Jim Truran 56Ni, SNeIa Luminosities, and Explosive Nucleosynthesis Astronomy and Astrophysics Enrico Fermi Institute University of Chicago and Argonne National Laboratory Conference on Nuclear Physics and Astrophysics Eilat, Israel, April 4th, 2011

Motivation: Understanding SNe Light Curves Doggett and Branch (1985) SNe Ia and Cosmology In 1998, SNe Ia played a major role in the “science breakthrough of the year:” Using SNe Ia as distance indicators (standard candles), two groups of astronomers found strong indications for an accelerating cosmic expansion. The Phillips relation compensates for the observed variation in peak luminosity to provide an effective “standard candle.” Brighter SNe Ia decline more slowly. Critical Factor: Synthesis of iron as 56Ni Peak luminosity  56Ni mass ejected.

56Ni Decay Signatures in SNe Light Curves 56Ni  56Co  56Fe  = 8.5 days  = 112 days Zeilik 2002 Doggett and Branch 1985 SNe Ia and SNe II both synthesized nuclei of mass A=56 as 56Ni For SN 1987A, Compton detected both 56Co and 57Co decay gamma rays, yielding 57Fe/56Fe Type Ia Supernovae: 1994D Type II Supernovae: 1987A What evidence have we that their explosive nucleosynthesis products are the same? Differ? High-z Supernova Search Team, HST, NASA

Supernovae: 56Ni  56Co  56Fe SNe II (Collapse)  = 8.5 days  = 112 days SN 1987A SNe II (Collapse) SNe Ia (Thermonuclear) SN 1994D SNe Ia and SNe II) both synthesize nuclei of mass A=56 as 56Ni (Ye~0.5). What evidence have we that their products are the same? Differ? “Standard model” (Hoyle & Fowler 1960): SNe II are the product of the evolution of massive stars 10 < M < 100 M. Evolution to criticality: Succession of nuclear burning stages yield a layered compositional structure and a core dominated by 56Fe. Collapse of the 56Fe core to NS . Gravitational energy released in the form of neutrinos, which help to drive explosion. Production of ≈ 0.1 M of 56Fe as 56Ni. “Standard model” (Hoyle & Fowler 1960): SNe Ia are thermonuclear explosions of C+O white dwarf stars. Evolution to criticality: Accretion from a binary companion (Whelan and Iben 1973) leads to growth of the WD. Complete incineration occurs within two seconds, leaving no compact remnant. Light curves powered by decay of 56Ni. Peak luminosity  M(56Ni).  Arnett Law

The “History” of Supernova Iron Production Hoyle (1946) identified the site of the production of iron-peak nuclei with a Nuclear Statistical Equilibrium process. Hoyle and Fowler (1960) identified Type II SNe with massive stars and SNe Ia with explosions of white dwarfs. These sources both focused on the production of a NSE abundance pattern centered on 56Fe. Cameron (1963) noted the dependence of iron-peak explosive nucleosynthesis products on neutron enrichment. Truran, Arnett and Cameron (1967) showed that typical SNe conditions yield 56Ni as the principle product in situ.

IRON “EQUILIBRIUM” PEAK

56Ni and SNe Ia Luminosities It is now generally accepted that the peak luminosities of SNe Ia are proportional to the mass ejected in the form of 56Ni. This view may be understood on the basis of energetic considerations: The conversion of 12C and 16O to 56Ni in 1.4 M releases 0.814 MeV/nucleon or ~ 1.8 x 1051 ergs. (Note 16O+20Ne 1.35 x 1051.) The gravitational binding energy of a 1.4 M white dwarf is approximately 5x1050 ergs. The kinetic energy associated with 1.4 M of ejecta moving at a velocity approaching 109 cm s-1 is ~1.4x1051 ergs. The bulk of the energy released in burning to 56Ni is thus expended in overcoming gravitational binding and imparting high velocities to the ejecta. It follows that it is the subsequent decay of 56Ni (=8.8 days) through 56Co (=111.3 days) to 56Fe that powers the light curve (Truran, Arnett, and Cameron 1967; Colgate and McKee 1969).

Production of Nuclei of Mass A=56 as 56Fe 254Cf (PASP 1956) (B2FH; Fowler 1966)

Why 56Ni ? 56Ni Production in Explosive Nucleosynthesis  Pre-explosion compositions involve nuclei of Z = N, viz. 12C, 16O, 28Si. Explosive burning at temperatures T > 4x109 oK typically occurs on timescales  seconds. Supernova nucleosynthesis sees reactions occurring on a dynamical timescale. Weak interactions proceed too slowly to convert any significant fraction of protons to neutrons. It follows that the main (in situ) iron-peak products of nucleosynthesis in supernovae are proton-rich nuclei of ZN, viz. 40Ca, 44Ti, 48Cr, 52Fe, 56Ni, 60Zn, and 64Ge.

56Ni Production in Nucleosynthesis 72Kr 56Ni Production in Nucleosynthesis Se 68 69 70 71 72 Explosive Nucleosynthesis of Iron Peak Nuclei As 67 Ge 64 65 66 70 72 Ga 63 69 Zn 60 61 62 64 66 67 68 Cu 59 63 65 Ni 56 57 58 60 61 62 64 Co 55 59 Fe 52 53 54 56 57 58 Mn 51 55 Cr 48 49 50 52 53 54 V 47 50 51 Ti 44 45 46 47 48 49 50 Sc 43 45 Ca 40 42 43 44 45 46 48

Solar System Iron Production The abundances of most of the iron-peak nuclei in Solar System matter were unquestionably formed under proton rich (Ye ≈ 0.5) conditions in explosive nucleosynthesis. This is reflected in isotopic production of even-Z elements. 48Ti and 49Ti formed in situ as 48Cr and 49Cr 50Cr as 50Cr: 52Cr and 53Cr formed as 52Fe and 53Fe 54Fe as 54Fe; 56Fe and 57Fe formed as 56Ni and 57Ni 58Ni as 58Ni; 60Ni,61Ni,62Ni formed as 60Zn,61Zn, and 62Zn 64Zn contributions from 64Ge ? 72Ge contributions from 72Kr ? Odd-Z: 51V (51Mn), 55Mn (55Co), 59Co (59Cu), 63Cu (63Ga)  The isotopic compositions of Cr, Fe, and Ni are derived from isotopes of different elements formed in explosive nucleosynthesis. This behavior strongly supported the formation of A=56 as 56Ni.

On the Observed Diversity of Type Ia Supernovae  Knowledge of the composition of SNe Ia ejecta provides us with important clues to the properties of their explosions e.g. : abundances of intermediate mass nuclei in SNe Ia ejecta (e.g. Branch et al. 1981) indicate that a pure detonation has not occurred; the absence of lines of high velocity iron group elements constrains models involving pure helium accretion onto CO white dwarfs with ignition at the base of the helium layer; and the composition of the ejecta as function of time informs us of the expansion and accompanying nucleosynthesis history as a function of mass. X-ray studies of old Type Ia remnants in our Galaxy – SN 1006, Tycho 1572, and Kepler 1604 – are providing knowledge of the concentrations of such iron-peak nuclei as Mn and Cr in their ejecta.

On the Observed Diversity of Type Ia Supernovae A more critical factor is the great diversity in peak brightness that is observed for SNe Ia in different stellar populations. This is in principle understandable when we recall the fact that the peak brightness is a direct function of the 56Ni mass ejected. These can be utilized as an effective probe of the natures of the underlying events and impose constraints on theoretical models. In Chandrasekhar mass models in which the flame begins as a deflagration, the early sub-sonic flame evolution yields pre-expansion and insures that the outer regions are not burned entirely to 56Ni. The top panel shows galaxies morphologically specified as spirals, while the lower panel shows SNe in ellipticals or S0s. (Sullivan et al. 2006) (Wang et al. 2007)

Clues from Simulations: Detonation vs Deflagration Early studies of Type Ia models focused on carbon detonation (Arnett 1969). This yields incineration of the entire core to 56Ni (Arnett, Truran, & Woosley 1971). In contrast, the carbon deflagration model of Thielemann, Nomoto, & Yokoi (1986) was shown to give rise to expansion of the outer regions prior to flame passage and synthesis of intermediate mass nuclei in outer layers.

Nickel Production in SNe Ia (Nomoto W7 Model) Intermediate Mass Nuclei Ye < 0.5 56Ni Production (Timmes, Brown, & Truran 2003)

Understanding Type Ia Supernova Diversity Supernova Ia show significant (factor ~5) dispersion/range in peak brightness. The distribution of peak brightness is dependent upon stellar population – e.g. spirals in general have brighter SNe Ia’s and the brightest SNe Ia’s are not found in ellipticals. Theory strongly confirms that a critical factor impacting SNe Ia peak luminosities is the 56Ni mass e.g. Arnett (1964). The challenge is then to identify – for specific populations/classes of SNe Ia event – the factors that dictate these distinctive behaviors.

Factors that can Influence 56Ni Mass It is understood that a critical factor impacting SNe Ia light curves is the 56Ni mass (Arnett 1984). There are several factors that can influence/constrain this mass: Primordial stellar composition (22Ne)  Metal-rich galaxies (ellipticals) may have fewer “bright” SNe Ia. (Timmes, Brown & Truran 2004) Neutronization of core in outburst  strongly dependent on density and electron capture rates Degree of pre-expansion prior to detonation in DDT or GCD models:  impacts the ratio Mintermediate mass nuclei /Miron Mass of the “sub-Chandrasekhar” CO dwarf. Dredge-up of 12C and 16O in He accreting systems.

Diversity and the 56Ni Mass For Chandrasekhar mass models, variations in the 56Ni mass can be effected e.g. by metallicity variations, by the details of the onset and occurrence of DDT, or by such details of the gravitationally confined detonation (GCD) trigger as the number of ignition points. For sub-Chandrasekhar models, diversity follows from the spread in mass of the CO white dwarf models that are triggered to detonation by helium acccretion. Constraints on models can be provided by observed variations in peak luminosities with metallicity or stellar population, by their consistency with observations of SNe Ia light curves, and by spectroscopic studies of the compositions of supernova ejecta (e.g. the relative abundances of iron peak elements and intermediate mass – Si to Ca – nuclei). Considerations of galactic chemical evolution can also provide relevant constraints, since both SNe Ia and SNe II contribute both to iron (56Ni) and to the Si to Ca elements. As a consequence of the fact that both SNe Ia and SNe II synthesize iron-peak nuclei with Ye ≅ 0.5, it remains difficult to identify distinct abundance features that might serve to differentiate Type Ia and Type II nucleosynthesis contributions.

Neutronization, 56Ni, and SNe Ia Peak Luminosities Neutronization of matter ejected in SNe Ia favors the production of neutron-rich isotopes (e.g.54Fe, 58Ni) at the expense of 56Ni. This can arise during the explosion via electron capture, or it can be a direct consequence of initial composition:  Hydrogen burning (CNO): initial CNO nuclei  14N Helium burns 14N to 22Ne: 14N(,)18F(e+,)18O(,)22N The white dwarf progenitors of Type Ia supernovae have a composition of 12C, 16O, and approximately 2.5 percent 22Ne.  High metallicity populations may be expected to exhibit lower peak Supernova Ia luminosities.  In the dense inner regions of the CO progenitor cores of Type Ia supernovae, electron captures can effect significant neutron enrichment and low 56Ni production

Clues from Simulations: Detonation vs Deflagration Early studies of Type Ia models focused on carbon detonation (Arnett 1969). This yields incineration of the entire core to 56Ni (Arnett, Truran, & Woosley 1971). In contrast, the carbon deflagration model of Thielemann, Nomoto, & Yokoi (1986) was shown to give rise to expansion of the outer regions prior to flame passage and synthesis of intermediate mass nuclei in outer layers.

Abundance Scatter Timmes, Brown, & Truran 2003 A scatter of a factor of 3 about the mean in the initial metallicity of the progenitor star (or its stellar population) leads to a variation of about 25% (0.13 Msun) in the mass of 56Ni ejected. The peak brightness variation caused by this variation in the mass of 56Ni ejected is MV ~ 0.3 mag. which doesn’t account for all the observed variation.

Galactic Nucleosynthesis Constraints on SNe Iron The gross trends in the evolution of the abundances of iron-peak nuclei reflect the integrated input from both SNe Ia and SNe II: Halo stars show high ratios of “intermediate mass nuclei” (IMN) Mg-Ca to Fe, consistent with the ejecta of Type II (massive star) supernovae. The trends in IMN/Fe evolve toward those of Solar matter over the evolution of the galactic disk. This is viewed as reflecting input from Type Ia events, with higher Fe/IMN ratios. This is also consistent with the fact that SNe Ia are systematically brighter in spiral galaxies.

Clues from Simulations: Detonation vs Deflagration Early studies of Type Ia models focused on carbon detonation (Arnett 1969). This yields incineration of the entire core to 56Ni (Arnett, Truran, & Woosley 1971). In contrast, the carbon deflagration model of Thielemann, Nomoto, & Yokoi (1986) was shown to give rise to expansion of the outer regions prior to flame passage and synthesis of intermediate mass nuclei in outer layers.

Concluding Comments on Type Ia Supernovae The “iron-peak” nuclei in galaxies and the Cosmos are products of explosive nucleosynthesis occurring in both SNeIa and SNeII, with 56Ni as the primary product. The fact that the peak brightness of a SNe Ia is proportional to the mass of 56Ni ejected helps to identify the factors that likely dictate SNe Ia diversity: (a) neutron enrichment attributable either to electron captures in the core or to a high metallicity of the progenitor stars; and (b) pre-expansion of the outer regions of the core in Chandrasekhar mass models or low densities in the outer regions of sub-Chandrasekhar models. Quantification of the consequences of these factors in the context of theoretical models of SNeIa is essential.

Constraints on Type Ia Supernovae Models  Chandrasekhar Mass Models: Carbon Detonation Model: Incineration to 56Ni. Low abundances of intermediate mass elements in ejecta. Pure Deflagration Models: Fail to yield observed homogeneity of ejecta and high velocity of ejecta. Deflagration-to Detonation Transition (DDT): Challenge to theoretical efforts to show how this occurs. The now classic W7 model of Nomoto that imposes a profile with increasing velocity demonstrates how such models can nicely fit observations of abundances of ejecta with time. “Gravitationally Confined Detonation” Model: Off center ignition, rise of driven leading to detonation A major challenge for all Chandrasekhar mass models is to understand how a 1.4 solar mass model involving a carbon-oxygen white dwarf can be formed. Sub-Chandrasekhar Mass Models:

Gravitationally Confined Detonation

Nickel Production in SNe Ia (Nomoto W7 Model) Intermediate Mass Nuclei Ye < 0.5 56Ni Production (Timmes, Brown, & Truran 2003)

Consequences of Metallicity Dependence The brightness of a Type Ia supernova at maximum is a function of the mass of 56Ni produced. 56Ni is formed in nuclear statistical equilibrium in matter characterized by a Ye approaching 0.5 (e.g. Z  N)  The critical region of formation in Chandrasekhar mass Type Ia events is that from  0.2-0.8 solar masses. The mass of 56Ni formed in NSE in this critical region is sensitive to the initial composition of the star (22Ne). The observed scatter in metallicity in the galactic disk, to values several times solar, introduces a scatter in peak brightness of order 30 percent.  Metal rich galaxies (e.g. E galaxies) may be expected to have fewer bright SNe Ia. (Timmes, Brown, and Truran 2004)