An analytic model for blue straggler formation in globular clusters

We present an analytic model for blue straggler formation in globular clusters. We assume that blue stragglers are formed only through stellar collisions and binary star evolution, and compare our predictions to observed blue straggler numbers. We can summarize our key results as follows. (1) Binary star evolution consistently dominates blue straggler production in all our best-fitting models. (2) In order to account for the observed sublinear dependence of blue straggler numbers on the core masses, the core binary fraction must be inversely proportional to the total cluster luminosity and should always exceed at least a few per cent. (3) In at least some clusters, blue straggler formation must be enhanced by dynamical encounters (either via direct collisions or by stimulating mass transfer to occur by altering the distribution of binary orbital parameters) relative to what is expected by assuming a simple population of binaries evolving in isolation. (4) The agreement between the predictions of our model and the observations can be improved by including blue stragglers that form outside the core but later migrate in due to dynamical friction. (5) Longer blue straggler lifetimes are preferred in models that include blue stragglers formed outside the core since this increases the fraction that will have sufficient time to migrate in via dynamical friction.

我们提出了一个球状星团中蓝离散星形成的分析模型。我们假设蓝离散星仅通过恒星碰撞和双星演化形成，并将我们的预测与观测到的蓝离散星数量进行比较。我们可以将主要结果总结如下：（1）在我们所有的最佳拟合模型中，双星演化始终在蓝离散星的产生过程中占主导地位。（2）为了解释观测到的蓝离散星数量对核心质量的次线性依赖关系，核心的双星比例必须与星团总光度成反比，并且应始终至少超过百分之几。（3）至少在一些星团中，相对于假设一群孤立演化的简单双星所预期的情况，动力学相遇（要么通过直接碰撞，要么通过改变双星轨道参数的分布来刺激物质转移发生）必须增强蓝离散星的形成。（4）通过纳入在核心之外形成但后来由于动力学摩擦而迁移进来的蓝离散星，可以提高我们模型的预测与观测结果之间的一致性。（5）在包含在核心之外形成的蓝离散星的模型中，更长的蓝离散星寿命更受青睐，因为这增加了有足够时间通过动力学摩擦迁移进来的比例。