COALA science: stellar astrophysics

Amanda Karakas

Stellar evolution and nucleosynthesis

One of the most important research fields in astrophysics is the study of the origin of the elements. The quest for the stellar sites that produced the elements is fundamental to modern science because they are linked to questions concerning the origins of planetary systems and life as well as the process of galaxy formation and evolution. Low and intermediate-mass red giant stars played an important role in this story, by producing much of the carbon, nitrogen, and fluorine, as well as up to half of all elements heavier than iron by the slow neutron capture process (the s process). Some of the topics being studied with COALA are outlined below.

Asymptotic Giant Branch (AGB) stars

For low and intermediate-mass stars (with initial masses between about 0.8 to 8 solar masses) the most important nucleosynthesis occurs when stars evolve off the main sequence to the giant branches. It is during the asymptotic giant branch (AGB) phase of stellar evolution that the richest nucleosynthesis occurs. This is driven by thermal instabilities of the helium-burning shell, the products of which are mixed to the stellar surface by recurrent mixing episodes. Strong stellar winds then expel this enriched matter into the interstellar medium, thus contributing to the chemical evolution of the interstellar medium. Amanda Karakas uses COALA to gain a deeper understanding of the evolution and nucleosynthesis of AGB stars, and to study the many uncertainties that affect predictions (e.g.,Karakas et al. 2008).

The slow neutron capture process

COALA is used to perform detailed nucleosynthesis calculations of the s-process in AGB stars. Understanding the s-process in AGB stars may allow us to place stronger constraints on the nature and origin of the rapid neutron capture process, very little of which is known about still today, as well as gain insights into the efficiency of mixing and mass loss in AGB stars. Figure 1 below shows an example of such a study from Karakas et al. (2009), where abundances from theoretical models were compared to the composition of planetary nebulae, the end product of low-mass stellar evolution.

Abundance predictions from stellar models (Karakas et al. 2009)

FIGURE 1 - AN EXAMPLE OF THE S-PROCESS ABUNDANCE PREDICTIONS NOW AVAILABLE FROM THE STELLAR MODELS (KARAKAS ET AL. 2009). ILLUSTRATED ARE THE SURFACE ABUNDANCE OF ZN (Z=30) THROUGH TO Y (Z=39) AT THE TIP OF THE AGB FOR A SELECTION OF MODELS WITH VARIOUS INITIAL COMPOSITIONS. THE ABUNDANCES ARE SHOWN AS LOGARITHMIC X/FE RATIOS, MEASURED RELATIVE TO THE SOLAR ABUNDANCE OF X/FE. HENCE [X/FE]=0 INDICATES NO INCREASE RELATIVE TO THE SOLAR ABUNDANCE OF X/FE, WHERE [X/FE] = 1 IS A FACTOR OF 10 INCREASE.

Carbon-enhanced metal-poor stars

Many of the most metal-poor ancient stars found in the Galactic halo have enrichments in carbon and s-process elements. This indicates that these stars obtained their unusual abundances via mass transfer from a previous red giant star, that has long evolved away. Karakas and her collaborators plan to compare the predicted abundance patterns of low-metallicity low-mass red giant stars to abundances derived from observations. This comparison may shed light on the many uncertainties that exist in the theoretical models, as well as increase our understanding of nucleosynthesis during the earliest times in the Universe. Figure 2 shows the predicted composition of a 2Msun, [Fe/H] = -2.3 AGB model compared to the abundance in the solar system. The abundance of Pb predicted at the surface of this model is about 9 times more than in the Sun, noting that the initial Pb abundance was 1/200th of the solar abundance!

Solar system composition from Asplund et al. (2009)

FIGURE 2 - NUCLEOSYNTHESIS PREDICTIONS FROM A LOW-METALLICITY AGB MODEL OF 2MSUN COMPARED TO THE COMPOSITION OF THE SOLAR SYSTEM. THE RED LINE AND POINTS SHOW THE SOLAR SYSTEM COMPOSITION FROM ASPLUND ET AL. (2009), WHEREAS THE BLUE LINE AND POINTS SHOW THE PREDICTED SURFACE COMPOSITION OF A 2MSUN MODEL. THE ABUNDANCES VERSUS THE AVERAGE ELEMENTAL ATOMIC MASS, AND ARE PLOTTED AS LOGARITHMIC X/H RATIOS, WHERE LOG N(X/H) = LOG(X/H) + 12.0, AND LOG N(H) = 12 BY DEFINITION. THE ABUNDANCE OF FE IS 200 TIMES LESS ABUNDANT IN THE MODEL THAN IN THE SUN. IN COMPARISON, THE OPERATION OF THE S-PROCESS HAS INCREASED THE ABUNDANCE OF HEAVY ELEMENTS, AND PB IN PARTICULAR, TO BE EQUAL OR HIGHER THAN THE SOLAR ABUNDANCE.

 

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