Letter to the Editor

A new imprint of fast rotators: low C/C ratios in extremely metal-poor halo stars

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 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 are produced. Hence, our goal is to investigate the consequence of fast rotation on the evolution of the ratio in the interstellar medium at low metallicity.

Methods.We compute the evolution of the ratio for the first time at very low metallicities upon the inclusion of fast rotators at .

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 ratios between 30–300. Without fast rotators, the predicted ratios would be ~ at , increasing to ∼ at around . 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 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 ~) are expected for giants at or turnoff stars already at . To test our predictions, challenging measurements of the in more extremely metal-poor giants and turnoff stars are required.