The Rapid Proton Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
The Rapid Proton Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star
复制标题
DOI：
10.1086/307837
复制
发表时间：
1999-05
期刊：
影响因子：
--
通讯作者：
H. Schatz;L. Bildsten;A. Cumming;M. Wiescher
中科院分区：
文献类型：
--
作者：
H. Schatz;L. Bildsten;A. Cumming;M. Wiescher
一颗吸积的中子星的地壳和海洋的温度和核组成取决于物质（灰烬）的混合，这些物质是由吸积的氢和氦在较低密度下融合产生的。氢/氦的燃烧在高吸积速率下是热稳定的，这种情况在吸积速率为bbb10 -8 M☉-1的弱磁（B≪1011 G）中子星和大多数吸积x射线脉冲星中都会遇到，在那里物质集中到磁极导致局部吸积速率高到足以稳定燃烧。对于以如此高速率吸积的中子星，我们计算了上层大气中氢和氦的稳态燃烧（ρ < 2 × 106 g cm-3），其中T≈(5-15)× 108 K。由于从“热”CNO循环中爆发的温度与稳定氦燃烧的温度相当（T > 5 × 108 K），氢总是通过华莱士和伍斯利的快速质子俘获（rp）过程燃烧。rp过程产生的原子核远远超过铁族，总是导致质量为a ~ 60-100的元素混合物。灰烬的平均核质量是由氦通过（α, p）反应燃烧的程度决定的，因为这些反应对温度敏感，所以取决于局部的吸积率。导致主要由铁组成的核统计平衡，只发生在非常高的局部吸积速率超过爱丁顿速率50倍的情况下。我们简要地讨论了我们的结果对中子星性质的影响。在固定的吸积速率下产生的核的范围很广，以及灰成分对局部吸积速率的敏感性，使得吸积中子星不可避免地有一个由各种各样的核组成的海洋和地壳。这对中子星地壳的热学和电学性质以及结构性质（剪切模量和粘度）产生了影响。我们的结果表明，不纯的地壳晶格在其大部分质量中都具有通过杂质散射设定的导电性，这使得磁场的欧姆扩散比以前估计的单核混合物更快。
The temperature and nuclear composition of the crust and ocean of an accreting neutron star depend on the mix of material (the ashes) that is produced at lower densities by fusion of the accreting hydrogen and helium. The hydrogen/helium burning is thermally stable at high accretion rates, a situation encountered in weakly magnetic (B ≪ 1011 G) neutron stars accreting at rates > 10-8 M☉ yr-1 and in most accreting X-ray pulsars, where the focusing of matter onto the magnetic poles results in local accretion rates high enough for stable burning. For a neutron star accreting at these high rates, we calculate the steady state burning of hydrogen and helium in the upper atmosphere (ρ < 2 × 106 g cm-3), where T ≈ (5-15) × 108 K. Since the breakout from the "hot" CNO cycle occurs at a temperature comparable to that of stable helium burning (T ≳ 5 × 108 K), the hydrogen is always burned via the rapid proton capture (rp) process of Wallace & Woosley. The rp-process makes nuclei far beyond the iron group, always leading to a mixture of elements with masses A ~ 60-100. The average nuclear mass of the ashes is set by the extent of helium burning via (α, p) reactions and, because these reactions are temperature sensitive, depends on the local accretion rate. Nuclear statistical equilibrium, leading to a composition of mostly iron, occurs only for very high local accretion rates in excess of 50 times the Eddington rate. We briefly discuss the consequences of our results for the properties of the neutron star. The wide range of nuclei made at a fixed accretion rate and the sensitivity of the ash composition to the local accretion rate makes it inevitable that accreting neutron stars have an ocean and crust made up of a large variety of nuclei. This has repercussions for the thermal and electrical properties and structural properties (the shear modulus and viscosity) of the neutron star crust. A crustal lattice as impure as implied by our results will have the conductivity throughout most of its mass set by impurity scattering, allowing for more rapid Ohmic diffusion of magnetic fields than previously estimated for mononuclear mixes.