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Stars and solar physics

Stars and solar physics

Flash Physics: Stars born near black holes, stormy superfluids, AAAS asks Trump to engage with scientists

29 Mar 2017 Sarah Tesh

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World‘s team of editors and reporters

Artist's impression of stellar formation within the powerful outflow of supermassive black holes
Extreme stars: stellar formation observed in the outflows of supermassive black holes. (Courtesy: ESO / M Kornmesser)

Stars born as supermassive black holes devour their surroundings

Stars are forming within the material blasted out by supermassive black holes. This extreme-environment stellar formation has been seen by astronomers using the European Space Observatory‘s (ESO) Very Large Telescope (VLT). Supermassive black holes exist at the centre of most galaxies and emit extremely powerful outflows of energy and material as they devour surrounding matter. “Astronomers have thought for a while that conditions within these outflows could be right for star formation,” says team leader Roberto Maiolino of the University of Cambridge in the UK, “but no one has seen it actually happening as it’s a very difficult observation.” The group used the VLT’s MUSE and X-shooter instruments to study the colossal jets emitting from a galaxy’s supermassive black hole 600 million light-years from Earth. By searching for the characteristic radiation signature of young stars, Maiolino and colleagues identified an infant stellar population within the outflow, containing stars less than a few tens of millions of years old. Due to the outflows’ extreme environment and fast-moving material, the young stars are hotter and brighter than typical stars and travelling at high velocity away from the galaxy centre. “The stars that form in the wind close to the galaxy centre might slow down and even start heading back inwards,” explains team member Helen Russell, “but the stars that form further out in the flow experience less deceleration and can even fly off out of the galaxy altogether.” The discovery, published in Nature, could lead to a better understanding of galaxy evolution.

Stormy superfluids could have boundary layers

Computer simulation of vortices

Despite having no viscosity, a superfluid will form a storm-like boundary layer as it flows along a rough solid surface. That is the surprising conclusion of George Stagg, Nick Parker and Carlo Barenghi of the University of Newcastle in the UK, who have done computer simulations of superfluid flow. A superfluid is a quantum state of matter that occurs at very low temperatures and is characterized by viscous free flow that, once started, can persist indefinitely. When an ordinary fluid flows along a surface, the liquid nearest the surface is slowed down by friction and this braking force is transmitted into the bulk of the fluid by its viscosity. This creates a boundary layer in which the flow increases gradually to that of the bulk fluid – something that should not occur in a superfluid with zero viscosity. Now, the Newcastle trio have calculated that vortices created when a superfluid flows over a rough surface could form a similar boundary layer. “Our computer simulations show that, remarkably, boundary layers can arise in viscosity-free superfluids, in a distinctly quantum mechanical form,” says Parker. While physicists expect these vortices to occur, they were surprised that their simulations suggest that the vortices tangle together tightly to form a swirling-storm-like layer that “sticks” to the surface. Furthermore, the flow velocity in the boundary layer increases smoothly to the bulk value – just like a conventional boundary layer. “This result points to the universality of boundary layers and sheds new light on the deep connection between superfluids and ordinary fluids,” says Parker. The simulations are described in Physical Review Letters.

AAAS asks Trump to engage with the scientific community on climate change

Rush Holt, the chief executive of the American Association for the Advancement of Science, has called on president Donald Trump and US policymakers to address the risks of climate change by engaging with the scientific community. The statement comes after Trump signed an executive order concerning the US Environmental Protection Agency (EPA) that aims to roll back current limits on carbon emissions. The order also calls for a review of restrictions on coal mining on federal land and a review of a policy that considers the “social cost of carbon” in the framing of federal regulations. “The scientific evidence is clear: climate change is happening – primarily due to human activities – and already impacting people and our environment,” says Holt, who is a physicist. “We encourage the White House and Congress to support the evidence on climate change, and welcome opportunities to bring scientists to meet with policymakers to discuss the state of the science, the degree of scientific understanding on climate change, and other areas of concern and interest.”

 

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