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Pristine abundances of C, N, and... mixing issues in the stellar interiors

1er octobre 2006 Pristine abundances of C, N, and... mixing issues in the stellar interiors

A fundamental tool to explore the formation history of our Galaxy is to observe its oldest stars. Since the interstellar gas from which stars are formed is more and more enriched in heavy elements ("metals") synthetised in the stellar interiors by nuclear reactions, the mean to detect the oldest stars is to observe those who are the poorest in metals. This is the ambitious program undertaken by an international team of researchers, and among them astronomers from Paris Observatory. In giant low mass stars, convection does not predict any mixing between the atmosphere of the stars and the deep layers where carbon is transformed into nitrogen. However, it seems that such a mixing takes place in some evolved very metal-poor giants...

Generally speaking, convection theory does not predict that the surface composition of a low-mass star should change during its lifetime. Its composition is thus a good tracer of Galactic matter at the time of the formation of the star. In the deep layers nuclear reactions will provide energy to the star, but the products of these reactions will remain in the stellar interior and will not contaminate the surface layers. An international team including several members of the Paris Observatory (and, in particular the PI) has studied on this basis with the VLT (Large Programme "First Stars" ID 165N-0276) a sample of very old, extremely metal-poor stars (they were born about 13 billion years ago) to constrain the early phases of Galactic evolution. Most elements in the atmosphere of the giant stars, show constant abundance ratios with a very small scatter, but carbon and nitrogen (two elements very abundant in the Universe) are an exception. For these elements the scatter from star to star is very large. It can reach a factor of 100 for the N/Fe ratio. Either this dispersion represents a true dispersion of C and N in the interstellar medium from place to place in the early Galaxy (primordial scenario), or the original C and N abundances in the atmosphere of the giant stars have been altered by variable degrees of mixing with the H burning layer (in situ scenario) where carbon is transformed into nitrogen at a temperature of about 2 107 K.

How to discriminate between these two scenarios ?

Figure 1 : Spectre d’une étoile ayant mille fois moins de métaux que le Soleil (croix grises) et (en bleu) spectre calculé pour différentes abondances de

Moreover, as expected, the C+N abundance in mixed and unmixed stars is about the same and that in the atmosphere of the mixed stars the lithium abundance is very low. This

References

  • First Stars VI - Abundances of C,N,O,Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements Spite, Monique, Cayrel, Roger, Plez, Bertrand, Hill, Vanessa, Spite, Francois, Depagne Eric, François Patrick et al. 2005, Astronomy & Astrophysics, 430, 655 First Stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C. ratio (Astronomy & Astrophysics, in press) Spite, Monique, Cayrel, Roger, Hill, Vanessa, Spite, Francois, Plez, Bertrand, Bonifacio, Piercarlo et al. http://fr.arxiv.org/abs/astro-ph/0605056

Contact

  • Monique Spite
    Observatoire de Paris, GEPI
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