Have you ever watched an ice skater performing a pirouette? As she draws in her arms, she automatically starts to rotate much faster. In fact, every rotating object that gets smaller will speed up its spin rate – it’s a law of nature. So the same is also true for clouds of gas and dust that contract to form new stars.
This has always been a puzzle for astronomers. If the contracting cloud starts to rotate faster and faster, the gas will eventually be flung outward – just like wet clothes are flung outward in a centrifuge. But if the gas is flung into space, it will never be able to reach the forming star. So how can young stars grow to a mature size at all?
A large group of Japanese astronomers has now found part of the answer. Together with French colleagues, they used ALMA to study a young ‘protostar’ called L1527. This star-in-the-making is surrounded by a rotating disk from which planets may eventually form. Beyond the disk is a much larger cloud of cold gas and dust from which the star has condensed in the first place. The large cloud surrounds the disk like an envelope.
The ALMA observations confirm that the material in the envelope is flowing inward. The closer it gets to the star, the faster it starts to whirl around. But at the outer edge of the disk, the gas and dust particles appear to pile up, like cars in a traffic jam. This is the region where you would expect the material to be flung into space. However, because of the ‘traffic jam’, that’s not easily possible.
Instead, the gas and dust is deflected upward and downward from the plane of the disk. It flows away from the protostar in a vertical direction. In doing so, it carries away part of the ‘rotational energy’ – officially known as angular momentum. As a result, the remaining gas and dust enters the disk with a much lower spin rate, and the central star is able to grow.
The astronomers point out that this is only a first step in solving the puzzle of star formation. Future ALMA observations of other protostars will hopefully provide an even better picture.
L1527 is a young protostar in the constellation Taurus, the Bull. It is located in a large cloud of cold, molecular gas at a distance of 450 light-years. The star-in-the-making is surrounded by a rotating disk of gas and dust from which planets may form. From the Earth, this disk is seen almost exactly edge-on. This makes it much easier to study the vertical structure of the disk, and of the surrounding ‘envelope’ of colder material. The border region between the inner disk and the outer envelope lies at a distance of some 15 billion kilometers from the central star – this is the region where the astronomers have detected the traffic jam-like pile-up of gas and dust, and where the vertical outflow originates.
The ALMA observations of L1527 were carried out by a large group of astronomers, led by Nami Sakai. Nami works at the RIKEN Star and Planet Formation Laboratory in Saitama, Japan. Most of Nami’s colleagues in this study are also from various institutes in Japan, but she also teamed up with astronomers from France. The scientists published their results in a professional astronomy magazine called Monthly Notices of the Royal Astronomical Society.