Though many people may not have known it was absent, astronomers say they have found the missing stellar sulfur. The cosmically abundant element is present in meteorites and dust particles from comets in the form of solid grains of iron sulfide (FeS). But direct evidence of such sulfide grains from objects similar to the early solar nebula was lacking, until now. According to a report published today in the journal Nature, FeS does indeed exist in the spectrum of young stellar objects. Problem was, it had been mistaken for something else.
Lindsay P. Keller of the NASA Johnson Space Center and his colleagues located the lost sulfur by comparing the infrared spectra of meteorites, cometary dust particles and laboratory standards containing FeS with recent spectra of protostars obtained by the Infrared Space Observatory (ISO). In so doing, they found there was an excellent match (in terms of peak position, width and shape) for a region in the ISO spectra that was previously attributed to iron oxide. These findings, the authors write, imply "that FeS grains are an important but previously unrecognized component of circumstellar dust." The scientists propose that the bulk of this FeS is formed either in the collapse phase of the molecular cloud or in the disk surrounding the young star. The team further determined that the grains of FeS must be minisculeless than 200 nanometers thickin order to produce the ISO spectra. Considering the difficulty many people have finding objects much larger than this, such as keys, the astronomers' find seems all the more impressive.