In their early stages of formation, the objects that will eventually become stars are small. They grow by gathering material from the surrounding cloud of gas. At least, that's what current theories tell us what happens. Due to the difficulty of resolving star systems during their formative years, most observations have been from later periods of their evolution, after the protostar has reached a substantial fraction of its final size and mass.
A new observation has revealed the youngest protostar yet observed. John J. Tobin and colleagues measured the properties of the newborn star and its environment, determining that it had only accreted about 20 percent of the matter surrounding it, and hasn't even begun nuclear fusion. Based on this, the protostar was likely no more than 300,000 years old at the time of observation, with the distinct possibility that it was even younger.
Interstellar space contains gas (mostly hydrogen) and dust, the name we give to heavier molecules and aggregates. Most of this material is very sparse, but about 5 percent of it is collected in cold, relatively dense clouds. If some instability hits—a nearby supernova or some other shock—portions of a cloud may collapse, creating a glowing aggregation of matter with a central core.
The process of matter falling onto the core emits a lot of infrared light, which heats things up sufficiently to break molecules apart and ionize the atoms of a surrounding region known as the envelope. This newborn object is known as a class 0 (zero) protostar.
Material falling onto the core doesn't tend to plunge straight in, but instead strikes at angles, creating a spinning disk of matter within the envelope. While most of the mass of the disk will also accrete onto the protostar, it may also fragment into planets. A significant amount of material also gets channeled away from the protostar through jets aligned with the axis of the spinning core.
It takes some time for the core to reach sufficiently high temperatures to begin nuclear fusion. At that point, the protostar becomes a bona fide star. (I should note that not all collapse clouds become protostars, and not all protostars become stars.) Though astronomers have observed many of the steps in this process, there are still some gaps, including the very earliest, class 0 protostar stage. Observations have found protostars that had drawn in over two-thirds of the envelope material, meaning they were nearly done with growth, if not quite at the point of switching on fusion.
By contrast, the protostar known as L1527 (full name Roberta J. L1527) has only about 20 percent of the mass of the surrounding envelope, marking it as the youngest yet observed. The researchers measured dust in the region surrounding L1527 using the Submillimeter Array (SMA) in Hawaii and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California. Previous observations, using one of the Gemini telescopes in Chile, determined the protostar had a disk, which appears edge-on from our perspective on Earth.
The newer results determined both the thermal emission—the broad spectrum of light directly emitted from the warm gas and dust—and absorption by carbon monoxide molecules in the disk. (Specifically, they measured 13CO, the version of carbon monoxide involving the isotope carbon-13.) The excellent resolution of the telescopes enabled the astronomers to measure the total size and mass of each component of the system.
The team determined the mass of the protostar to be about 19 percent of the Sun's mass, but spread across a volume nearly seven times the Sun's. The disk surrounding it is about 180 astronomical units (AU) in diameter, comparable in size to our Solar System. (One AU is the average distance between Earth and the Sun; Neptune orbits at about 30 AU.) The disk mass is small (only about 0.7 percent of the Sun's mass), while the total mass of the envelope is about 5 times the mass of the protostar.
Together, these measurements point to a very young system: almost none of the mass of the envelope has been transferred to what will eventually be the star and its planets. On the other hand, the brightness of the protostar indicates that the accretion is actively going on, and is very rapid. This means the protostar is likely 300,000 years old, assuming a constant rate of accretion. A more realistic accretion model, in which initial rate of mass transfer is higher, would indicate an even younger age. L1527 appears to be a remarkably young protostar system.
Nature, 2012. DOI: 10.1038/nature11610 (About DOIs).
reader comments
21