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Cosmic Evolution of Isotopic Abundances: Basics

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Part of the book series: Astrophysics and Space Science Library ((ASSL,volume 453))

Abstract

The description of the tempo-spatial evolution of the composition of cosmic gas on galactic scales is called ‘galactic chemical evolution’. It combines the knowledge about cosmic sources of nuclei (that is their internal workings and nucleosynthesis yields, and their properties such as frequency of occurrence and spatial distribution), with knowledge about the formation and evolution of these sources in the greater context of a galaxy, as well as transport processes of gas within galaxies. It provides a useful framework, allowing us to interpret the large amount of observational data concerning the chemical composition of stars, galaxies and the interstellar medium.

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Notes

  1. 1.

    Stars of the Galactic disk with ages comparable to, or much younger than, the Sun (4.5 Gy) are called Population I. They are the only stellar population which contains massive (hence short-lived) stars observable today. Stars of the Galactic halo are much older (> 10 Gy) and are called Population II.

  2. 2.

    In principle, the IMF may depend on time, either explicitly or implicitly (i.e. through a dependence on metallicity, which increases with time); in that case one should adopt a Star Creation Function C(t, M) (making the solution of the GCE equations more difficult). In practice, however, observations indicate that the IMF does not vary with the environment, allowing to separate the variables t and M and adopt C(t, M) = Ψ(t)Φ(M).

  3. 3.

    Massive stars also eject part of their mass through a wind, either in the red giant stage (a rather negligible fraction) or in the Wolf-Rayet stage (an important fraction of their mass, in the case of the most massive stars).

  4. 4.

    This is the so called Instantaneous Mixing Approximation, not to be confused with the Instantaneous Recycling Approximation (IRA), to be discussed in Sect. 11.1.3.3.

  5. 5.

    An exception to that rule is fragile D, already burned in the Pre-Main Sequence all over the star’s mass; Li isotopes are also destroyed, and survive only in the thin convective envelopes of the hottest stars.

  6. 6.

    Schmidt (1959) describes the distributions of gas and young stars perpendicularly to the galactic plane (z direction) in terms of volume densities ρ Gas ∝ exp(−zh Gas) and ρ Stars ∝ exp(−zh Stars) with corresponding scaleheights (obervationally derived) h Gas=78 pc and h Stars=144 pc∼2 h Gas; from that, Schmidt deduces that \(\rho _{Stars} \propto \rho _{Gas}^2\), that is N=2.

  7. 7.

    Metallicity dependent lifetimes have to be taken into account in models of the spectrophotometric evolution of galaxies, where they have a large impact. In galactic chemical evolution calculations, they play an important role in the evolution of s-elements, which are mostly produced by long-lived AGB stars of ∼1.5–2 M.

  8. 8.

    LIMS are defined as those stars evolving to white dwarfs. However, there is no universal definition for the mass limits characterizing Low and Intermediate Mass stars. The upper limit is usually taken around 8–9 M, although values as low as 6 M have been suggested (in models with very large convective cores). The limit between Low and Intermediate masses is the one separating stars powered on the Main Sequence by the p-p chains from those powered by the CNO cycle and is ∼1.2–1.7 M, depending on metallicity.

  9. 9.

    In active galaxies the central supermassive black hole also plays a role, and even dominates over the impact from massive stars for central regions, and for entire galaxies in late (largely-processed) evolution such as at low redshifts.

  10. 10.

    In this book we only address the scales and processes within a galaxy, as it is driven by massive stars and can be traced by radioactive material; in this and further sections of this chapter, the broader (cosmic evolution) context is also relevant.

  11. 11.

    AGB stars form colorful planetary nebulae, massive-star winds form gas structures within the HII-regions created by the ionizing radiation of the same stars, and thus a similarly-rich variety of colorful filamentary structure from atomic recombination lines results. Supernova remnants are the more violent version of the same processes.

  12. 12.

    Feedback from supermassive black holes is small by comparison, but may become significant in AGN phases of galaxies and on the next-larger scale (clusters of galaxies, see (e)). Galaxy interactions and merging events are also important agents over cosmic times, their overall significance for cosmic evolution is a subject of many current studies.

  13. 13.

    Gaseous flows in the plane of a galactic disk, due e.g. to viscosity, are called inflows; for simple GCE models they also constitute a form of infall.

  14. 14.

    Alternatively, the theory of general relativity may fail at small accelerations.

  15. 15.

    In fact, it is difficult to account for the chemical and kinematical properties of all three galactic components in the monolithic collapse scenario.

  16. 16.

    G-type stars are bright enough for a reasonably complete sample to be constructed and long-lived enough to survive since the earliest days of the disk; the same problem is encountered if F- or K- type stars are used.

  17. 17.

    In fact, the data can be corrected for scaleheight effects only in the framework of a model of the vertical dynamical equilibrium of the local disk, which requires as input the “weight” of each stellar population as a function of its metallicity, i.e. a chemo-dynamical model of GCE is required. Thus, the best approach consists, not in comparing the GCE model with the corrected data, but in (a) calculating the local vertical structure by using the GCE data (surface density vs metallicity) as input, (b) correcting the GCE model metallicity distribution (which refers to the total surface area of the local cylinder), by taking into account the vertical disk structure calculated in (a), and finally (c) comparing the corrected MD of the GCE model to the original (local volume) data. Such an approach is used e.g. Sommer-Larsen and Dolgov (2001).

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Correspondence to Roland Diehl .

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Diehl, R., Prantzos, N. (2018). Cosmic Evolution of Isotopic Abundances: Basics. In: Diehl, R., Hartmann, D., Prantzos, N. (eds) Astrophysics with Radioactive Isotopes. Astrophysics and Space Science Library, vol 453. Springer, Cham. https://doi.org/10.1007/978-3-319-91929-4_11

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