Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have for the first time directly imaged a snow line at another star.
This is an artist’s impression of the snow line in TW Hydrae showing water ice covered dust grains in the inner disc, shown in blue, and carbon monoxide ice covered grains in the outer disc, shown in green (B. Saxton / A. Angelich / NRAO / AUI / NSF / ALMA / ESO / NAOJ / NRAO)
On Earth, snow lines form at high altitudes where falling temperatures turn the moisture in the air into snow. This line is clearly visible on a mountain, where the snow-capped summit ends and the rocky face begins.
The snow lines around young stars form in a similar way. Starting from the star and moving outwards, water is the first to freeze, forming the first snow line. Further out from the star, as temperatures drop, more exotic molecules can freeze and turn to snow, such as carbon dioxide, methane, and carbon monoxide.
These different snows give the dust grains a sticky outer coating and play an essential role in helping the grains to overcome their usual tendency to break up in collisions, allowing them to become the crucial building blocks of planets and comets. The snow also increases how much solid matter is available and may dramatically speed up the planetary formation process.
Each of these different snow lines – for water, carbon dioxide, methane and carbon monoxide – may be linked to the formation of particular kinds of planets.
Around a Sun-like star in a planetary system like our own, the water snow line would correspond to a distance between the orbits of Mars and Jupiter, and the carbon monoxide snow line would correspond to the orbit of Neptune.
The snow line spotted by the astronomers is the first glimpse of the carbon monoxide snow line around TW Hydrae. It is a young star located in the constellation Hydra about 176 light-years away.
Astronomers believe TW Hydrae shares many of the same characteristics of the Solar System when it was just a few million years old.
This image shows the region where carbon monoxide snow has formed around TW Hydrae (ALMA / ESO / NAOJ / NRAO)
“We’ve had evidence of snow lines in our own Solar System, but now we’re able to see one with our own eyes. That is exciting,” said Prof Edwin Bergin from the University of Michigan, co-author of the study published online in Science Express.
“ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of the Solar System,” added lead author Dr Chunhua ‘Charlie’ Qi from the Harvard-Smithsonian Center for Astrophysics.
“We can now see previously hidden details about the frozen outer reaches of another planetary system similar to our own.”
______
Bibliographic information: Chunhua Qi et al. Imaging of the CO Snow Line in a Solar Nebula Analog. Science Express, published online July 18, 2013; doi: 10.1126/science.1239560
