Snow forecast for distant star proves correct

Snow was always in the forecast out on the sun-like star TW Hya. It’s just that astronomers were never sure they would actually find it and “see” it.

That changed recently when an international team of researchers trained a powerful array of radiotelescopes on this solar system precursor, located about 175 light years from Earth. What they chronicled may answer questions about how the collision and accretion of celestial particles led to comets and planets in our own solar system.

The findings of the team, led by the Harvard-Smithsonian Center for Astrophysics and University of Virginia, were published online Thursday in SciencExpress.

The “snow” lines that form in a dusty disc around young stars are basically bands of different compounds and elements that freeze at varied distances from their star, in a fashion similar to the snow lines that form as temperature drops in high mountains.

By helping particles clump instead of shatter, these frozen bands help form planets – rocky ones like ours in near areas, big gaseous planets elsewhere.

Water, the snow we know, forms the first line. Farther out, carbon dioxide, methane and carbon monoxide form their own frozen lines.

The band that most interests astronomers is carbon monoxide, largely because the compound is a root for the more complex organic molecules that can lead to life. That band was located about the same distance from its star as Neptune is from our sun.

At these distances, observation is never easy. But the team, which included members from the National Autonomous University of Mexico, University of Michigan, Caltech, the Leiden Observatory in the Netherlands, and the Max Planck Institute in Germany, found a clever way of defining what they couldn’t see: diazenylium.

This celestial ion, also called protonated dinitrogen (NH2+), has a complicated relationship with carbon monoxide. When carbon monoxide is in its gaseous form, as it is at normal atmospheric pressures on Earth, it thwarts the formation of diazenylium by stealing a proton from its hydrogen isotope. So, there would be less of it where carbon monoxide is still a gas. But in the thermal layers where carbon monoxide vapor condenses to ice, you’d expect to see more diazenylium than usual.

That simple astrochemical relationship was enough for the team to calculate the distribution of CO and establish the boundary of the ice band. The result: a chemical “picture” of the band at a radius of 30 astronomical units – 30 times the distance from Earth to the sun.

And that turned out to be precisely what calculations had predicted.