NASA’s Spitzer Space Telescope has helped solve an infrared mystery surrounding baby star systems that has puzzled astronomers since the 1980s.
A star is born from the gravitational collapse of clouds of dust and gas. When the compressed star-forming cloud reaches a certain density, fusion reactions are triggered in the core — a young star is born. However, during this collapse, there is a natural rotation in the cloud, a rotation that is preserved by the star as it reaches maturity. Any leftovers from the stellar formation accumulates around the new star forming a spinning, flat protoplanetary disk that creates rocky bodies like asteroids and eventually planets.
During the 1980s, the Infrared Astronomical Satellite (IRAS) mission surveyed young star systems measuring the infrared light they emitted. The protoplanetary disk of gas and dust generates a strong infrared signal — the young star heats up the disk, which radiates in infrared wavelengths.
However, even during those early observations, astronomers noticed a discrepancy; the young star systems were generating too much infrared radiation.
Over the years, further infrared observations and more refined models have suggested that the simple “flat” structure of protoplanetary disks may need to be revisited. Revised theoretical models included a modification of the ‘classic’ protoplanetary disk, adding a halo of dusty material encapsulating the young, hot star. By doing this, more dust is heated than the disk scenario and could perhaps explain the excess in infrared radiation.
However, with the help of Spitzer and new 3-D models, astronomers think they have a more refined answer.
As the star-forming cloud collapses, the new star not only retains the angular momentum of the spinning cloud, it also collapses any magnetic fields contained within it. The magnetic field will thread through the protoplanetary disk creating huge loops, trapping gas, dust and plasma, enhancing the disk’s atmosphere. These huge arcs — like the bright coronal loops that are filled with hot plasma reaching high above the sun’s photosphere — could be what is responsible for the excess; starlight is blocked by the huge arcs, which are then heated to generate more infrared radiation.
“If you could somehow stand on one of these planet-forming disks and look at the star in the center through the disk atmosphere, you would see what looks like a sunset,” said Neal Turner of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. The disk, in this case, is not smooth or flat — the magnetic fields crate a fuzziness, forcing the starlight to heat more dust.
“The starlight-intercepting material lies not in a halo, and not in a traditional disk either, but in a disk atmosphere supported by magnetic fields,” said Turner. “Such magnetized atmospheres were predicted to form as the disk drives gas inward to crash onto the growing star.”
Astronomers now hope to continue refining this model by observing more protoplanetary systems with observatories like NASA’s SOFIA telescope, the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile and NASA’s James Webb Space Telescope.