Astronomers have come to realize during the past decade that most newborn stars are surrounded by disks of gas and dust, material that both helps the young star to grow, and that might evolve into planets around the star. No one yet knows exactly how or when these disks originate, but gravity begins assembling them sometime even before the star itself starts to burn its nuclear fuel.
Astronomers also now realize that the
processes of star formation, once thought to involve just the simple gravitational coalescence of material, is much more complex. As they grow, for example, stars can disgorge dramatic outflows of material -- narrow jets that squirt outward in opposing directions perpendicular to these disks. The role of the disk in the formation of stars and planetary systems has obvious importance for scientists trying to unravel the conditions in the solar system when the earth formed.
A team of four SAO astronomers, Chin-Fei Lee, Paul Ho, Tyler Bourke, and Qizhou Zhang, together with three of their colleagues from the Academia Sinica Institute of Astronomy and Astrophysics, have now found evidence that the matter in these jets is not simply ejected outward, it spirals outward. This twisting motion addresses a long-standing puzzle about star formation. New stars grow by accreting matter, but this is not always easy to do. The disk of material from which a star forms is rotating, and rotating matter has angular momentum. Angular momentum -- the tendency for a spinning object to continue spinning -- inhibits the gas from falling inward, and until the gas can sheds its angular momentum, the star cannot easily grow.
Using the Submillimeter Array (SMA), the scientists observed a young star with a known bipolar jet located about 1,000 light-years away in the constellation Perseus. The central young star is only about 20,000 years old; its mass is currently only six percent the mass of our sun, but it may grow via accretion into a star like the sun. The astronomers found clear evidence for rotation in this bipolar jet. While the gas blasts away from the star at a velocity greater than 200,000 miles per hour, it also swirls around at speeds of more than 3,000 miles per hour. It is this spiral motion in the jets that can dissipate angular momentum from the accretion disk, thereby helping the star continue to grow. As gas falls in towards the star, a fraction is apparently trapped by the jet and ejected outward, carrying angular momentum away with it. The new results go a long way toward resolving the problem of how new stars can dissipate the angular momentum that inhibits their growth.