For the first time ever, Japanese researchers have been able to directly observe an astronomical process that has been elusive up until now—an early phase in stellar evolution that involves the formation of the gas disk that feeds nascent stars and births planets.
The observations were of a baby star (protostar) named TMC-1A, which is located 450 light years away from Earth, in the constellation Taurus. Like other stars, TMC-1A formed when an enormous gas cloud collapsed under the weight of its own gravity, and as it’s still forming, it’s still surrounded by the remains of that gas cloud.
Of course, baby stars draw on this sort of blanket of gas on to grow. However, the gas doesn’t flow directly into the protostar from outside; instead, it accumulates in a spiral, forming a hot, swirling disk around that star that then feeds into it.
But researchers are uncertain about when these gas disks form, and how they later evolve with the growing star, as previous technology (radio observations) has lacked the sensitivity and resolution to allow researchers to view this phenomenon.
“The disks around young stars are the places where planets will be formed,” said lead author Yusuke Aso, a graduate student at the University of Tokyo, in a Phys.org statement. “To understand the formation mechanism of a disk, we need to differentiate the disk from the outer envelope precisely and pinpoint the location of its boundary.”
Which is where the researchers stepped in. According to the paper in The Astrophysical Journal, they have successfully used the Atacama Large Millimeter/submillimeter Array (ALMA), an array of radio telescopes in Chile, to observe this boundary between the inner rotating disk (which “feeds” the star) and the outer infalling gas envelope (which provides the gaseous “nutrients” to the disk).
For TMC-1A in particular, this boundary happens to be located 90 astronomical units from the baby star’s center—three times longer than Neptune’s orbit of the Sun. The gas infall rate was measured to be a millionth of the mass of the Sun per year—at a speed of 1 kilometer per second (roughly 2,240 miles per hour). And, while gravity is causing this gas to fall inward, the gas isn’t falling as quickly as it would in normal freefall, indicating something (perhaps a magnetic field around the star) is slowing the gas down.
Moreover, the disk itself was observed to obey Keplerian rotation—as in the materials which orbit closer to the baby star revolve faster than material farther out. And, after measuring TMC-1A’s speed of rotation, they were able to calculate its mass: Currently, it’s 0.68 times the mass of the Sun.
“We expect that as the baby star grows, the boundary between the disk and the infall region moves outward,” said Aso. “We are sure that future ALMA observations will reveal such evolution.”
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Image credit: ALMA
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