Spitzer Spots Protostar's 'Crystal Rain'

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I never meant to cause you any sorrow I never meant to cause you any pain I only wanted one time to see you laughing I only wanted to see you laughing in the purple rain. –Prince, “Purple Rain” (1984)

Okay, so maybe NASA’s Spitzer Space Telescope hasn’t found purple rain, like in Prince’s classic pop tune. But it has observed a strange kind of greenish “crystal rain” falling onto a protostar called HOPS-68 in the constellation Orion.

The crystals in question are known as forsterite, a kind of silicate that belongs to the olivine family of minerals — the same substance found on Hawaii’s green sandy beaches, or in peridots, a semiprecious gemstone. They’ve been detected on certain comets, and in the gassy disks where planets form, but this is the first time forsterite crystals have been spotted in the outer collapsing cloud of a star-in-process.

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That’s because that cloud in which a protostar forms is much colder — 170 degrees Celsius below zero, to be exact, which is usually not conducive to the formation of such crystals. You need hot temperatures on a par with lava for forsterite to form. So spotting the mineral in the core cloud, in turn, gives astronomers a clue as to what’s going on inside those clouds, which are “dark” and can only be spotted through infrared-sensitive instruments like those aboard the Spitzer Space Telescope.

At such low temperatures, gases start to group together densely like molecules, usually in the form of carbon monoxide and H2. When the density reaches a certain point, the cloud core — which is denser than the outer cloud — collapses under its own weight, due to gravity.

As the core collapses, it fragments into clumps, which then forms into a protostar. All of this happens over the course of around 10 million years.

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So how did the crystals get there? A new paper in Astrophysical Journal Letters suggests that jets of gas are responsible, shooting away from the star about to be born. The crystals form near the surface of the protostar, then are carried into the surrounding cloud by the jets, where they cool and then fall back down onto the protostar like tiny bits of glitter.

It would be quite a sight, apparently, from the view inside the core cloud. “If you could somehow transport yourself inside this protostar’s collapsing gas cloud, it would be very dark,” says lead author Charles Poteet (University of Toledo) in NASA’s official press release. “But the tiny crystals might catch whatever light is present, resulting in a green sparkle against a black, dusty backdrop.”

Comets, too, might contain the crystals for similar reasons. These balls of ice formed in the very cold regions at the edge of our solar system, even colder than the core clouds of protostars, so the presence of forsterite crystals is unusual there, too. The best theory so far is that the crystals formed in a similar planet-forming disk back when the solar system was young and then migrated to the colder outer regions.

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But Poteet and his co-authors now propose that it’s likely this migration process is helped along by jets lifting the crystals into the collapsing cloud. Then the crystals rained down into those outer regions and eventually froze into comets.

At any rate, it’s an intriguing new insight into the complex interplay of raw materials swirling around the cosmic soup that eventually coalesce into stars, planets and comets.

Image credit: NASA

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