The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

¹ Sezione di Astronomia, Dipartimento di Fisica, Universitá di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy e-mail: cescutti@oats.inaf.it
² Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland e-mail: cristina.chiappini@unige.ch
³ INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 15 January 2010
Accepted: 27 February 2010

Abstract

Aims. We use the growing data sets of very-metal-poor stars to study the impact of stellar winds of fast rotating massive stars on the chemical enrichment of the early Galaxy.

Methods. We use an inhomogeneous chemical evolution model for the Galactic halo to predict both the mean trend and scatter of C/O and N/O. In one set of models, we assume that massive stars enrich the interstellar medium during both the stellar wind and supernovae phases. In the second set, we consider that in the earliest phases (Z < 10^-8), stars with masses above 40  only enrich the interstellar medium via stellar winds, collapsing directly into black holes.

Results. We predict a larger scatter in the C/O and N/O ratios at low metallicities when allowing the more massive fast-rotating stars to contribute to the chemical enrichment only via stellar winds. The latter assumption, combined with the stochasticity in the star formation process in the primordial Galactic halo can explain the wide spread observed in the N/O and C/O ratios in normal very-metal-poor stars.

Conclusions. For chemical elements with stellar yields that depend strongly on initial mass (and rotation) such as C, N, and neutron capture elements, within the range of massive stars, a large scatter is expected once the stochastic enrichment of the early interstellar medium is taken into account. We also find that stellar winds of fast rotators mixed with interstellar medium gas are not enough to explain the large CNO enhancements found in most of the carbon-enhanced very-metal-poor stars. In particular, this is the case of the most metal-poor star known to date, HE 1327–2326, for which our models predict lower N enhancements than observed when assuming a mixture of stellar winds and interstellar medium. We suggest that these carbon-enhanced very metal-poor stars were formed from almost pure stellar wind material, without dilution with the pristine interstellar medium.