Schedule Aug 21, 2009
Trends in 44 Ti and 56 Ni from Core-Collapse Supernovae
Frank Timmes (ASU)

We investigate trends in the yields of ux{44}{Ti} and ux{56}{Ni} as material with a specified initial electron fraction both cools and rarefies along exponential, power-law, and post-processing thermodynamic trajectories. We use the exponential and power-law expansion profiles to identify 6 distinct regions in the peak temperature-density plane and extensively probe the sensitivity of the isotopic yields to the input nuclear physics. The 6 regions are largely driven by different phase transitions within the quasi-statistical equilbrium cluster as the material undergoes freeze-out. Most Si-group and Fe-group isotopes are affected by the transitions, with the exception of a small group centralized around 56Ni. Reactions that affect all yields globally are the 3-alpha, p(e-,nu_e)n and n(e+, nubar_e)p, while 44Ti is mainly affected by 44Ti(a,p)47V, 40Ca(a,g)44Ti and 45V(p,g)46Cr. Trends due to electron fraction variations may be largely explained in terms of an empirical rule for the major nuclear flows where the most abundant isotopes at any given time in the thermodynamic evolution are generally the ones whose individual proton to nucleon ratio is within a small range from the current value of the electron fraction in combination with a minimization of the Helmholtz free energy. We compare the yields of 44Ti and 56Ni produced from post-processing the thermodynamic trajectories from three different core-collapse models -- a Cas A progenitor, a double shock hypernova progenitor, and a rotating 2D explosion -- with the yields from the exponential and power-law trajectories. The peak temperatures and densities achieved in these core-collapse models span several of the 6 regions we identify, resulting in different trends in the 44Ti and 56Ni yields for different mass elements. The 44Ti and 56Ni mass fraction profiles from the exponential and power-law profiles generally explain the tendencies of the post-processed yields, depending on which regions are traversed by the model. We find integrated yields of 44Ti and 56Ni from the exponential and power-law trajectories are generally within a factor 2 or less of the post-process yields. Our analysis suggests that not all 44Ti need be produced in an alpha-rich freeze-out in core-collapse events, and that that reaction rate equilibria in combination with timescale effects for the expansion profile may account for the paucity of 44Ti observed in supernovae remnants.

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