Explosive H-burning, the rp-process, and X-ray bursts

Abstract

The major astrophysical events which involve explosive H-burning are novae and type I X-ray bursts. Both are related to binary stellar systems with hydrogen accretion from a binary companion onto a compact object, and the explosive ignition of the accreted H-layer. High densities cause the pressure to be dominated by the degenerate electron gas, preventing a stable and controlled burning. In the case of novae the compact object is a white dwarf, in the case of X-ray bursts it is a neutron star. Explosive II-burning in novae has been discussed in many recent articles [1, 2, 3, 4, 5]. Its processing is limited due to maximum temperatures of similar to 3X10(8)K. Only in X-ray bursts temperatures larger than 4x10(8)K are possible, which permit a break-out from the hot CNO-cycle, leading to a further temperature increase beyond 10(9)K, the onset of an rp-process (a sequence of proton captures and beta-decays) and burning of H and He to Fe/Ni and beyond. Here we investigate the rp-process by making use of a complete and updated nuclear reaction network from H to Sn. In particular we consider 2p-capture reactions that can bridge proton unbound nuclei and therefore accelerate the reaction flow. In a simplified one dimensional, one-cone X-ray burst model we find that for a 25 s burst the reaction flow reaches Cd. The consequences for energy production, final composition of the ashes, and fuel consumption are discussed.