Early chemical enrichment of the universe and the role of very massive population III stars (original) (raw)
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Potential contributions of Pop III and intermediate-mass Pop II stars to cosmic chemical enrichment
Astronomy & Astrophysics
Context. We propose a semi-analytic model that is developed to understand the cosmological evolution of the mean metallicity in the Universe. In particular, we study the contributions of Population III (Pop III) and Population II (Pop II) stars to the production of Fe, Si, Zn, Ni, P, Mg, Al, S, C, N, and O. Aims. We aim to quantify the roles of two different models in the chemical enrichment of the Universe. The first model (A) considers both stars with Pop III and Pop II yields. For the second model (B), the yields involved are only for Pop II stars. Methods. We start by describing the cosmic star formation rate (CSFR) through an adaptation of a scenario developed within the hierarchical scenario of structure formation with a Press-Schechter-like formalism. We adapt the formalism to implement the CSFR to the standard chemical evolution scenario to investigate the course of chemical evolution on a cosmological basis. Calculations start at redshift z ∼ 20, and we compare the results ...
Pop III stars and the earliest phases of the evolution of galaxies and IGM
Proceedings of the International Astronomical Union, 2005
We discuss the effects of very massive Population III stars on the chemical evolution of the Milky Way, elliptical galaxies and the intergalactic medium (IGM) at high reshift. It is shown that the effects produced by Pop III stars on the early evolution of the most common chemical abundances (C, N, O, α-elements, Fe) are negligible if these stars formed only for a very short period of time, corresponding to the suggested threshold metallicity (Z th r ∼ 10 −4 Z ). For a higher threshold metallicity and therefore a longer period of time, the predicted results are at variance with observations. It is also concluded that the IGM at high redshift (z = 5.0) cannot have been enriched only by very massive Pop III stars, but that the contribution of lower mass stars is necessary. The same conclusion holds for DLA systems at high redshift.
Astronomy & Astrophysics, 2005
We have investigated the poorly-understood origin of nitrogen in the early Galaxy by determining N abundances in 35 extremely metal-poor halo giants (22 stars have [Fe/H]<-3.0) using the C and O abundances determined in Paper V. Because any dredge-up of CNO processed material to the surface may complicate the interpretation of CNO abundances in giants, we have also measured the surface abundance of lithium. Our sample shows a clear dichotomy between two groups of stars. The first group shows evidence of C to N conversion through CN cycling and strong Li dilution, a signature of mixing. The second group shows no evidence for C to N conversion, and Li is only moderately diluted, and we conclude that their C and N abundances are very close to those of the gas from which they formed in the early Galaxy. These "unmixed" stars reflect the abundances in the early Galaxy: the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio is close to solar at low metallicity, favouring a high C production by massive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatter widely. The larger values of these ratios define a flat upper plateau ([N/Mg]= 0.0, [N/Fe]= +0.1), which could reflect higher values within a wide range of yields of zero-metal Sne II. Alternatively, by analogy with the DLA's, the lower abundances ([N/Mg]= -1.1, [N/Fe]= -0.7) could reflect generally low yields from the first Sne II, the other stars being N enhanced by winds of massive Asymptotic Giant Branch (AGB) stars. At present it cannot be decided whether primary N is produced primarily in SNe II or in massive AGB stars, or in both. The stellar N abundances and [N/O] ratios are compatible with those found in Damped Lyman-alpha (DLA) systems.
Cosmological Effects of the First Stars: Evolving Spectra of Population III
The Astrophysical Journal, 2003
The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z > 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z ∼ 6. The metal-free composition of the first stars restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Pop III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Pop II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Pop II in the He II (E > 4 Ryd) continuum. We suggest that large Lyα equivalent widths (W Lyα > 400 Å) may provide a means of detecting metal-free stellar populations at high redshift, and that He II recombination lines (λ1640, λ4686) may confirm identifications of Population III. While Pop III clusters are intrinsically bluer than their Pop II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Pop III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Pop III for the reionization of the IGM in both H and He.
Evolution and chemical and dynamical effects of high-mass stars
We review general characteristics of massive stars, present the main observable constraints that stellar models should reproduce. We discuss the impact of massive star nucleosynthesis on the early phases of the chemical evolution of the Milky Way (MW). We show that rotating models can account for the important primary nitrogen production needed at low metallicity. Interestingly such rotating models can also better account for other features as the variation with the metallicity of the C/O ratio. Damped Lyman Alpha (DLA) systems present similar characteristics as the halo of the MW for what concern the N/O and C/O ratios. Although in DLAs, the star formation history might be quite different from that of the halo, in these systems also, rotating stars (both massive and intermediate) probably play an important role for explaining these features. The production of primary nitrogen is accompanied by an overproduction of other elements as 13 C, 22 Ne and s-process elements. We show also how the observed variation with the metallicity of the number ratio of type Ibc to type II supernovae may be a consequence of the metallicity dependence of the line-driven stellar winds.
The effects of Population III stars and variable IMF on the chemical evolution of the Galaxy
New Astronomy, 2006
We have studied the effects of a hypothetical initial stellar generation (Population III) containing only massive (M > 10M ⊙ ) and very massive stars (M > 100M ⊙ , Pair-Creation Supernovae) on the chemical evolution of the Milky Way. To this purpose, we have adopted a chemical evolution model -the two-infall model from -which successfully reproduces the main observational features of the Galaxy. Several sets of yields for very massive zero-metallicity stars have been tested: these stars in fact produce quite different amounts of heavy elements, in particular α-elements and iron, than lower mass stars. We have focused our attention on the chemical evolution of α-elements, carbon, nitrogen and iron. It was found that the effects of Population III stars on the Galactic evolution of these elements is negligible if only one or two generations of such stars occurred, whereas they produce quite different results from the standard models if they continuously formed for a longer period. Also the effects of a more strongly variable IMF were discussed and to this purpose we have made use of suggestions appeared in the literature to explain the lack of metal-poor stars in the Galactic halo with respect to model predictions. In these cases the predicted variations in the abundance ratios, the SN rates and the G-dwarf metallicity distribution are more dramatic and always in contrast with observations, so we have concluded that a constant or slightly varying IMF remains the best solution. Our main conclusion is that if very massive stars ever existed they must have formed only for a very short period of time (until the halo gas reached the suggested threshold metallicity of 10 −4 Z ⊙ for the formation of very massive objects); in this case, their effects on the evolution of the elements studied here was negligible also in the early halo phases. In other words, we cannot prove or disprove the existence of such stars on the basis of the available data on very metal poor stars. Because of their large metal production and short lifetimes very massive primordial stars should have enriched the halo gas to the metallicity of the most metal poor stars known ([Fe/H] ∼ −5.4) and beyond in only a few million years. This fact imposes constraints on the number of Pair-Creation Supernovae: we find that a number from 2 to 20 of such SNe occurred in our Galaxy depending on the assumed stellar yields.
The effects of the initial mass function on Galactic chemical enrichment
Astronomy & Astrophysics, 2021
Context. We have been seeing mounting evidence that the stellar initial mass function (IMF) might extend far beyond the canonical Mi ∼ 100 M⊙ limit, but the impact of such a hypothesis on the chemical enrichment of galaxies is yet to be clarified. Aims. We aim to address this question by analysing the observed abundances of thin- and thick-disc stars in the Milky Way with chemical evolution models that account for the contribution of very massive stars dying as pair instability supernovae. Methods. We built new sets of chemical yields from massive and very massive stars up to Mi ∼ 350 M⊙ by combining the wind ejecta extracted from our hydrostatic stellar evolution models with explosion ejecta from the literature. Using a simple chemical evolution code, we analysed the effects of adopting different yield tables by comparing predictions against observations of stars in the solar vicinity. Results. After several tests, we set our focus on the [O/Fe] ratio that best separates the chemic...
Chemical enrichment in the early Galaxy
Proceedings of the International Astronomical Union, 2008
The chemical enrichment by the first sources of light in the universe ultimately set the stage for the subsequent evolution of the Milky Way system. The oldest and, usually, the most-metal poor stars are our ‘near-field’ link to this ancient epoch as they, apart from tracing the chemical enrichment itself, also indirectly hold information on, e.g., the conditions for star formation and feed-back effects in the early universe. In particular, I will discuss the possible origins of the relatively large number of carbon enhanced metal-poor stars in the Galactic halo. Furthermore, I will argue that the apparent absence of the chemical signature of so-called pair-instability supernovae (PISNe), which are a natural consequence of current theoretical models for primordial star formation at the highest masses, may arise from a subtle observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, early PISN enrichment is predicted to ‘overshoot’, reachi...
Metal-Poor Stars and the Chemical Enrichment of the Universe
Planets, Stars and Stellar Systems, 2013
Metal-poor stars hold the key to our understanding of the origin of the elements and the chemical evolution of the Universe. This chapter describes the process of discovery of these rare stars, the manner in which their surface abundances (produced in supernovae and other evolved stars) are determined from the analysis of their spectra, and the interpretation of their abundance patterns to elucidate questions of origin and evolution. More generally, studies of these stars contribute to other fundamental areas that include nuclear astrophysics, conditions at the earliest times, the nature of the first stars, and the formation and evolution of galaxies-including our own Milky Way. We illustrate this with results from studies of lithium formed during the Big Bang; of stars dated to within ∼1 Gyr of that event; of the most metal-poor stars, with abundance signatures very different from all other stars; and of the build-up of the elements over the first several Gyr. The combination of abundance and kinematic signatures constrains how the Milky Way formed, while recent discoveries of extremely metal-poor stars in the Milky Way's dwarf galaxy satellites constrain the hierarchical build-up of its stellar halo from small dark-matter dominated systems. We discuss two areas needing priority consideration. The first is improvement of abundance analysis techniques. While one-dimensional, Local Thermodynamic Equilibrium (1D/LTE) model atmospheres provide a mature and precise formalism, proponents of more physically realistic 3D/non-LTE techniques argue that 1D/LTE results are not accurate, with systematic errors often of order ∼0.5 dex or even more in some cases. Self-consistent 3D/non-LTE analysis as a standard tool is essential for meaningful comparison between the abundances of metal-poor stars and models of chemical enrichment. The second need is for larger samples of metal-poor stars, in particular those with [Fe/H] <-4 and those at large distances (20 − 50 kpc), including the Galaxy's ultra-faint dwarf satellites. With future astronomical surveys and facilities these endeavors will become possible. This will provide new insights into small-scale details of nucleosynthesis as well as large-scale issues such as galactic formation. Subject headings: Galaxy: formation − Galaxy: halo − stars: abundances − early Universe − nuclear reactions, nucleosynthesis, abundances 1.2.3. Nomenclature Chemical abundance is one of the basic parameters that define stellar populations, to which we shall return in Section 2.1. Here we define terms that we shall use in the present work. Baade (1944), in his seminal paper on the subject, defined two groups of stars, Type I and Type II, which today are referred to as Population I and