November 30, 2012
Cosmic Dust Around A Failed Star Measured By ALMA
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered that the outer region of a dusty disc encircling a brown dwarf contains solid grains like those found in denser discs of newborn stars.The find challenges theories of how rock, Earth-scale planets form, suggesting the planets may be more common in the Universe than expected.
Scientists believe rocky planets form through random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains are similar to very fine soot or sand.
In the outer regions around a brown dwarf, astronomers believe that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Theories also say that any grains that manage to form should move quickly towards the central brown dwarf.
“We were completely surprised to find millimeter-sized grains in this thin little disc,” said Luca Ricci of the California Institute of Technology, who led a team of astronomers based in the United States, Europe and Chile. “Solid grains of that size shouldn´t be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do."
He said they are not sure if a whole rocky planet could develop there, but that they are seeing the first steps, so they have to change the assumptions about conditions required for solids to grow.
The telescope's increased resolution compared to previous telescopes allowed the team to pinpoint carbon monoxide gas around the brown dwarf. This discovery suggests that the disc is much more similar to the ones around young stars than previously expected.
The team pointed ALMA at the young brown dwarf ISO-Oph 102 in the Rho Ophiuchi star-forming region in the constellation of Ophiuchus. The brown dwarf is about 60 times the mass of Jupiter, but only 0.06 times that of the Sun. It has too little mass to ignite the thermonuclear reactions, but emits heat released by its slow gravitational contraction.
ALMA was able to collect light with wavelengths around a millimeter, emitted by disc material that was warmed by the brown dwarf. The grains in the disc do not emit much radiation, so a characteristic drop-off in brightness can be measured at longer wavelengths.
“ALMA is a powerful new tool for solving mysteries of planetary system formation,” according to Leonardo Testi from ESO, a member of the research team. “Trying this with previous generation telescopes would have needed almost a month of observing – impossibly long in practice. But, using just a quarter of ALMA's final complement of antennas, we were able to do it in less than one hour!” he said.
The telescope will eventually be powerful enough to make detailed images of the discs around Rho-Oph 102 and other objects.
“We will soon be able to not only detect the presence of small particles in discs, but to map how they are spread across the circumstellar disc and how they interact with the gas that we´ve also detected in the disc. This will help us better understand how planets come to be," Ricci said.