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Issue A&A
Volume 479, Number 1, February III 2008
Page(s) L9 - L12
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078698



A&A 479, L9-L12 (2008)
DOI: 10.1051/0004-6361:20078698

Letter

A new imprint of fast rotators: low $\mathsf{^{12}}$C/ $\mathsf{^{13}}$C ratios in extremely metal-poor halo stars

C. Chiappini1, 2, S. Ekström1, G. Meynet1, R. Hirschi3, A. Maeder1, and C. Charbonnel1, 4

1  Observatoire Astronomique de l'Université de Genève, 1290 Sauverny, Switzerland
    e-mail: Christina.chiappini@obs.unige.ch
2  Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italia
3  Keele University, Lennard-Jones Lab., Keele ST5 5BG, UK
4  LATT, CNRS UMR 5572, OMP, Université Paul Sabatier Toulouse 3, 14 Av. E. Belin, 31400 Toulouse, France

(Received 18 September 2007 / Accepted 11 December 2007)

Abstract
Context.Fast stellar rotation is currently the most promising mechanism for producing primary nitrogen in metal-poor massive stars. Chemical evolution models computed with the inclusion of the yields of fast rotating models at a metallicity Z=10-8 can account for the high N/O abundances observed in normal metal-poor halo stars. If, as believed, intermediate mass stars did not have enough time to contribute to the interstellar medium enrichment at such low metallicities, the above result constitutes a strong case for the existence of fast rotators in the primordial Universe.
Aims.An important result of stellar models of fast rotators is that large quantities of primary $^{13}{\rm C}$ are produced. Hence, our goal is to investigate the consequence of fast rotation on the evolution of the $^{12}{\rm C}/^{13}{\rm C}$ ratio in the interstellar medium at low metallicity.
Methods.We compute the evolution of the $^{12}{\rm C}/^{13}{\rm C}$ ratio for the first time at very low metallicities upon the inclusion of fast rotators at Z= 10-8.
Results.We predict that, if fast rotating massive stars were common phenomena in the early Universe, the primordial interstellar medium of galaxies with a star formation history similar to the one inferred for our galactic halo should have $^{12}{\rm C}/^{13}{\rm C}$ ratios between 30-300. Without fast rotators, the predicted $^{12}{\rm C}/^{13}{C}$ ratios would be ~4500 at $\rm [Fe/H] = -3.5$, increasing to $\sim $$31\,000$ at around $\rm [Fe/H] = -5.0$. Current data on very metal-poor giant normal stars in the galactic halo agree better with chemical evolution models including fast rotators. The expected difference in the $^{12}{\rm C}/^{13}{\rm C}$ ratios, after accounting for the effects of the first dredge-up, between our predictions with/without fast rotators is of the order of a factor of 2-3. However, larger differences (a factor of ~60-90) are expected for giants at $\rm [Fe/H]=-5$ or turnoff stars already at $\rm [Fe/H] = -3.5$. To test our predictions, challenging measurements of the $^{12}{\rm C}/^{13}{\rm C}$ in more extremely metal-poor giants and turnoff stars are required.


Key words: stars: abundances -- stars: rotation -- Galaxy: evolution -- Galaxy: abundances -- Galaxy: halo -- early Universe



© ESO 2008


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