Do massive stars form by "feeding" off a surrounding disk of dust and gas -- or are they created when two smaller stars combine? Two independent teams of astronomers have now found new evidence that points to the first scenario, which is also the way in which small stars, like our Sun, are thought to form. The results could help settle the dispute between the two conflicting theories of high-mass star formation.
High-mass stars — those that are more than eight times the mass of the Sun — are relatively rare, but are important because they create heavy elements (beyond carbon) through the process of nucleosynthesis. However, they are not as well understood as smaller stars. Moreover, they are difficult to observe because they are generally much further away than lower-mass stars, being typically 7000 light years from Earth. Two exceptions are the Orion constellation (1500 light years away) and Cepheus-A — a breeding ground for young, high-mass stars that is about 2400 light years from Earth
There are currently two rival theories for how massive stars form. The first — accretion — is a “scaled-up” version of what happens when small stars form and involves the gravitational collapse of a dense cloud of gas and dust. The second occurs when lower mass stars within crowed star-forming regions collide and merge. Some astronomers have been reluctant to believe in the accretion scenario because the intense pressure of radiation created by larger stars should stop the influx of new material, therefore putting an upper limit on a star created this way. Another argument in favour of merger is the fact that young high-mass stars are often found in very dense clusters of stars.
Using the eight radio telescopes that form the Submillimeter Array in Hawaii, Nimesh Patel of the Harvard Smithsonian Center for Astrophysics in the US and colleagues have observed a high-mass protostar (called HW2) in the Cepheus-A region that is about 15 times the mass of the Sun (Nature 437 109). They discovered a rotating disk of dust and gas, which itself is quite massive — about one to eight times the mass of the Sun — at far-infrared wavelengths of 0.9-mm.
“These observations provide direct evidence favouring the accretion model for the formation of massive stars, at least those up to 15 solar masses,” says Patel. “Observing at submillimeter wavelengths — a relatively unexplored region of the electromagnetic spectrum — was crucial for this discovery.” These observations are currently only possible with the newly commissioned Submillimeter Array.
Meanwhile, a team led by Zhibo Jiang of the Purple Mountain Observatory in Nanjing, China, has discovered a similar disk structure around the Becklin-Neugebauer object — a star that is seven times more massive than the Sun (Nature 437 112). This team used high-resolution near-infrared imaging polarimetry to obtain its results. “Our work suggests that stars up to seven solar masses can be formed though gravitational collapse and subsequent mass accretion,” says Jiang.
Both teams now plan to make further observations to see if stars that are more massive can also be formed this way.
