Morning fog disappears when the sun rises. Can you imagine what happens when the sun would be much hotter and brighter? The fog will disappear much sooner, of course.
Astronomers believed that the same would be true for newborn stars. They are often surrounded by disks of dust and gas. So you would expect that the gas – a form of cosmic fog – disappears much faster when the star is hotter and brighter. Surprisingly, ALMA has found something different.
Astronomers used ALMA to study 24 young stars, with ages between 5 and 10 million years. Dust particles around these stars have already started to accumulate into asteroids and comets. Collisions between those small bodies produce smaller debris. That’s why the stars are surrounded by so-called debris disks.
Three of the debris disks observed by ALMA turned out to contain large amounts of carbon monoxide gas. Strangely enough, those were disks surrounding the heftier stars – about two times the weight of our own sun. These stars are also much hotter and brighter than the sun. In contrast, the smaller, cooler and fainter stars – more like our own sun – had debris disks without gas.
This is the opposite of what you would expect. It’s unclear where the carbon monoxide gas comes from. Maybe the heavy stars haven’t been able to blow away the gas as efficiently as you would expect. Or maybe the disks are replenished with new gas: the carbon monoxide may have been released by the collisions of comets.
The new observations shed new light on the birth of giant planets. If the debris disks of massive stars can contain large amounts of gas for millions of years, there’s also more time for the formation of gas planets like Jupiter or Uranus.
The 24 stars that have been studied by ALMA are part of a stellar nursery in the constellations Scorpius and Centaurus. This star-forming region contains a few hundred stars, aged between 5 and 10 million years. Most are surrounded by debris disks. Many stars are relatively small and light-weight, like our own sun. Some are much more massive – they weigh around twice as much as the sun.
The new observations were carried out during six nights between December 2013 and December 2014, by a team of astronomers led by Jesse Lieman-Sifry. Jesse is an undergraduate student at Wesleyan University in Middletown, Connecticut. He worked together with Meredith Hughes, also at Wesleyan University, and with four other astronomers: John Carpenter (California Institute of Technology, Pasadena), Uma Gorti (SETI Institute, Mountain View, California), and Antonio Hales and Kevin Flaherty (National Radio Astronomy Observatory, Charlottesville, Virginia). The discovery has been published in The Astrophysical Journal.