Explosive nucleosynthesis in core-collapse

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Explosive nucleosynthesis in core-collapse supernovae and the titanium problem To cite this article: M Eichler et al 2018 J. Phys.: Conf. Ser. 940 012040

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Nuclear Physics in Astrophysics Conference (NPA VII) IOP Conf. Series: Journal of Physics: Conf. Series 1234567890 940 (2017) 012040

IOP Publishing doi:10.1088/1742-6596/940/1/012040

Explosive nucleosynthesis in core-collapse supernovae and the titanium problem M Eichler1 , A Perego2 , M Hempel1 , C Fr¨ ohlich3 , K Ebinger1 , J 3 1 Casanova , M Liebend¨ orfer and F-K Thielemann1 1

Department f¨ ur Physik, Universit¨ at Basel, CH-4056 Basel, Switzerland Institut f¨ ur Kernphysik, Technische Universit¨ at Darmstadt, D-64289 Darmstadt, Germany 3 Department of Physics, North Carolina State University, Raleigh NC 27695, USA 2

E-mail: [email protected] Abstract. Based on a new mechanism to drive spherically symmetric core-collapse supernovae, PUSH, we perform full network nucleosynthesis calculations for different progenitors. While the 56−58 Ni yields match the observational data very well for certain progenitors, the ejected titanium masses in our calculations are lower than the values inferred from observations. We demonstrate the dependence of ejecta composition on the progenitor structure and the mass cut. Furthermore, we discuss possible solutions to the well-known problem of titanium underproduction.

1. Introduction Recently, the PUSH method for triggering core-collapse supernovae in 1D has been presented ([1], Fr¨ ohlich et al., and Ebinger et al., this volume). In this approach, a fraction of the energy of the heavy flavour neutrinos is deposited in the gain region to artificially increase the neutrino heating efficiency in otherwise non-exploding spherically symmetric models. Apart from studying explosion characteristics for a broad range of progenitors, this method provides a good framework for nucleosynthesis studies in core-collapse supernovae. One of the long-standing problems associated with explosive nucleosynthesis in core-collapse supernovae is the discrepancy in the amount of 44 Ti ejected between observations and simulations, the latter usually being significantly lower than the former. While observations of SN 1987A have reported values between (0.5 − 4) × 10−4 M [2, 3, 4, 5, 6, 7, 8], simulations (e.g., [9, 10]) typically predict around one order of magnitude less. We give a short overview of our nucleosynthesis network and compare the composition of the ejecta from two different progenitors in our calculations with the observed values from SN 1987A in section 2. In section 3 we summarize the titanium underproduction problem and discuss possible solutions. 2. Composition of the ejecta The PUSH method is calibrated on observables (explosion energy, 56−58 Ni, and 44 Ti yields) of SN 1987A. To that end, we explore the progenitor space between 18 M and 21 M , as it is known that the progenitor of SN 1987A is in that mass bracket. After applying a fallback of 0.1 M , many progenitors in that mass range are found to reproduce the 56 Ni yield from

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SN 1987A well enough, however only about half of them also produce 57 Ni and 58 Ni to an amount that is needed to match the observational data. The reason can be found in the initial Ye profiles of the progenitors and the position of the cutoff mass in the supernova simulations. The ejecta composition is plotted against mass coordinates in Figure 1 for two of the progenitors we consider (18.0 M and 20.6 M ) [11]. For the 18.0 M model, the cutoff mass is 1.56 M and a large part of the silicon shell is ejected. In this shell, the initial matter composition is slightly neutron-rich (due to a small contribution from 56 Fe) with Ye 0.498 and the conditions for the production of 57 Ni and 58 Ni are favourable. The transition from silicon shell to oxygen shell for the 20.6 M model happens around 1.74 M , i.e., inside the mass cut. Therefore, the matter ejected by this model is less neutron-rich and contains less 57 Ni and 58 Ni (see also [12]). 10

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Figure 1. Ejecta composition of the 18.0 M (left) and 20.6 M (right) progenitors vs mass coordinates.

3. Titanium-44 With a fallback of 0.1 M onto the proto-neutron star, the amount of 56−58 Ni in the ejecta and the explosion energies agree very well with observations of SN 1987A for two of the progenitors we considered (18.0 M and 19.4 M ). However, the inclusion of fallback drastically reduces the ejected amount of 44 Ti, as it is produced in the innermost part of the ejecta. In Figure 2 the ejected amounts of 44 Ti (a) without and (b) with fallback are compared for four different progenitors and five different PUSH parameter sets, plotted against the respective explosion energies. The error box represents the observational values from Seitenzahl et al. (2014) [8]. Too low 44 Ti yield predictions have been a long-standing problem in supernova nucleosynthesis calculations (e.g., [9, 10]). Several uncertainties are connected to the production, destruction, and ejection of 44 Ti. First, ejecta in a supernova may be subject to convective overturn, which cannot be simulated in our 1D approach. To account for this, we can assume homogeneous mixing up to the outer boundary of the silicon shell before cutting off the fallback material (e.g., [13]). As a consequence, the ejected 44 Ti mass for the 18.0 M progenitor increases to 2.70 × 10−5 M compared to the previous yield of 1.04 × 10−5 M . Moreover, there are uncertainties in the main nuclear reactions that are responsible for the production and destruction of 44 Ti. The final amount of produced 44 Ti depends mainly on two reactions: 40 Ca(α,γ)44 Ti and 44 Ti(α,p)47 V. Recent measurements of the 44 Ti(α,p)47 V reaction rate concluded that it may be considerably smaller than previous theoretical predictions [14]. Using this smaller cross section, our yield of ejected 44 Ti for the 18.0 M progenitor rises to 1.49 × 10−5 M with fallback and 5.65 × 10−5 M without fallback (see Figure 3). If we include both the new cross section and homogeneous mixing (corresponding to the red line in Figure 3), the amount of 44 Ti in the

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Figure 2. Ejected 44 Ti masses for different progenitors and PUSH parameter sets (a) without and (b) with the inclusion of 0.1 M fallback. ejecta is 3.99 × 10−5 M including fallback, just within the error range of the observational value reported in Ref. [8]. Figure 3. 44 Ti yields of the ejecta for the 18 M star as in Figure 1 (black solid line), and with an updated Ti 44 Ti(α,p)47 V rate (black Ti with mixing dashed line). The red line represents the distribution when in addition to the updated rate homogeneous mixing is assumed. The dashed vertical line indicates the 1.85 1.90 mass cut if 0.1 M fallback is applied.

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