than before.

But the team found a study that helped explain things?

Yes: Suppression of X-rays from radiative shocks by their thin-shell instability, published in Monthly Notices of the Royal Astronomical Society in 2014. It seems that, in the case of large stars in close proximity, their colliding winds can emit so much energy that the shocked material becomes unstable and the X-ray emissions dim – almost artificially. It's this unexpected dip that allows the emissions to rise over time following a spike in stellar activity in these conditions.

So this is a case of life imitating art?

If by life you mean observed astrophysical phenomenon and if by art you mean theoretical papers on shock collisions in astrophysics, then absolutely. The 2014 report was written by researchers from the University of Delaware and Pennsylvania State University – Worthington Scranton and based purely on planar simulation and analysis. But it appears the phenomenon observed from HD 5980 bears out that theory. We love when a planar simulation comes together.

Source: European Space Agency

1 comment
There are a lot of cosmic structures - cavities with tenuous, high temperature gas. The nomenclature of these cavities is not finally established yet, but in general there are cavities with a characteristic size of 10 pc – bubbles, about 100 pc – superbubbles, and extended up to 1 kpc or more – supershells. There are a lot of global outflows in our and other star-forming galaxies that percolate galactic disk and are streaming away in galactic halo. These are so called chimneys, fountains, winds and high velocity clouds. All of these structures have an enigmatic nature.