Critical Nuclear Physics Needs for Astrophysical Nucleosynthesis Studies

1. Critical Nuclear Physics Needs for Astrophysical Nucleosynthesis Studies James W. Truran Physics Division Argonne National Laboratory and Department of Astronomy and Astrophysics University of Chicago March 23rd, 2010

2. Cosmic Nucleosynthesis Perspective CRL-41 • The Universe emerged from the Big Bang with a composition consisting of hydrogen, helium, 2D, 3He, and 7Li. The first stars and galaxies were born with this “primordial” composition. • The heavy elements with which we are familiar - from carbon and oxygen, to iron, .. to uranium - are the products of nuclear processes associated with the evolution of stars and supernovae of Types Ia and II. • The classic papers by Burbidge, Burbidge, Fowler, and Hoyle (1957) and Cameron (1957) provided the perspective from which we now view this nucleosynthesis history. • The respective efforts by Cameron and Fowler to improve the nuclear physics input for nucleosynthesis studies largely reflect today’s needs for both theory and experiment: • Fowler and his collaborators focused during the 1950s and 1960s on nuclear experiments involving light element reactions relevant to early stages of stellar energy generation - efforts which have established the empirical foundations of stellar evolution theory. • Strongly motivated by Paul Merrill’s identification of the presence of technetium in red giant stars, Cameron focused rather on the mechanisms of heavy element synthesis and the need for theoretical estimates of reaction rates and other nuclear properties.

3. “Cosmic” Abundances of the Elements Signatures of nuclear systematics: -particle nuclei are dominant 12C --- 40Ca dominance of unstable  -nuclei decay products: 44Ti44Ca, 48Cr48Ti52Fe52Cr, 56Ni56Fe60Zn60Ni, 64Ge64Zn 68Se68Zn, 72Kr72Ge nuclear statistical equilibrium centered on mass A=56 (56Niin situ) neutron shell structure reflected in the heavy element region at magic numbers N=50, 82, and 126 (s- and r-processes) r-process s-process Massive Stars & SNe

4. Outline • Helium Burning Reactions • CNO hydrogen burning in novae and X-ray bursts • Explosive Nucleosynthesis in supernovae: SNe Ia and SNe II • Some interesting long lived radioactivities • Heavy element (A>60) nucleosynthesis mechanisms • s-process • r-process • p-process • Clues from abundance studies

5. 12C synthesis realized in red giant (AGB) stars in ‘incomplete’ helium burning regions (helium burning shells). Stellar helium burning proceeds mainly by the two reactions: 3  12C and 12C(,)16O.

6. target beam bubbles  + 16O --> 12C +  • Superheated water will nucleate from a and 12C recoils • The detector is insensitive to g-rays from the beam. • The target density is 1000xhigher than that of previous experiments. • g-ray beam provided by the HIgS facility at Duke University. • Water has been depleted in heavy isotopes of oxygen and deuterium. • Backgrounds such as a-emitters can be rejected effectively. • First experiment has been scheduled for April 2010

7. Outline • Helium burning reactions • CNO Hydrogen Burning and “breakout:” Novae and X-ray Bursts • Explosive Nucleosynthesis in supernovae: SNe Ia and SNe II • Some interesting long lived radioactivities • Heavy element (A>60) nucleosynthesis mechanisms • s-process • r-process • p-process • Clues from abundance studies

8. Novae & X-Ray Bursts: Standard Models • Classical Novae and X-Ray Bursters involve thermonuclear explosions in accreted hydrogen-rich envelopes of white dwarfs and neutron stars, respectively, in close binary systems. • Accretion of matter from their companion leads to growth of the envelope until a critical pressure is achieved at its base to trigger a thermonuclear runaway. • The violence of the ensuing outbursts depends strongly on the degree of degeneracy at the base of the envelope and the composition of the available nuclear fuel. The early stages of the outbursts of both novae and X-ray bursts witness rapid energy release on a dynamic time scale, moderated in a critical way by the operation of the hot (+-limited) CNO cycles. X-ray Burst GS 1826-24 Nova V1974 Cygni 1992

9. Hot CNO Cycles and Breakout Reactions Break-Out Reactions 14O(α,p)17F(p,g)18Ne(β+ν)18F(p,α)15O 15O(a, g)19Ne(p,γ)20Na CNO Cycles) (T > 80 million K)  +-Decay Constrained Implications for: novae and X-ray bursts

10. Nucleosynthesis Processes Supernovae? Supernovae np-process

11. Courtesy: H. Schatz

12. Classical Nova Explosions under Extreme Conditions • Townsley and Bildsten (2004) have shown that their exists a subclass of nova events occurring on massive white dwarfs accreting at extremely low rates and characterized by high densities and low central temperatures (T ≈ 5 million Kelvin). • Glasner and Truran (2009; 2010) have shown that such conditions lead to violent nova events with peak temperatures exceeding 400 million Kelvin, such that “breakout” from the conventional CNO cycles can occur. This may explain the anomalous abundance patterns seen in the ejecta of such systems. • Glasner and Truran have also demonstrated the sensitivities of breakout to critical reaction rates in the hot CNO sequences. We find a particularly strong dependence on the rate of the 15O(α,γ)19Ne and a weaker sensitivity to the rate of the 14O(α,γ)17F reaction. Also we note a constraint on flow past the iron peak at 64Ge.

13. “Breakout,” the 15O(α,γ)19Ne Rate, and Nova Abundances (The accreted matter was assumed to include no elements past oxygen.) (Glasner and Truran 2009) Note that heavier elements synthesized in nova events are observable in nebular remnants > a direct observational test. (Glasner and Truran 2010)

14. Outline • Helium burning reactions • CNO hydrogen burning in novae and X-ray bursts • Explosive Nucleosynthesis in SNe Ia and SNe II • Heavy element (A>60) nucleosynthesis mechanisms • s-process • r-process • p-process • Clues from abundance studies

15. Synthesis of Intermediate Mass and Iron-Peak Nuclei • Following helium burning, the elements from neon to zinc are formed in late stages of energy generation (carbon, neon, oxygen, and silicon burning) in massive stars (M ≥ 10 M) and in supernovae Type Ia and Type II. • The critical first step here involves the fusion reactions 12C(12C,p)23Na and 12C(12C,α)20Ne. • Currently utilized reaction rates are obtained by extrapolation from higher energy ranges that do not involve resonances. Recent experimental investigations of the 12C+12C rate (e.g. Zickefoose et al. 2010) are focusing on extending the studied region down to the vicinity of the Gamow window. (See also Spillane et al. 2007). • Challenges remain to confirm the production levels of intermediate mass and iron nuclei formed under explosive burning conditions in Type Ia and Type II supernova events.

16. COSMIC ABUNDANCES The abundance history of the “intermediate mass” nuclei Si-Ca relative to “iron-peak” nuclei (56Fe/56Ni) provides important constraints on SNe Ia and SNe II models.

17. “Explosive Iron-Peak Nucleosynthesis” • Explosive nucleosynthesis is a complicated process. The nuclear products are sensitively dependent upon: • thermonuclear reaction rates (level densities are critical), • weak interaction rates (which serve to convert protons to neutrons - neutronization - and thereby reduce Ye and decrease the abundances of proton rich nuclei), and • the (short) dynamic timescales appropriate to supernovae. • Cameron (1963) recognized that the products of heavy element synthesis under explosive conditions were sensitive to the relative rates of strong and weak interactions and to the timescale on which synthesis occurred. This has ultimately led to the appreciation of the fact that the production of iron-peak nuclei in supernovae occurs under neutron-poor conditions (e.g. Ye ~ 0.5) such that the dominant products are proton-rich nuclei (e.g. 56Ni). • Critical input nuclear physics includes experimental and theoretical rates and/or lifetimes for nuclei near the alpha-line, for both nuclear reactions and weak interactions.

18. Why 56Ni ? 56Ni Production in Explosive Nucleosynthesis Pre-explosion compositions involve largely nuclei of Z  N, viz. 12C, 16O, 28Si (for SNe II). • Explosive burning at temperatures T > 4x109 K typically occurs on timescales  seconds. • Supernova nucleosynthesis sees reactions occurring on a dynamical timescale. (Experimental rate determinations and Hauser-Feshbach calculations are critical input.) • Weak interactions proceed too slowly to convert any significant fraction of protons to neutrons. (Weak interaction rates are critical input.) • It follows that the main (in situ) iron-peak products of explosive nucleosynthesis in supernovae are proton-rich nuclei of ZN, viz. 44Ti, 48Cr, 52Fe, 56Ni, 60Zn, and 64Ge (Cameron 1963; Truran, Arnett, and Cameron 1967).

19. Explosive Nucleosynthesis of Fe-Peak Nuclei 68Se 72Kr 67As 64Ge 65Ge 66Ge 70Ge 72Ge 74Ge 63Ga 69Ga 71Ga 60Zn 61Zn 62Zn 64Zn 67Zn 68Zn 70Zn 66Zn 59Cu 63Cu 65Cu 56Ni 57Ni 58Ni 60Ni 61Ni 62Ni 64Ni  55Co 59Co 52Fe 53Fe 54Fe 56Fe 57Fe 58Fe Z 51Mn 55Mn 48Cr 49Cr 50Cr 52Cr 53Cr 54Cr 47V 50V 51V 44Ti 45Ti 46Ti 47Ti 48Ti 49Ti 50Ti Iron peak nucleosynthesis occurs at Ye≈0.5 in both SNe Ia and SNe II 43Sc 45Sc 40Ca 42Ca 43Ca 44Ca 46Ca N  48Ca

20. Type Ia Supernovae: Thermonuclear explosions of CO white dwarfs. Type II Supernovae: Core collapse driven events in massive stars. In both instances,the formation of iron peak elements in explosive nucleosynthesis occurs under neutron-poor conditions.This is reflected in the 56Ni56Co56Fe signatures in both Type Ia and Type II supernova light curves … and in the isotopic compositions of iron-peak elements in Solar matter. Supernova Nucleosynthesis Contributions SNe Ia SNe 1987A (Iwamoto et al. 1999) (Thielemann et al. 1992)

21. 56Ni Production in SNe Ia / Nomoto Neutron Rich Matter Intermediate Mass Elements: Si - Ca 56Ni (Ye≅ 0) (Timmes, Brown, and Truran 2003)

22. Interesting Long Lived Radioactivities • Long lived radioactivities that are products of explosive nucleosynthesis (e.g. 7Be,22Na, 26Al, 44Ti, 56Ni, 57Ni, 60Fe, .. 232Th,235U, 238U ..) can test and constrain models. Both 7Be (53.28 d) and 22Na (2.604 a) are anticipated products of classical nova explosions. Their abundance levels are sensitive functions of the convective/temperature history. The three long lived actinide isotopes 232Th,235U, 238U constitute important chronometers of the earliest stages of star formation activity in our Galaxy. • Supernovae represent the source of the interesting radioactivities 44Ti, 56Ni, and 57Ni in nucleosynthesis. Detections of gamma rays from 56Ni and 57Ni have informed us both of the mass of 56Fe in Type II supernova ejecta and of the 56Fe/57Fe ratio emerging from these events. We also understand the role of 56Ni in powering the light curves of Type Ia supernovae, utilized as distance indicators. • 44Ti decay gamma rays have been detected from the Cas A Type II supernova remnant (see figure). The measured flux implies that if 44Ti and 56Ni were formed in a ratio consistent with the solar 44Ca/56Fe ratio, the Cas A supernova should have been brighter. Alternatively, the 44Ti may simply have been overabundant relative to mass 56 in that Type II event (the Ti/Ca ratio seen in halo stars is ≈ 3 times solar). Experimental determinations of the rates of production and destruction of 44Ti remain critical input.

23. Outline • Helium burning reactions • CNO hydrogen burning in novae and X-ray bursts • Explosive Nucleosynthesis in supernovae: SNe Ia and SNe II • Some interesting long lived radioactivities • Heavy Element (A>60) Nucleosynthesis Mechanisms • s-process • r-process • p-process • Clues from abundance studies

24. Neutron-Capture Processes of Heavy Element Synthesis • Our picture of the processes of heavy element synthesis has become considerably more complicated with time. Early work identified the s- and r-processes, with a relatively small contribution from the p-process. • We now understand that there are two distinguishable contributions (a “weak” and a “main” component) to both of the neutron capture processes: the s-process and the r-process. • The challenges to theorists include the need to identify the astrophysical sites in which this nucleosynthesis occurs and to explain the distinctive features of the diverse components. (Cameron 1963)

26. Neutron Capture Cross Sections: theory/experiment Rauscher, Thielemann and Kratz (1996) Truran and Cameron (1965) Macklin and Gibbons (1957) Thielemann, Arnould and Truran (1987) Bao and Kappeler (1986) Note systematic trends through positions of shell closures arising from level density variations. Neutron capture cross section uncertainties (Käppeler et al. 2007)

27. Identifying the r-Process Component in Solar System Matter Neutron Capture Cross Sections

28. Search for a Second r-Process “weak” r-process • The robust r-process abundance pattern in the regime from barium through the actinides does not extend below mass A ≈ 130, although the classic r-process model extends down to mass A ≈ 70. A second component is thus essential. Two possibilities are: • the νp-process (Fröhlich 2007) • an r-process associated with the mass shells ejected in Type II events CS 22892-052

29. Nucleosynthesis Beyond Fe-group: νp-process • Proton-rich matter is ejected under the influence of neutrino interactions. • Nuclei form at distances where a substantial antineutrino flux is present. • True rp-process is limited by slow  decays, e.g. (64Ge). • Antineutrinos help bridging long waiting points via (n,p) reactions: (Frebel et al. 2005) • With neutrinos • Without neutrinos

30. Numerical Effects of Rate Variations Fröhlich, Tang, and Truran (2010) (p,γ) rates (n,p) rates

31. Penning Trap Mass Measurements (υp=process regime) N = Z CANADIAN Penning TRAP at ANL N = 50 Xe 110 Xe 111 Xe 112 Xe 113 Xe 114 I 108 I 109 I 110 I 111 I 112 I 113 52 Te 105 Te 106 Te 107 Te 108 Te 109 Te 110 Te 111 Te 112 SHIPTRAP 51 Sb 103 Sb 104 Sb 105 Sb 106 Sb 107 Sb 108 Sb 109 Sb 110 Sb 111 Z = 50 50 Sn 100 Sn 101 Sn 102 Sn 103 Sn 104 Sn 105 Sn 106 Sn 107 Sn 108 Sn 109 Sn 110 49 In 98 In 99 In 100 In 101 In 102 In 103 In 104 In 105 In 106 In 107 In 108 In 109 JYFLTRAP 48 Cd 97 Cd 98 Cd 99 Cd 100 Cd 101 Cd 102 Cd 103 Cd 104 Cd 105 Cd 106 Cd 107 Cd 108 47 Ag 93 Ag 94 Ag 95 Ag 96 Ag 97 Ag 98 Ag 99 Ag 100 Ag 101 Ag 102 Ag 103 Ag 104 Ag 105 Ag 106 Ag 107 46 Pd 91 Pd 92 Pd 93 Pd 94 Pd 95 Pd 96 Pd 97 Pd 98 Pd 99 Pd 100 Pd 101 Pd 102 Pd 103 Pd 104 Pd 105 Pd 106 45 Rh 89 Rh 90 Rh 91 Rh 92 Rh 93 Rh 94 Rh 95 Rh 96 Rh 97 Rh 98 Rh 99 Rh 100 Rh 101 Rh 102 Rh 103 Rh 104 Rh 105 44 Ru 87 Ru 88 Ru 89 Ru 90 Ru 91 Ru 92 Ru 93 Ru 94 Ru 95 Ru 96 Ru 97 Ru 98 Ru 99 Ru 100 Ru 101 Ru 102 Ru 103 Ru 104 p – process 43 Tc 85 Tc 86 Tc 87 Tc 88 Tc 89 Tc 90 Tc 91 Tc 92 Tc 93 Tc 94 Tc 95 Tc 96 Tc 97 Tc 98 Tc 99 Tc 100 Tc 101 Tc 102 Tc 103 Mo 83 Mo 84 Mo 85 Mo 86 Mo 87 Mo 88 Mo 89 Mo 90 Mo 91 Mo 92 Mo 93 Mo 94 Mo 95 Mo 96 Mo 97 Mo 98 Mo 99 Mo 100 42 59 60 41 Nb 81 Nb 82 Nb 83 Nb 84 Nb 85 Nb 86 Nb 87 Nb 88 Nb 89 Nb 90 Nb 91 Nb 92 Nb 93 Nb 94 Nb 95 Nb 96 Nb 97 Nb 98 Nb 99 REFERENCE NUCLIDES 40 Zr 78 Zr 79 Zr 80 Zr 81 Zr 82 Zr 83 Zr 84 Zr 85 Zr 86 Zr 87 Zr 88 Zr 89 Zr 90 Zr 91 Zr 92 Zr 93 Zr 94 Zr 95 Zr 96 Zr 97 58 39 Y 78 Y 79 Y 80 Y 81 Y 82 Y 83 Y 84 Y 85 Y 86 Y 87 Y 88 Y 89 Y 90 Y 91 Y 92 Y 93 Y 94 Y 95 Y 76 Y 77 67 38 Sr 73 Sr 74 Sr 75 Sr 76 Sr 77 Sr 78 Sr 79 Sr 80 Sr 81 Sr 82 Sr 83 Sr 84 Sr 85 Sr 86 Sr 87 Sr 88 Sr 89 Sr 90 Sr 91 Sr 92 55 56 37 Rb 72 Rb 73 Rb 74 Rb 75 Rb 76 Rb 77 Rb 78 Rb 79 Rb 80 Rb 81 Rb 82 Rb 83 Rb 84 Rb 85 Rb 86 Rb 87 Rb 88 Rb 89 53 54 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 (adapted from C. Weber)

32. Main r-Process Nucleosynthesis Contributions • Significant efforts have been made to understand the extremely robust r-process abundance pattern observed in the mass range from barium to uranium that characterizes both Solar matter and the oldest (and r-process rich) stars in our Galaxy’s halo. • This robustness strongly suggests – perhaps even demands – that significant fission recycling enables an approach to a stable pattern. This emphasizes the need for a more reliable treatment of fission including an understanding of the fragment distribution. • Critical nuclear input also includes masses (SN) for nuclei along the capture path, a more refined treatment of nuclear level densities, greater predictability of the behavior near shell closures, and a more extensive study of the beta-decay systematics of deformed nuclei (Möller, Nix, and Kratz 1997).

33. CPT Fission Fragment Measurements • Ongoing program of measurements since March 2008, target 15 keV uncertainty • 38 species, 31 have never been measured by method other than β-endpoint • Typically improved precision by factor of 5-10 • Adds to 30 measurements taken at CPT in past years with small gas catcher and source New CPT / Previous CPT measurements

34. “Site-ing” r-Process Nucleosynthesis Helium Shells of SNe II Low Mass ( Prompt Expl.) SNe II R-Process Models Identification of the r-process sites remains a major challenge. Neutron Star- Neutron Star Collisions • - Driven Winds SNe II

35. Synthesizing the r-Process Mass Region A = 110-238 Farouqi et al. (2010) r-process calculations in the context of the ‘high entropy wind’ model for two choices of mass model. The possibility of achievement of such high entropy conditions in Type II supernova environments remains a challenging issue (Panov and Janka 2009).

36. Calculated Fit to Solar r-Process Pattern Kratz et al. (2004)

37. Synthesizing the r-Process Mass Region A = 110-238 • The two most obvious constraints imposed on the r-process mechanism(s) by spectroscopic studies are: First – that there exist at least two distinct astrophysical sites. Second – that the process responsible for the synthesis of elements past Z = 56 is quite exttraordinarily robust. This points to fission recycling. Panov et al. (2004) provide beta-delayed and neutron-induced fission rates and argue that fission leads to the termination of the r-process. Petermann et al. (2009) have performed representative r-process calculations for different neutron-to-seed ratios that provide a measure of the importance of fission recycling to the robustness of this process. Panov et al. (2004) Petermann et al. (2009)

38. Fission Product Yields from Californium 252Cf Source at ANL 78Ni (110 +100-60 ms) 130Cd (162 ±30 ms) (Hosmer et al. 2005) (Hannawald et al. 2001)

39. Concluding Remarks-I • The nuclear physics input requirements for nucleosynthesis studies continue to reflect the importance of both experimental studies of identified “critical” reaction links and theoretical studies built upon statistical properties of nuclei. • In the light mass regime, the 12C(α,γ)16O reaction is of particular importance. The triple-alpha reaction and the 12C + 12C reaction also warrant further experimental scrutiny. • Studies of “breakout” from the hot CNO cycles, relevant to both X-ray bursts and novae, will be greatly helped by an improved experimental determination of the rate of the critical 15O(α,γ)19Ne reaction. • Calculations of nucleosynthesis of Ne to Zn nuclei in massive stars and both SNe II and SNe Ia are sensitive both to nuclear reaction rates and to weak interaction rates. Both experimental and theoretical efforts are required. Observations of halo stars provide clues to and constraints upon the ratio of intermediate mass nuclei (Si-Ca) to iron peak nuclei while the range in peak brightness of SNe Ia also reflects variations in the 56Ni/(Si-Ca) abundance ratio.

40. Concluding Remarks-II • In the regime past the iron peak, a number of processes can contribute. We now know that both the s-process and the r-process patterns demand contributions from at least two different sites. • A critical factor historically has been the separation of the s- and r-components using theory together with experimental neutron capture cross determinations. This may be more complicated in the mass regime A ≅ 60-90 where two s-process contributions may overlap. The dependence of the ‘weak’ s-process on neutron capture cross sections has recently been examined by Pignatari et al. (2010). • The sites of the two (?) r-process contributions remain to be identified as do the range of physical conditions that they may reflect. Required experimental studies include both mass determinations on the proton-rich side relevant to the operation of the νp-process in the mass range A ≅ 60-100 and those on the neutron-rich side relevant to determination of the beta-decay timescales at the waiting points along the r-process path in the vicinity of e.g. 130Cd, critical to the fission recycling that yields a robust r-process pattern in the extended mass range A ≅ 130-238 such as is observed in “r-process rich” and iron-poor halo stars. Further theoretical studies of beta-delayed and neutron-induced fission are also of interest.