Present-day cosmic elemental abundances from massive stars in the solar neighbourhood

Massive unevolved B-type stars are ideal indicators for present-day cosmic abundances as they conserve their pristine chemical composition and do not migrate far beyond their birth environments unlike older stars such as the Sun. Scientists from the Max Planck Institute for Astrophysics and the University of Erlangen-Nuremberg have now achieved a breakthrough in the spectral modelling of this kind of stars. Their investigations have shown that the present-day chemical composition of the matter in our Galactic neighbourhood is highly uniform. The use of these present-day cosmic abundances instead of the canonical solar abundances has interesting implications on the models of stellar and Galactic evolution.

Fig. 1: A schematic sketch of the Galactic neighbourhood around the Sun, out to about 1500 light years. The 29 sample stars are distributed in the Orion, Scorpius-Centaurus and other associations shown in blue/white as well as in the field within this area. Credit & copyright: Linda Huff & Priscilla C. Frisch.

Fig. 2: Abundance distribution of various chemical elements in the B-type star sample (red histograms) in comparison with data from the literature (black histograms). The much narrower distribution indicates a much higher uniformity in the present-day chemical composition of the cosmic matter in our Galactic neighbourhood than suggested by all previous work. Photospheric and protosolar abundances from the most recent study by M. Asplund and colleagues linkPfeil.gif The shining make-up of our star are indicated by coloured bars, representing the uncertainty range. The new solar abundances (for example for carbon and nitrogen) are more similar to this work than older solar values.

Fig. 3a: Evolution of the carbon/oxygen and nitrogen/oxygen ratios over cosmic times, represented by the incremental increase of the oxygen abundance. Green symbols represent long-lived solar-type stars, black symbols B-type stars from the literature. Solar photospheric abundances of Asplund and colleagues are indicated by the red circle.

Fig. 3b: Same as Fig 3a, but with the black symbols representing the B-stars of the present work. The smaller spread of abundance values clearly shows that the spectral modelling of the B-type stars has been improved.

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 linkPfeil.gif 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