The formation and evolution of all objects in the universe, galaxies, stars, interstellar gas and dust, planetary systems, and even life, are tightly related to the origin and evolution of the chemical elements and therefore to the cosmic cycle of matter. Theories related to these phenomena hence need to be anchored to certain reference values for chemical abundances, traditionally chosen to be those of the Sun. However, whenever the current state of the chemical evolution of cosmic matter is of interest, B-type stars are better indicators. They allow an accurate spatial and temporal mapping of element abundances, providing a snapshot of the chemical composition of nearby stellar nurseries at the present day. B-stars are also preferable over some other present-day abundance indicators accompanying massive star formation, the H II regions. This is because the heavy elements in these luminous gaseous nebulae are partially depleted onto dust grains, which is difficult to quantify.

Nieva and Przybilla have performed a comprehensive study of a carefully selected sample of early B-type stars in the solar neighbourhood, shown schematically in Fig. 1. Various telescopes in the Northern and Southern hemisphere were used to obtain high-quality spectra of the sample stars. With this new data, sophisticated non-local thermodynamic equilibrium models, and a novel self-consistent analysis method, the stellar parameters and chemical abundances of these 29 massive stars could be derived with unprecedented accuracy and precision.

One of the most surprising results is that the stars analysed in this study show a high level of chemical homogeneity (varying only about 10%, see Fig. 2). This is independent of their location, does not depend on whether they are members in an OB association or field stars, and is also regardless of their temperature (15000 to 35000 K), mass (6 to 20 solar masses) and age (about 5 to 50 million years). This high degree of chemical homogeneity agrees with studies of absorption lines in the interstellar medium, but challenges all previous work on B-type stars in our Galactic vicinity, which claim a scatter of elemental abundances by a factor of 2 to 3.

The high degree of homogeneity lead Nieva and Przybilla to propose a present-day cosmic abundance standard (CAS) based on B-stars in the solar neighbourhood. Comparing these to the equally accurate solar abundances, one can identify similarities and differences in the abundances for individual elements that are of relevance for various aspects of astrophysical research. A few examples are detailed below.

One crucial test concerns the evolution of massive stars, which for the first time has been passed successfully for this kind of stars. The abundances of carbon, nitrogen and oxygen at the surface of the sample stars follow tightly the expected nuclear path specified by the CNO-cycle for initial CAS values.

The CAS also puts tight constraints on the composition and evolution of the local interstellar medium (ISM). By comparing ISM gas-phase abundances and the CAS, the amount of metals incorporated in the interstellar dust can be inferred, as only the ISM is affected by depletion onto dust grains. Moreover, such a study provides important constraints on the injection and mixing timescales of metals in the ISM.

As the CAS marks the present-day endpoint of chemical evolution in our galaxy, it provides highly accurate reference values for models of Galactic evolution (Fig. 3). The improvements achieved in spectral modelling of the B-type stars are evident in the smaller spread of abundance values, turning them into valuable precision tools for astrophysics. Together with results from older, solar-type stars of various ages, the CAS fits rather well into the established picture of nucleosynthesis over cosmic time.

When comparing the CAS to abundances of our Sun, the differences are most prominently seen in the C/O ratio with a disagreement of about 50%. On the other hand, there is an unexpected agreement between CAS and solar values for elements such as magnesium, silicon and iron (see Fig. 2). These elements should be enriched today due to the continuing nucleosynthesis since the formation of the Sun. This suggests that the Sun and probably many other older stars in the local vicinity were not formed here but have migrated to their current environment, supporting recent findings by colleagues from MPA Exploring the history of the Milky Way that stellar migration is as an essential ingredient for galactic evolution.

Maria Fernanda Nieva and Norbert Przybilla

Further information:

Nieva, M. F. and Przybilla, N., "Present-Day Cosmic Abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution", submitted to Astronomy and Astrophysics

Nieva, M. F. and Simon-Diaz, S., 2011, "The chemical composition of the Orion star-forming region. III. C, N, Ne, Mg and Fe abundances in B-type stars revisited", Astronomy and Astrophysics, in press