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

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 M 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.

Modeling the chemical evolution of the Galaxy halo

“…Hence, we confirm that it is fundamental to include both a gas infall and outflow during the halo formation in modeling the chemical evolution of the Galaxy, to explain the observed MDF of halo stars (see also Prantzos 2003). Moreover, the value of the parameter λ = 14 is the same as suggested by Cescutti & Chiappini (2010) in their inhomogeneous simple model for the Galactic halo. Furthermore we obtain that the halo formation happened on a timescale (τ H = 0.2 Gyr) much shorter than the age of the Galaxy (∼14 Gyr), suggesting a fast formation for the halo.…”

“…The effect of the delayed iron production is to create an overabundance of α-elements relative to iron ( In addition, the stellar yields of nitrogen adopted here are strongly dependent on the rotational velocities of metal-poor stars, so it is possible to understand the apparent observational contradiction consisting in a large scatter in [N/Fe] and an almost complete lack of scatter in [α/Fe] ratios found in the same very metal-poor halo stars. Although the observed scatter could be related to the distribution of stellar rotational velocities as a function of metallicity, that the neutron-capture elements in the same stars also show a large scatter pointed to a strong variation in the stellar yields with the mass range of the stars responsible of the synthesis of these elements (Cescutti & Chiappini 2010). Cescutti (2008) explains simultaneously the observed spread in the neutron capture elements and the lack of scatter in the α-elements as being caused by the stochasticity in the formation of massive stars, combined with the fact that massive stars belonging to different mass ranges are responsible for the synthesis of different chemical elements, namely: only massive stars with masses between 12 and 30 M contribute to the neutron capture elements, whereas the whole mass range of massive stars (10 to 100 M ) contribute to the production af α-elements.…”

“…Previous papers (i.e. Hartwick 1976;Prantzos 2003;Cescutti & Chiappini 2010) have already suggested a gas outflow from the halo, but either this hypothesis was tested in very simple models or not tested against all the observational constraints (see discussion above). Furthermore, we test in our model the effects of a possible Population III of first massive stars with zero-metal content.…”

First stars and reionization: Spinstars

“…We have shown (Cescutti & Chiappini 2010) that a large C, N, O scatter is obtained when assuming that stars with masses above 40 solar masses do contribute to the chemical enrichment of the early Universe via stellar winds, before collapsing directly to black holes (as predicted to happen above this mass limit Heger et al 2003). In these models the contribution to the n-capture elements comes from massive stars below that limit (Cescutti 2008), via supernovae explosion.…”

“…4, 5, and 7 the CEMP-no shows no distinction with respect to the normal (non-CEMP) stars. On the other hand, as shown in Cescutti & Chiappini (2010) the same models cannot not explain the locus of the CEMP-no stars in the nitrogen diagrams, even when using stellar yields from the winds only (and not the SN ones), which produce a lot more nitrogen. At that time we suggested (Meynet et al 2010) these objects to have been born directly from the gas expelled by the wind of fast rotating massive stars without mixing with the surrounding ISM.…”