星团中造父变星多重性的动力学演化及其对B型星诞生时多重性的启示
On the dynamical evolution of Cepheid multiplicity in star clusters and its implications for B-star multiplicity at birth
AI总结：
本文通过星团中双星演化与动力学交会模拟，研究造父变星多重性演化，发现星团环境提高双星比例、并合普遍且模拟双星比例仅为观测值一半，表明中等质量B型星诞生时可能普遍形成于三合星及更高阶系统。
摘要：
经典造父变星为中等质量恒星在数千万至数亿年间的双星演化提供了独特的探针。我们研究了造父变星的双星与多重性性质，假设所有中等质量B型星都在星团内部以双星形式诞生。这些双星系统同时受到恒星演化以及与星团中其他恒星动力学交会的影响。动力学星团环境导致留在星团中的造父变星具有比逃逸到场中的造父变星更高的双星比例（约60%对约35%）。在星团中，双星、三合星和多重系统的比例随星团质量增大而下降。质量更大的星团中的双星轨道周期比低质量星团和场造父变星更短。并合非常普遍，约30%的中等质量B型星因与伴星相互作用而未能演化成造父变星。约40%的造父变星已与伴星并合，并合事件会影响恒星演化；由质量推断的造父变星年龄可能与其宿主星团年龄不同。我们的模型预测，五分之一的造父变星是由质量低于造父变星质量下限的恒星并合产生的；在星团中，这些天体出现的时间明显晚于由其质量所预期的时间。约3%至5%的造父变星拥有致密伴星（约0.15%的造父变星伴有黑洞）。我们的模拟得到的双星比例（42%）比观测到的造父变星双星比例低约一倍。这表明B型星诞生时的真实多重性比例可能远大于1，因此中等质量B型星可能通常形成于三合星及更高阶系统中。
英文摘要：
Classical Cepheid variable stars provide a unique probe to binary evolution in intermediate-mass stars over the course of several tens to hundreds of Myr. We studied the binary and multiple properties of Cepheids, assuming that all mid-B stars form in binaries inside star clusters. The binaries were subjected both to stellar evolution and dynamical encounters with other stars in the cluster. The dynamical cluster environment results in a higher binary fraction among the Cepheids that remain in star clusters ($\approx 60$%) than among the Cepheids which have escaped to the field ($\approx 35$%). In clusters, the binary, triple, and multiple fraction decreases with increasing cluster mass. More massive clusters have binaries of shorter orbital periods than lower mass clusters and field Cepheids. Mergers are very common with $\approx 30$% of mid-B stars not evolving to Cepheids because of the interaction with their companion. Approximately $40$ % of Cepheids have merged with their companion, and the merger event impacts stellar evolution; the age of Cepheids expected from their mass can differ from the age of their host cluster. Our models predict that one in five Cepheids is the result of a merger between stars with mass below the lower mass limit for Cepheids; in clusters, these objects occur substantially later than expected from their mass. Approximately $3$ to $5$ % of all Cepheids have a compact companion ($\approx 0.15$ % of all Cepheids are accompanied by a black hole). The binary fraction derived from our simulations (42%) underestimates the observed binary Cepheid fraction by approximately a factor of 2. This suggests that the true multiplicity fraction of B-stars at birth could be substantially larger than unity and, thus, that mid-B stars may typically form in triple and higher order systems.
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