Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) have made the first direct observations delineating the gas disk around a baby star from the infalling gas envelope. The new findings fill an important missing piece in our understanding of the early stages of a baby star.
Stars form in dense gas clouds. Baby stars, in particular, grow by taking in the surrounding gas, like a fetus receiving nutrition from the mother's placenta. In this process, gas can't flow directly into the star. Instead, it accumulates and forms a disk around the star. Then, this disk feeds into the star. However, scientists are still unsure when in the process of star formation this disk appears and how it evolves.
"The disks around young stars are the places where planets will be formed," said Yusuke Aso, a graduate student at the University of Tokyo and lead author of the paper. "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."
Using ALMA, the team directly observed the boundary between the inner rotating disk and outer infalling envelope with high accuracy for the first time. Since gas from the outer envelope is continuously falling into the disk, it had been difficult to identify the transition region in previous studies.
In this latest study, the researchers found that the boundary between the disk and the envelope is located 90 astronomical units from the central baby star. This distance is three times longer than the orbit of Neptune, which is the outermost planet in the solar system. In addition, the disk obeyed Keplerian rotation, which means that the material orbiting closer to the central star revolves faster than material farther out.
"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."
The findings reveal a bit more about how baby stars are born and how they may eventually form planets over time. By better understanding this disk of material, researchers are understanding the very foundations of our own solar system.
