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A first look at a forming world

Astronomers have got a view of the water-snow line around a star, which they call V 883 Ori, located in the Orion Nebula Cluster.

July 13, 2016 11:22 pm | Updated July 14, 2016 10:16 am IST

An artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Photo: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

An artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Photo: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Astronomers using the Atacama Large Millimeter/Submillimeter Array (ALMA) have got a view of the water-snow line around a star, which they call V 883 Ori, located in the Orion Nebula Cluster. This is a discovery that can inform research on the formation of planets around a protostar - a star in the making. The research will be published in journal Nature on July 14.

The snow line or frost line marks the distance from the protostar where volatile substances condense. There are different snow lines for water, carbon monoxide, ammonia, methane etc. While carbon monoxide-snow lines have been identified earlier, this is the first time a water-snow line has been found. Its location can determine the architecture of planetary systems: rocky planets like the earth form inside this line, while gaseous and giant planets form outside.

The discovery has implications for habitability. For instance, how the earth is really a very dry planet. As Lucas A. Cieza, first author of the paper says, “At the location of the proto-Earth, water was in the form of vapor and couldn't be accreted by the planet. The water of the Earth came from comets and asteroids that have a lot of water precisely because they formed outside of the water snow-line.”

The researchers also find that the position of the snow line can vary a lot. “When the Solar System formed, the water snow-line was mostly between the orbits of Mars and Jupiter, if it had been much farther away from the Sun, perhaps the Earth wouldn't have received enough water-rich comets and asteroids to form the ocean and we wouldn't have any life on Earth (at least no life as we know it),” says Dr Cieza.

The water-snow line is difficult to observe under normal circumstances because it lies close to the protostar, at about 5 AU (Astronomical Unit – one AU is equal to the average distance between the sun and the earth.). In the case of protostar V 883 Ori, the line has moved away from the centre, to a distance of about 42 AU, because the star itself is undergoing an outburst. The outburst is due to a temporary in crease in the rate at which the star grows and consumes the material around it and causes an increase in luminosity.

The team discovered it almost by accident. “We expected to see clumps, for example, the forming planets, and spiral arms. Instead, we saw a ring in the disk, so at the beginning we were very confused and a little disappointed! At the end, it turned out that the temperature of the disk at the location of this ring was consistent with the expected temperature of the water-snow line and we had obtained the first-ever image of the water snow line in a protoplanetary disk,” says Dr Cieza.

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