Nuclear-reaction rates in the thermonuclear runaway phase of accreting neutron stars
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THE EUROPEAN PHYSICAL JOURNAL A
Nuclear-reaction rates in the thermonuclear runaway phase of accreting neutron stars M. Wiescher1,a , V. Barnard1 , J.L. Fisker2,3 , J. G¨ orres1 , K. Langanke3 , G. Martinez-Pinedo2,3 , F. Rembges2 , 4 2 H. Schatz , and F.K. Thielemann 1 2 3 4
Department of Physics, University of Notre Dame, Notre Dame, IN 46556-5670, USA Department of Physics, University of Basel, CH-4056 Basel, Switzerland Institute of Physics and Astronomy, University of ˚ Arhus, DK-8000 ˚ Arhus, Denmark NSCL & Department of Physics, Michigan State University, East Lansing, MI 48824, USA Received: 21 March 2002 / c Societ` Published online: 31 October 2002 – a Italiana di Fisica / Springer-Verlag 2002 Abstract. The rp-process has been suggested as the dominant nucleosynthesis process in explosive hydrogen burning at high temperature and density conditions. The process is characterized by a sequence of fast proton capture reactions and subsequent β-decays. The reaction path of the rp-process runs along the drip line up to Z ≈ 50. Most of the charged-particle reaction rates for the reaction path are presently based on statistical Hauser-Feshbach calculations. While these rates are supposed to be reliable within a factor of two for conditions of high density in the compound nuclei, discrepancies may occur for nuclei near closed shells or near the proton drip line where the Q-values of proton capture processes are typically very small. It has been argued that the thermonuclear runaway is less sensitive to the reaction rates because of the rapid time-scale of the event. However, since these processes may operate at the same time-scale as fast mixing and convection processes, a change in reaction rates indeed may have a significant impact. In this paper we present two examples, the break-out from the hot CNO cycles, and the thermonuclear runaway in X-ray bursts itself, where changes in reaction rates have a direct impact on time-scale, energy generation and nucleosynthesis predictions for the explosive event. PACS. 21.60.Cs Shell model – 26.30.+k Nucleosynthesis in novae, supernovae and other explosive environments
1 Introduction The thermonuclear runaway in accreting binary star systems is triggered by hydrogen ignition of the accreted material through the pp chains and the hot CNO cycles. Thermonuclear explosions on the surface of accreting neutron star binary components have been identified as X-ray bursts [1]. The thermonuclear explosion itself is powered by a sequence of proton capture reactions and β-decays on short-lived neutron-deficient nuclei [2] after break-out from the CNO cycles [3] and is characterized by the ≈ 1-2 s lasting thermonuclear runaway phase at electron degenerate conditions. After degeneracy is lifted, a much longer cooling phase sets in which is typically associated with radiation-driven expansion. The endpoint of the reaction path during the entire X-ray burst is associated with reaction cycling in the Sn-Sb-Te range [4]. While the nucleosynthesis aspects of the cooling phase have been discu
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