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More nitrogen in a cluster

Stars of different ages and compositions form the oldest collections of matter in galaxies

DALCANTON, WILLIAMS & CALDWELL / ADAM EVANSIn the Milky Way and other galaxies, the oldest existing star systems, which emerged between 12 and 10 billion years ago, are enormous collections of matter containing hundreds of millions of stars. There are about 160 systems of this type that we know of in the Milky Way, distributed in the form of a halo around the galaxy. These formations, technically known as globular clusters, may hold the key to understanding some of the mysteries of the primordial Universe. Until the end of the past decade, the idea prevalent among astrophysicists was that all the stars in a cluster were formed at once and had the same basic chemical composition. More recent observations, however, have cast doubt on this model by showing that some globular clusters have several generations of stars, with different ages and different abundances of certain elements in the periodic table. In other words, the process of cluster formation is likely not to have been as simple as it was thought in the past.

A scientific article published on October 10, 2013 in Astrophysical Journal Letters by Brazilian astrophysicist Ricardo Schiavon, a professor at Liverpool John Moores University in England, lends additional force to the current suspicion. In the paper, Schiavon presents a kind of law that appears to govern the dynamics at play in the emergence of clusters: the larger the mass of this type of formation, the greater the quantity of nitrogen present in its stars. This correlation is being interpreted as evidence that, in reality, there are several generations of stars within the clusters and that the younger stellar populations are richer in nitrogen than the older ones. “For the first time, a solid empirical correlation has been established between a global parameter for globular clusters—their mass—and the chemical composition of their stars,” says Schiavon. “This link strongly suggests that the clusters did in fact undergo an internal chemical evolution.” With time, the interstellar medium of the clusters, made up of dust and gas, is thought to have become richer in nitrogen—produced and ejected by the first generations of stars formed there—and the greater amount of that element was gradually incorporated into the composition of the subsequent stellar populations within those systems.

Alongside colleagues in the United States and Canada, the Brazilian researcher found this correlation after having measured the integrated light—the average luminosity of all the stars in 72 clusters in Andromeda, the largest spiral galaxy in the vicinity of the Milky Way. In addition to studying the abundance of nitrogen, the researchers analyzed the amounts of carbon, iron, magnesium and calcium in the clusters. But they found no clear connection between mass and any of these elements. Although the clusters in our own galaxy are much closer, the researchers chose to work with the neighboring galaxy. “In a certain sense, it’s easier to study the Andromeda clusters than the ones in our own galaxy because we don’t need to look through the forest of stars sitting in the “primary plane” of our vision,” says astrophysicist Charlie Conroy of the University of California at Santa Cruz, a coauthor of the article. “But our findings should apply to clusters in any galaxy, including the Milky Way.”

Medium-sized stars
Nitrogen is synthesized in large quantities by medium-sized stars—those having a mass four to eight times greater than that of the Sun. Since the only correlation found was between cluster mass and the presence of that element in the stars within them, the astrophysicists suspect that the chemical enrichment process within that type of star formation came about through the incorporation of matter ejected by medium-sized stars. When they reach middle age, such stars eject a large quantity of mass in the form of star winds. This matter, greatly enriched with nitrogen, contaminated the gas where the younger generations were formed, making them richer in that element.

In the opinion of astrophysicist Beatriz Barbuy of the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP), a specialist in the chemical characterization of stellar populations (who was not involved in the study of the globular clusters), the work of Schiavon and his colleagues was well done and shows consistent results. “The correlation found between mass and nitrogen abundance is important in view of the considerable past resistance to the idea of self-enrichment in clusters,” Barbuy says. “It also confirms the current evidence that there are several subsequent stellar populations in clusters.”

Scientific article
SCHIAVON, R.P. et al. Star clusters in M31. V. Evidence for self-enrichment in old M31 clusters from integrated spectroscopy. The Astrophysical Journal Letters. V. 776, No. 1. Oct. 10, 2013.