Chemical Evolution across Space & Time - ACS Publications

Chapter 19

Online Tools for Understanding Galactic Chemical Evolution

Downloaded by RUTGERS UNIV on January 12, 2018 | http://pubs.acs.org Publication Date: February 15, 2008 | doi: 10.1021/bk-2008-0981.ch019

Allen Parker and Bradley S. Meyer Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978

Due to element formation in stars, the abundances of the isotopes in our Galaxy have evolved over time. While study of this evolution can be an excellent classroom activity for illustrating principles of chemistry, astronomy, and nuclear physics, it requires a number of complicated ingredients. We are developing some online tools for investigating these inputs and for demonstrating the chemical evolution of the Galaxy. We recommend that persons interested in these tools subscribe visit www.webnucleo.org for more information.

Introduction Our Galaxy is a collection of gas and stars nearly 13 billion years in age. Throughout its history, gas has condensed into stars. These stars live their lives in stable configurations by burning nuclear fuel which creates new nuclei (/). Once the usable nuclear fuel is exhausted, the star dies and expels its outer layers, thereby enriching the gas between the stars in new isotopes. This gas then can form into new stars, and the cycle repeats. One of the new stars that formed in the Galaxy 4.5 billion years ago is our Sun, and the chemical elements and their isotopes in our Solar System are thus the result of billions of years of Galactic history. © 2008 American Chemical Society Zaikowski and Friedrich; Chemical Evolution across Space & Time ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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344 This story of how the Galaxy has enriched itself in the abundance of its isotopes over time is known to astronomers as Galactic chemical evolution. Strictly speaking, Galactic chemical evolution is a misnomer since it is not about the evolution of the chemistry of the Galaxy but rather the evolution of the Galactic abundances of the elements and isotopes. Perhaps the proper term would be Galactic Abundance Evolution. Nevertheless, the term galactic chemical evolution has stuck, and we use it here. As a pedagogical theme, Galactic chemical evolution is an excellent means of introducing students to the chemical elements and their isotopes, Galactic history and astronomy, element formation in stars, and the interstellar medium. The difficulty is that the ingredients for modeling Galactic chemical evolution at any level above the most superficial are numerous and complex. For example, to follow the evolution of oxygen in the Galaxy, one needs the yield of the oxygen isotopes in various generations of stars, the star formation rate, and a model of the evolution of the gas in the Galaxy with time. Moreover, one needs a good appreciation of the concepts underlying these ingredients. Gathering and explaining the necessary ingredients can be daunting, which limits the usefulness of Galactic chemical evolution as an organizing idea in the classroom. Because of the complexity of the inputs for studying Galactic chemical evolution, we are developing a number of tools that make these ingredients and the underlying concepts more accessible to educators, students, and the general public. These tools are available over the World Wide Web at the URL http://www.webnucleo.org This is the web site of the Clemson University nuclear astrophysics research group. Each available tool has background material explaining the underlying concepts as well as tutorials demonstrating correct usage of the tool. Underlying code modules are available for download. A mail list allows users to report bugs, request new features, and to receive announcements regarding new tools.

T h e A v a i l a b l e Tools In this section, we briefly review some of the available tools, their features, and possible pedagogical uses. Other tools not necessarily related to Galactic chemical evolution are also available, and the interested reader is invited to explore them at webnucleo.org.

Zaikowski and Friedrich; Chemical Evolution across Space & Time ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

345 Solar Abundances Tool


Webnucleo's Solar Abundances Online Tool allows an internet user to interactively study the abundance of the chemical elements and their isotopes, that is, the Solar Abundance Distribution (SAD). Users can use default by Anders and Grevesse (2) or upload their own. The tool then allows the user to view, sort, and plot the abundances in various ways. Common plots or tables generated show how the abundance, fractional abundance, or massfractionsof the species vary with atomic number or mass number. Figure 1 is a plot of abundance as a function of the species' mass number, A. From a plot like this we can easily see how the abundances fall off for the more massive species.

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Figure L This is a plot of the solar abundance distribution generated by the Solar Abundances Tool. This plot shows the abundance of each species as a function of its mass number, A.

Students can use this tool to explore the variety of elements and isotopes, understand the idea of abundances and mass fractions, compare elemental and isotopic abundances (for example, how much gold is there compared to iron or how much 0 there is compared to 0), and appreciate the different number of nucleosynthetic processes that have built up the Solar System's supply of isotopes. In a soon to be released update of the tool, users will also be able to convert between the various abundance notations used throughout the scientific literature such as the cosmochemical and logarithmic notations, fractional 17

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346 abundances, and the mass fractions of the elements. In this way, the language of astrophysicists and chemists can be easily reconciled.


Nuclear Decay Tool Webnucleo's Nuclear Decay Tool allows an internet user to follow isotopic changes that occur due to α-decay and β-decay. In its current version, the user inputs an initial abundance distribution and a decay interval. The Webnucleo server computes the abundance changes and then emails the user the final results. A soon-to-be-released version of the tool will allow will users to upload their own decay rates. Students can use this tool to understand radioactive decay, nuclear decay chains, and geological dating via radioactive isotopes (such as the Mg-Al, K-Ar, Rb-Sr, and Pb-Pb systems).

Nuclear Data Tool Webnucleo's Nuclear Data Tool allows an internet user to explore some of the key properties of nuclei that govern how the species behave in nuclear reactions. These are the atomic number Z, the neutron number N , the mass number A, the mass excess, the ground-state spin, and the nuclear partition function. Students can use the Nuclear Data Tool to explore these quantities and then to study questions such as what reactions among nuclei are energetically possible or even to simply look up needed quantities for their homework.

Galactic Chemical Evolution Machine Webnucleo's Galactic Chemical Evolution Machine lets users interactively explore the chemical evolution of the Galaxy. With this tool, users may explore in detail the éjecta from a massive star (3) too see how the abundances of the elements are distributed throughout the éjecta. The isotopic yields from a generation of stars can be explored, or one can use these yields to run Galactic chemical evolution models. For example, Figure 2 shows how the isotopes of oxygen change through time as a result of chemical evolution and clearly demonstrates the difference between primary and secondary isotopes. 0 is a primary isotope and increases linearly with time while 0 and 0 are secondary isotopes and vary with the cube of the time. Students can follow the tutorial and the help links in this tool to learn more about this concept and many of the basics of Galactic chemical evolution. 16

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347 Oxygen Isotopes 1

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Chemical Evolution across Space & Time - ACS Publications

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