Behemoth star destroys potential solar systems

A massive star in the Orion Nebula is obliterating some planet nurseries while sparing others. Measurements of the dust swirling around Orion’s young stars reveal a zone of destruction near the massive star, one of the brightest in the nebula. Yet despite the carnage, many more planet-building disks survive in the harsh environment near the bright star than expected, a finding that could help researchers determine where solar systems can thrive in our galaxy.

The Orion Nebula, about 1,300 light-years away, is one of the closest star-forming neighborhoods. The nebula’s proximity makes it an attractive laboratory for understanding how stars and planets are born. The nebula envelops the Trapezium Cluster, a bundle of blue behemoths known as O stars, some of which shine roughly 250,000 times as bright as the sun.

Astronomers have long suspected that solar systems cannot form near O stars, whose abundant ultraviolet light should erode nearby planet nurseries. Rita Mann, an astronomer at the Herzberg Institute of Astrophysics in Victoria, Canada, decided to test this idea. She and her colleagues used the Atacama Large Millimeter/submillimeter Array, or ALMA, telescope in Chile to, for the first time, measure the faint radiation emitted by interstellar dust in the disks orbiting 48 young stars in the nebula. The micrometer-sized dust particles, composed of silicon- and carbon-rich compounds, are thought to be the building blocks of planets.

“We found that the vicinity where disks are destroyed is quite small,” Mann says. Within about 0.1 light-years of the O star Theta ¹ Orionis C, all the disks were almost completely demolished. But disks that are farther away appeared intact, although truncated. Since most stars form near massive stars, she says, the finding may help clarify where around O stars planetary disks can survive and solar systems can form. The observations appear in the March 20 Astrophysical Journal .

C. Robert O’Dell, an astronomer at Vanderbilt University in Nashville, hopes the findings help researchers crack a decades-old conundrum:  Given the generally accepted age of the nebula — about 2 million years — and  previous measurements of how quickly the disks are falling apart, there shouldn’t be any disks at all near the O stars in the Trapezium Cluster. “There’s something mysterious going on,” he says.

One possibility is that, until very recently, the cluster was shrouded in a dust shell that shielded the disks from the destructive UV radiation. Lynne Hillenbrand, a Caltech astronomer, has another idea: The stars that are holding on to their disks may be visiting Trapezium stars like Theta¹ Orionis C for the first time. Because the disk-enshrouded stars loop around the Trapezium Cluster on roughly million-year-long orbits, she says, they may have been exposed to the O stars’ radiation for only 100,000 years.

The new findings sampled only four dozen stars within about three light-years from Theta¹ Orionis C. Mann wants to expand her analysis once she receives data from a broader investigation of 300 other stars within a five light-year bubble around the O star. The Orion Nebula is important to astronomers, Mann says, because it’s probably similar to the environment where the solar system was born. Studying how massive stars influence what’s around them can give researchers a better handle on the origin of the sun and Earth.