ALMA sees ‘snow’ in planet-forming disk around newborn star

ALMA sees ‘snow’ in planet-forming disk around newborn star

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Why is there snow on high mountaintops but not in the valleys between the mountains? Easy: it’s much colder at higher altitudes. That’s why you see a ‘snow line’ on the slope of the mountain. The snow line marks the place where the temperature drops below zero degrees.

Something similar happens in a disk of gas and dust around a newborn star. The material close to the young star is hot. Farther from the star, the gas and dust is much colder. Where the temperature is low enough, you would expect a snow line to form.

In fact, planet-forming disks around newborn stars have a number of different snow lines. First, water vapor condenses into ice crystals, comparable to regular snow. Further out, where the temperature is lower still, other gases also freeze out: carbon dioxide, methane and carbon monoxide.

Using ALMA, astronomers have now for the first time ever pinned down the location of the carbon monoxide snow line in a proto-planetary disk around the newborn star TW Hydrae. This was done before the observatory was completed. Future observations may also reveal other snow lines.

The astronomers could not see the carbon monoxide snow directly. Instead, they mapped the millimeter radiation of another gas, called diazenylium. Diazenylium is easily destroyed by carbon monoxide gas. Therefore, if you see diazenylium, you can be sure that carbon monoxide gas must have frozen out into ice crystals.

Knowing more about ice in planet-forming disks is important. The ice crystals attach to dust particles, making them rougher. The result is that the dust particles stick to each other more easily. Eventually, they grow into pebbles, rocks and planets.

Studying carbon monoxide ice is also important because it is needed to form molecules of methanol – a building block of life.

TW Hydrae is a star at a distance of 175 light-years in the constellation the Water Serpent. It is less than ten million years old, and it is somewhat comparable to our own sun. So observing TW Hydrae is like looking at a baby photo of the sun!

This study was led by Charlie Qi of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Karin Oberg of Harvard University and the University of Virginia in Charlottesville. Charlie and Karin worked together with many other astronomers in the United States, Mexico and Europe. The results were published on the scientific website SciencExpress on 18 July 2013.