It's a puzzle that has bedeviled astronomers and theorists for years: Do massive stars form in the same way as our sun or by some other process? Now a team of astronomers has gone a long way toward providing the answer by catching a massive star in the act of condensing. Their verdict is that this massive star, at least, seems to follow the same mechanism as smaller stars.
Based on decades of observations, astronomers know that sun-sized stars condense from vast clouds of dust and gas. But finding nascent massive stars has proven difficult. They are rare and form within just a few hundred thousand years, so it's unlikely that astronomers will spot one that is condensing and close enough to observe. Also, no star, heavy or light, forms out in the open. They are born in a cosmic womb of obscuring dust and gas, which prevents optical observations altogether and makes studies in other wavelengths tricky.
So the researchers used a new tool--the Near-Infrared Integral Field Spectrograph (NIFS) on the Gemini North telescope at Mauna Kea, Hawaii--to study one of the few, well-known, young and massive stars. With its ability to detect infrared light, NIFS pierced the gas and dust obscuring the star, known as W33A and located about 12,000 light-years away in the constellation Sagittarius. The detailed spectrum of the light allowed the astronomers to measure the relative speed and direction of motion of the material surrounding W33A.
The team found that W33A, which is about 10 times more massive than the sun, not only is surrounded by a disk of gas and dust from which it's condensing but is also ejecting fast-moving jets of particles from its north and south poles. As the team reports in an upcoming Monthly Notices of the Royal Astronomy Society, the disk and the jets are very similar to what is seen around young, low-mass stars. Moreover, the jets are the "smoking gun" that W33A is still growing, says astronomer and lead author Ben Davies of the University of Leeds in the United Kingdom. The next step, he says, is to see whether the jets are common to all massive protostars.
The observations are "beautiful," says astronomer Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. They "clearly show that at least some massive stars grow by accreting material from a circumstellar disk of gas and dust, just like our sun did roughly 4.5 billion years ago." The finding is "an exciting new result that further strengthens the case for a universal mechanism of star formation," says astronomer Jonathan Tan of the University of Florida, Gainesville. One question, Tan says, is how efficiently the process works for massive versus light stars, because that's likely to be the mechanism that determines stellar mass.