New Insights into the Early Stage of the Galactic Chemical Evolution (original) (raw)
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Galactic chemical abundance evolution in the solar neighborhood up to the iron peak
Astronomy & Astrophysics, 2001
We have developed a detailed standard chemical evolution model to study the evolution of all the chemical elements up to the iron peak in the solar vicinity. We consider that the Galaxy was formed through two episodes of exponentially decreasing infall, out of extragalactic gas. In a first infall episode, with a duration of sim\simsim 1 Gyr, the halo and the thick disk were assembled out of primordial gas, while the thin disk formed in a second episode of infall of slightly enriched extragalactic gas, with much longer timescale. The model nicely reproduces the main observational constraints of the solar neighborhood, and the calculated elemental abundances at the time of the solar birth are in excellent agreement with the solar abundances. By the inclusion of metallicity dependent yields for the whole range of stellar masses we follow the evolution of 76 isotopes of all the chemical elements between hydrogen and zinc. Those results are confronted with a large and recent body of observational data, and we discuss in detail the implications for stellar nucleosynthesis.
The Nucleosynthetic Imprint of 15–40 M ☉ Primordial Supernovae on Metal-Poor Stars
The Astrophysical Journal, 2010
The inclusion of rotationally-induced mixing in stellar evolution can alter the structure and composition of presupernova stars. We survey the effects of progenitor rotation on nucleosynthetic yields in Population III and II supernovae using the new adaptive mesh refinement (AMR) code CASTRO. We examine piston-driven spherical explosions in 15, 25 and 40 M ⊙ stars at Z = 0 and 10 −4 Z ⊙ with three explosion energies and two rotation rates. Rotation in the Z = 0 models resulted in primary nitrogen production and a stronger hydrogen burning shell which led all models to die as red supergiants (in contrast to the blue supergiant progenitors made without rotation). On the other hand, the Z = 10 −4 Z ⊙ models that included rotation ended their lives as compact blue stars. Because of their extended structure, the hydrodynamics favors more mixing and less fallback in the metal free stars than the Z = 10 −4 models. As expected, higher energy explosions produce more enrichment and less fallback than do lower energy explosions, and at constant explosion energy, less massive stars produce more enrichment and leave behind smaller remnants than do more massive stars. We compare our nucleosynthetic yields to the chemical abundances in the three most iron-poor stars yet found and reproduce the abundance pattern of one, HE 0557-4840, with a zero metallicity 15 M ⊙ , 2.4 × 10 51 erg supernova. A Salpeter IMF averaged integration of our yields for Z = 0 models with explosion energies of 2.4 × 10 51 ergs or less is in good agreement with the abundances observed in larger samples of extremely metal-poor stars, provided 15 M ⊙ stars are included. Since the abundance patterns of extremely metal-poor stars likely arise from a representative sample of progenitors, our yields suggest that 15-40 M ⊙ core-collapse supernovae with moderate explosion energies contributed the bulk of the metals to the early universe.
The Nucleosynthetic Imprint of 15-40 Solar Mass Primordial Supernovae on Metal-Poor Stars
2009
The inclusion of rotationally-induced mixing in stellar evolution can alter the structure and composition of presupernova stars. We survey the effects of progenitor rotation on nucleosynthetic yields in Population III and II supernovae using the new adaptive mesh refinement (AMR) code CASTRO. We examine piston-driven spherical explosions in 15, 25 and 40 M ⊙ stars at Z = 0 and 10 −4 Z ⊙ with three explosion energies and two rotation rates. Rotation in the Z = 0 models resulted in primary nitrogen production and a stronger hydrogen burning shell which led all models to die as red supergiants (in contrast to the blue supergiant progenitors made without rotation). On the other hand, the Z = 10 −4 Z ⊙ models that included rotation ended their lives as compact blue stars. Because of their extended structure, the hydrodynamics favors more mixing and less fallback in the metal free stars than the Z = 10 −4 models. As expected, higher energy explosions produce more enrichment and less fallback than do lower energy explosions, and at constant explosion energy, less massive stars produce more enrichment and leave behind smaller remnants than do more massive stars. We compare our nucleosynthetic yields to the chemical abundances in the three most iron-poor stars yet found and reproduce the abundance pattern of one, HE 0557-4840, with a zero metallicity 15 M ⊙ , 2.4 × 10 51 erg supernova. A Salpeter IMF averaged integration of our yields for Z = 0 models with explosion energies of 2.4 × 10 51 ergs or less is in good agreement with the abundances observed in larger samples of extremely metal-poor stars, provided 15 M ⊙ stars are included. Since the abundance patterns of extremely metal-poor stars likely arise from a representative sample of progenitors, our yields suggest that 15-40 M ⊙ core-collapse supernovae with moderate explosion energies contributed the bulk of the metals to the early universe.
Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy
Astronomy & Astrophysics, 2001
We present revised strontium, barium and europium abundances for 63 cool stars with metallicities [Fe/H] ranging from −2.20 to 0.25. The stellar sample has been extracted from Fuhrmann's lists (1998, 2001). It is confined to main-sequence and turnoff stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40000 to 60000. The element abundance ratios reveal a distinct chemical history of the halo and thick disk compared with that of the thin disk. Europium is overabundant relative to iron and barium in halo and thick disk stars suggesting that during the formation of these galactic populations high-mass stars exploding as SNe II dominated nucleosynthesis on a short time scale of the order of 1 Gyr. We note the importance of [Eu/Mg] determinations for halo stars. Our analysis leads to the preliminary conclusion that Eu/Mg ratios found in halo stars do not support current theoretical models of the r-process based on low-mass SNe; instead they seem to point at a halo formation time much shorter than 1 Gyr. A steep decline of [Eu/Fe] and a slight decline of [Eu/Ba] with increasing metallicity have been first obtained for thick disk stars. This indicates the start of nucleosynthesis in the lower mass stars, in SN I and AGB stars, which enriched the interstellar gas with iron and the most abundant s-process elements. From a decrease of the Eu/Ba ratio by ∼ 0.10 . . . 0.15 dex the time interval corresponding to the thick disk formation phase can be estimated. The step-like change of element abundance ratios at the thick to thin disk transition found in our previous analysis is confirmed in this study: [Eu/Ba] and [Eu/Fe] are reduced by ∼ 0.25 dex and ∼ 0.15 dex, respectively; [Ba/Fe] increases by ∼ 0.1 dex. This is indicative of an intermediate phase before the early stage of the thin disk developed, during which only evolved middle and low mass (< 8M ) stars contributed to nucleosynthesis. Our data provide an independent method to calculate the duration of this phase. The main s-process becomes dominant in the production of heavy elements beyond the iron group during the thin disk evolution. We find that in the thin disk stars Ba/Fe ratios increase with time from [Ba/Fe] = −0.06 in stars older than 8 Gyr to [Ba/Fe] = 0.06 in stars that are between 2 and 4 Gyr old.
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
Nucleosynthesis and Evolution of Massive Metal-Free Stars
The Astrophysical Journal, 2010
The evolution and explosion of metal-free stars with masses 10 − 100 M ⊙ are followed, and their nucleosynthetic yields, light curves, and remnant masses determined. Such stars would have been the first to form after the Big Bang and may have left a distinctive imprint on the composition of the early universe. When the supernova yields are integrated over a Salpeter initial mass function (IMF), the resulting elemental abundance pattern is qualitatively solar, but with marked deficiencies of odd-Z elements with 7 ≤ Z ≤ 13. Neglecting the contribution of the neutrino wind from the neutron stars that they make, no appreciable abundances are made for elements heavier than germanium. The computed pattern compares favorably with what has been observed in metal-deficient stars with [Z] −3. The amount of ionizing radiation from this generation of stars is ∼ 2.16 MeV per baryon (4.15 B per M ⊙ ; where 1 B = 1 Bethe = 10 51 erg) for a Salpeter IMF, and may have played a role in reionizing the universe. Most of the stars end their lives as blue supergiants and make supernovae with distinctive light curves resembling SN 1987A, but some produce primary nitrogen by dredge up and become red supergiants. These make brighter supernovae like typical Type IIp's. For the lower mass supernovae considered, the distribution of remnant masses clusters around typical modern neutron star masses, but above 20 M ⊙ to 30 M ⊙ , with the value depending on explosion energy, black holes are copiously formed by fallback, with a maximum hole mass of ∼ 40 M ⊙. A novel automated fitting algorithm is developed for determining optimal combinations of explosion energy, mixing, and initial mass function in the large model data base to agree with specified data sets. The model is applied to the low metallicity sample of Cayrel et al. (2004) and the two ultra-iron-poor stars HE0107-5240 and HE1327-2326. Best agreement with these low metallicity stars is achieved with very little mixing, and none of the metal-deficient data sets considered show the need for a high energy explosion component. To the contrary, explosion energies somewhat less than 1.2 B seem to be preferred in most cases.
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.
Monthly Notices of the Royal Astronomical Society, 1998
We analyse two recent computations of type II supernova nucleosynthesis by Woosley & Weaver (1995, hereafter WW95) and Thielemann, Nomoto, & Hashimoto (1996, hereafter TNH96), focusing on the ability to reproduce the observed [Mg/Fe]-ratios in various galaxy types. We show that the yields of oxygen and total metallicity are in good agreement. However, TNH96-models produce more magnesium in the intermediate and less iron in the upper mass range of type II supernovae than WW95-models. To investigate the significance of these discrepancies for chemical evolution, we calculate Simple Stellar Population-yields for both sets of models and different IMF slopes. We conclude that the Mg-yields of WW95 do not suffice to explain the [Mg/Fe] overabundance neither in giant elliptical galaxies and bulges nor in metal-poor stars in the solar neighbourhood and the galactic halo. Calculating the chemical evolution in the solar neighbourhood according to the standard infall-model (e.g. Matteucci & Greggio 1986; Timmes, Woosley, & Weaver 1995; Yoshii, Tsujimoto, & Nomoto 1995), we find that using WW95 and TNH96 nucleosynthesis, the solar magnesium abundance is underestimated by 29 and 7 per cent, respectively. We include the relaxation of the instantaneous mixing approximation in chemical evolution models by splitting the gas component into two different phases. In additional simulations of the chemical evolution in the solar neighbourhood, we discuss various timescales for the mixing of the stellar ejecta with the interstellar medium. We find that a delay of the order of 10 8 years leads to a better fit of the observational data in the [Mg/Fe]-[Fe/H] diagram without destroying the agreement with solar element abundances and the age-metallicity relation.
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...
Heavy element production in the early galaxy
AIP Conference Proceedings, 2015
We present some general features of the heavy element signature as a function of metallicity indicating the chemical evolution of the Galaxy. The aim is to emphasize that the heavy element production in the Galaxy is a powerful tool to understand how the r-process and s-process nucleosynthesis have contributed to the production of the heavy elements in the course of the galaxy evolution. We also discuss some current problems encountered in modeling the stellar sites of the above processes.