NUCLEAR ASTROPHYSICS An Overview

1. NUCLEAR ASTROPHYSICS An Overview Brian Fulton University of York

2. PART 1 What is nuclear astrophysics and how do you do it?

3. The aim of nuclear astrophysics is to: Identify and study the nuclear reactions that occur in stars and other astrophysical sites Understand how these give rise to the energy generation which powers these objects Understand how these processes lead to the abundances of the elements that we see around us

4. Any astrophysical site contains a hot, gaseous mix In this the nuclei are moving with a spread of velocities (energies) and continually colliding. For some of the fastest, the collisions may be so energetic that the Coulomb repulsion between the nuclei is overcome and they get close enough for nuclear reactions to occur – these release energy and cause new elements to be created

5. How could we model this? Assume a mix of different nuclei (from spectroscopic measurements) |Assume a distribution of energies (Maxwell-Boltzmann) Carry out nuclear physics experiments to measure the reaction probability (cross section) at all energies for all nuclei Fold these distributions to get the average reaction rate Set up a set of coupled equations which track how the number of each type of nuclei changes as the reactions occur and which also determine the rate at which energy is released in these nuclear reactions.

6. Nuclear Reaction rates Astrophysical conditions (pressure, temperature and abundances) Abundances + Light Curves MODEL Hydrodynamical development Observational Tests

7. Beam Target Measuring the cross section Know number of beam particles Know number of target nuclei Measure number of nuclei created Probability > Cross section

8. Fortunately we don’t have to measure the cross section at all energies The combination of the falling number of particles at high energy (tail of Maxwell Boltzmann) with the low probability of penetrating the Coulomb barrier means the bulk of reactions occur in a narrow range of energies Gammow Window

9. However we have to be careful as Nature can be cunning As well as direct reactions, there can be resonant reactions if the collision energy matches that needed to excite a state in one of the nuclei. In this case the cross section is greatly enhanced

10. PART 2 What are the current interests?

11. There are basically three “classes” of nucleosynthesis Nucleosynthesis in the Big Bang Nucleosynthesis in stars Nucleosynthesis in explosive sites (Novae, X-ray Bursters, SN etc)

12. Nucleosynthesis in the Big Bang Fairly “textbook” stuff Few, light nuclei (p, d, t, 3He, a) involved High(!) temperature so energies well above barrier and so cross sections large

14. Still some discrepancies to track down. Are these a problem with the reaction cross section measurements, or do they indicate a problem with the standard model of the BB?

15. Inhomogeneous BB models (Matsuura et al.; Phys.Rev.D (2005)) If density fluctuations exist in the early universe, additional forms of nucleosynthesis can occur T=1 x 107 and h = 10-6 T=1 x 109 and h = 10-4 Great interest to see if this helps resolve light element discrepancies and also explain observations of heavy elements and very large z

16. Nucleosynthesis in stars A much more complex situation, with many different types of reactions occurring at different stages in the star’s lifecycle: Proton burning (like BB nucleosynthesis) Helium burning (producing C and O nuclei) CNO-cycle (catalytic processing of H > He) Reactions with heavier nuclei (C, O, Ne Mg, Si) Neutron induced reactions (s-process)

17. Hydrogen burning Helium burning CNO-cycle Then a sequence of reactions involving heavier and heavier nuclei until we reach iorn, the most tightly bound nucleus

18. So what are the problems? Firstly, although these are “hot” sites, the energies of the particles are still very low (<< 1 MeV/u) and so well below the Coulomb barrier. Typical temperatures: T ~106-108 K typical interaction energies: E~10-300 keV (i.e. sub-Coulomb energies) So the reactions only proceed through quantum mechanical tunnelling and the cross sections are extremely low. This in turn makes the measurements very long and we run into problems because of background events in the detector systems. 1 event/ 3000 y 10-18 barn <  < 10-9 barn 35 events/h

19. (E) = S(E)·exp(-2) /E S(E) = E·(E)·exp(2) 2 = 31.29 Z1 Z2 (/E)0.5 Often you can’t even get close to the Gamow window and have to extrapolate from higher energies One solution – take your accelerator to an underground laboratory to get rid of the background

20. LUNA accelerator facility in the Gran Sasso Laboratory

21. (E) = S(E)·exp(-2) /E S(E) = E·(E)·exp(2) ? 2 = 31.29 Z1 Z2 (/E)0.5 Three reactions measured into Gamow range 3He(a, g)7Be 3He(3He,2p)a 14N(p,g)15O

22. Experimental Low-Energy Nuclear Astrophysics PRD bid to STFC (Edinburgh, Sheffield, RAL) PI Dr Marialuisa Aliotta (Edinburgh)

23. Nucleosynthesis in explosive sites Nova Herculis 1934: AAT SN1999BE: CGCG 089-013 One week after outburst X-ray burster in NGC 6624: HST

24. The nucleosynthesis in these sites is radically different Because of the very high energies, the reactions proceed very rapidly and many short-lived, radioactive nuclei are formed These nuclei then undergo further reactions – indeed the reactions involving these “exotic” nuclei dominate the process.

25. http://arxiv.org/abs/astro-ph/0607624 X-ray binaries, H. Schatz and K.E. Rehm

26. But this creates a problem for us – these nuclei don’t live long enough for us to make a target out of them to use to measure the cross sections Separator + Ion Source Primary accelerator Primary accelerator Target High intensity stable beam A c c e l e r a t o r The technology required to solve this problem was developed in the last decade – accelerator facilities where short lived nuclei can be produced and then accelerated quickly in a second accelerator to provide a beam of radioactive nuclei.

27. Louvain-la-Neuve The first two-stage radioactive beam facility New generation now coming on line e.g TRIUMF (Canada) and GANIL (France) where we can do more complex measurements

28. The ESFRI Roadmap identified 35 large scale facilities for construction of which 2 are new Radioactive Beam faclities FAIR (GSI in Germany) SPIRAL-2 (GANIL in France)

29. Testing models of novae New gamma-ray observatories (e.g. INTEGRAL) are capable of identifying specific nuclei from their characteristic gamma energies (e.g. 22Na, 26Al, 44Ti) This will provide a stringent test on models (22Na) 26Al

30. But note that COMPTEL didn’t observe any – e.g. measurements of 22Na in Her1991 and Cyg1992 are below expected limits Models wrong? Nuclear physics wrong? Recent calculations (Jordi et al.) suggest a 1.25 solar mass White Dwarf Nova can eject 6.3 x 10-9 solar mass of 22Na T 1/2 = 3.75 yr Eg = 1.275 MeV With INTEGRAL should be able to detect out to 1 kpc (about two thousand, million, million miles) BUT: uncertainties in reaction rates give an uncertainty in production rate (and so chances of observing)

31. Production and destruction of 22Na Combined “Hot” and “Cold” Ne-Na cycles Key reactions: 21Na+p > 22Mg+g (increased production) 22Na+p > 23Mg+g (decreased production)

32. Production reaction C Ruiz et al. Phys Rev C 65 (2002) 042801R S Bishop et al. Phys Rev Lett 90 (2003) 165501 Cross section is larger than thought at low energies Updated calculations (J.Jose) show 22Na production occurs earlier while envelope is still hot and dense enough for it to be destroyed So lower final abundance of 22Na

33. Destruction reaction D G Jenkins et al. Phys. Rev. Letts 92 (2004) 031101 Reaction rate two orders of magnitude larger than thought So again, a lower final abundance of 22Na

34. Feed through same Nova model gives factor of 3 less 22Na So chances of observing a 22Na signal from Nova from satellite missions may be considerably less that anticipated THE END