Stellar Collisions and Ultracompact X-Ray Binary Formation
Stellar Collisions and Ultracompact X-Ray Binary Formation
复制标题
DOI：
10.1086/499938
复制
发表时间：
2005-09
期刊：
影响因子：
--
通讯作者：
James C. Lombardi;Z. Proulx;K. Dooley;E. M. Theriault;Natalia Ivanova;F. Rasio
中科院分区：
文献类型：
--
作者：
James C. Lombardi;Z. Proulx;K. Dooley;E. M. Theriault;Natalia Ivanova;F. Rasio
我们报告了亚巨星或轻微演化的红巨星与中子星 (NS) 之间抛物线碰撞的新 SPH 计算结果。这种碰撞可能为今天在古老球状星团中观察到的超紧凑 X 射线双星 (UCXB) 提供了主要的形成机制。特别是，我们计算了 1.4 M☉ NS 与初始质量为 0.8 和 0.9 M☉ 的真实建模母星的碰撞，每个母星处于三个不同的演化阶段（对应于三个不同的核心质量 mc 和半径 R）。初始轨道的最近接近距离从 rp = 0.04R（几乎迎面）到 1.3R（掠过）不等。这些碰撞导致形成紧密的双星，由 NS 和次巨星或红巨星核心组成，嵌入一个极其弥散的共同包层 (CE)，通常质量约为 0.1-0.3 M☉。我们的计算遵循二进制数数百个轨道，确保我们在计算结束时确定的轨道参数接近最终结果。在我们考虑的情况下，最初在巨人包膜中的一些流体（从 0.003 到 0.023 M☉）被留在 NS 中。所得双星的偏心率范围从我们大多数擦边碰撞的约 0.2 到几乎正面碰撞的约 0.9。在我们考虑的几乎所有情况下，仅引力辐射就会引起足够快的轨道衰变，从而在哈勃时间内形成 UCXB，而且通常在更短的时间尺度上。我们的流体动力学代码实现了 Springel & Hernquist 和 Monaghan 通过变分法导出的最新 SPH 运动方程。通过执行与 SPH 粒子的平滑长度和密度相关的解析约束方程来减少数值噪声。我们对这些新方法进行了测试，以帮助证明其准确性的提高。
We report the results of new SPH calculations of parabolic collisions between a subgiant or slightly evolved red giant star and a neutron star (NS). Such collisions are likely to provide the dominant formation mechanism for ultracompact X-ray binaries (UCXBs) observed today in old globular clusters. In particular, we compute collisions of a 1.4 M☉ NS with realistically modeled parent stars of initial masses 0.8 and 0.9 M☉, each at three different evolutionary stages (corresponding to three different core masses mc and radii R). The distance of closest approach for the initial orbit varies from rp = 0.04R (nearly head-on) to 1.3R (grazing). These collisions lead to the formation of a tight binary, composed of the NS and the subgiant or red giant core, embedded in an extremely diffuse common envelope (CE) typically of mass ~0.1-0.3 M☉. Our calculations follow the binary for many hundreds of orbits, ensuring that the orbital parameters we determine at the end of the calculations are close to final. Some of the fluid initially in the giant's envelope, from 0.003 to 0.023 M☉ in the cases we considered, is left bound to the NS. The eccentricities of the resulting binaries range from about 0.2 for our most grazing collision to about 0.9 for the nearly head-on cases. In almost all the cases we consider, gravitational radiation alone will cause sufficiently fast orbital decay to form a UCXB within a Hubble time, and often on a much shorter timescale. Our hydrodynamics code implements the recent SPH equations of motion derived with a variational approach by Springel & Hernquist and by Monaghan. Numerical noise is reduced by enforcing an analytic constraint equation that relates the smoothing lengths and densities of SPH particles. We present tests of these new methods to help demonstrate their improved accuracy.