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Imaging the water snow line within a protoplanetary disc

Research using the ALMA telescope by scientists including Leiden's John Tobin and Steven Bos has produced the first images of the water snow line within a protoplanetary disc. Publication in Nature on 14 July.

Spectacular increase in brightness

The snow line indicates where the temperature in the disc around a young star has dropped far enough to allow snow to form. No previous images have been made of this line. As a result of a spectacular increase in the luminosity of the young star V883 Orionis, the core of the disc heated up rapidly, shifting the snow line much further outwards than is normal for a protostar. Using ALMA (the powerful Atacama Large Millimeter/submillimeter Array in Chile), the researchers were able to make the first-ever observations of the snow line.

Looking for something different

The observation came as a surprise; the astronomers were looking for something quite different. Lead author Lucas Cieza (Universidad Diego Portales, Santiago, Chile) explained: ‘The ALMA observations came as a surprise to us. Our observations were designed to look for disc fragmentation leading to planet formation. We saw none of that; instead, we found what looks like a ring at 40 au (roughly 6 billion kilometres, or the size of the orbit of the planet Pluto in our solar system, ed.). This is a good illustration of ALMA's versatility, that is able to provide some highly exciting results, even when you are looking for something else.'

Directly from gas to solid

Young stars are often surrounded by a dense, rotating disc made up of gas and dust from which planets can be formed: a protoplanetary disc. The heat produced by an average young solar-like star means that the water within the protoplanetary disc remains gaseous at distances of up to roughly 3 au from the star. This corresponds with three times the average distance between the Earth and the Sun, or around 450  million kilometres. Further out, as a result of the extremely low pressure, water molecules transition rapidly from gas to solid matter, forming a patina of ice on dust and other particles. The area in the protoplanetary disc where water changes from a gas into ice is called the snow line. 

Exceptional star behaviour

But the star V883 Orionis exhibits some exceptional behaviour. A spectacular increase in brightness has caused the snow line to shift to a distance of around 40 au. This distant location and ALMA's high resolution at long base lines made it possible for the astronomers to observe for the first time the water snow-line in a protoplanetary disc. 

Brighter and hotter than the sun

Sudden outbursts such as those of V883 Orionis occur when large amounts of material from the surrounding disc land on the star. V883 Orionis is only 30% more massive than the Sun, but the present outburst means it is a staggering 400 times more luminous - and much hotter.

Snow crucial for planet formation

The bizarre idea of snow orbiting in space is fundamental to planet formation. The presence of water ice regulates the efficiency of the coagulation of dust grains — the first step in planet formation. Within the snow line, where water is vapourised, smaller, rocky planets like our own are believed to form. Outside the water snow line, the presence of water ice allows the rapid formation of cosmic snowballs, which eventually go on to form massive gaseous planets such as Jupiter.

A common phenomenon?

The discovery that these outbursts may blast the water snow line to about 10 times its typical radius is very significant for the development of good planetary formation models. Such outbursts are believed to be a stage in the evolution of most planetary systems, so this may be the first observation of a common occurrence. In that case, this observation from ALMA could contribute significantly to a better understanding of how planets throughout the Universe formed and evolved.

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