The sign of an invisible star
Modelling suggests an unseen second star explains an unusual gas disc 800 light years away. Andrew Masterson reports.
Is this the birth of a planet, or the signature of an unknown twin?
This image, captured by the Atacama Large Millimetre/submillimetre Array radio telescopes in Chile in 2016, shows a distinctive horseshoe-shaped dust cloud surrounding a star called HD142527.
The cloud has been of intense interest to astronomers ever since it was first discovered in 2013, and now researchers led by Daniel Price of Monash University in Australia think they might have at last worked out how it arises, and how it develops its shape.
HD142527, in the constellation of Lupus, about 800 light years from Earth, is a very young star, perhaps only about one million years old. Because of its age, it has a vast amount of dust orbiting it, in a formation known as a protoplanetary disc – an accretion of matter out of which planets form. The disc has a mass equivalent to about 15% of our sun.
ALMA imaging has shown a number of peculiarities about HD142527’s disc, including an enormous cavity in the middle, fast flowing material across it, visible streamers of gas, and spiral patterns.
Understanding how these arise has been the focus of several research projects, but now Price and his colleagues believe they may have found an answer – HD142527 has a twin.
To make their finding, the researchers ran simulations based on ALMA data through a supercomputer. The model that best recreated the look and activity of the protoplanetary cloud included the influence of second star.
“For the first time, we have shown how the dust horseshoe, cavity, fast flows, streamers, spirals and shadows could be explained with one simple answer: the disc orbits two stars not one,” explains Price.
“We showed how the second star carves a hole in the middle of the disc, creates a giant `horseshoe’ in the dust, produces spirals and streamers feeding across the hole and even the shadows that are seen.”
Binaries – two stars locked together by gravity – are not unusual, but Price’s team shows that the HD142527 set-up differs from most.
“Normally we would expect the two stars and the gas disc to orbit in the same direction — like the planets in our solar system all orbit in almost the same plane,” he says.
“In this case, the models show the two stars are orbiting in a plane which is almost 90 degrees to the disc. We used to think of planet formation as this quiet, slow process, but the new observations and models tell us that in some cases it may be chaotic and violent.”