AI 中文总结
本研究用MESA模拟主序星向超大质量黑洞的质量转移，揭示四个演化阶段，质量转移使轨道膨胀、引力波频率降低，在人马座A*周围最大信噪比约600。
AI 中文摘要
我们考虑围绕超大质量黑洞（SMBH）近距离运行的主序星，如果它们的轨道周期约为小时量级，则它们是LISA的潜在引力波（GW）源。在如此短的轨道周期下，恒星向超大质量黑洞的质量转移会发生。半长轴和引力波频率的演化取决于恒星质量和半径的演化。我们使用MESA研究从零龄主序（MS）开始向类似于人马座A*的超大质量黑洞转移质量的恒星。我们确定了4个演化阶段。（I）初始质量大于$2 M_\odot$的恒星在质量和半径减小时仍停留在主序上。（II）低于$2 M_\odot$时，间距足够小，以至于引力波时标太短，恒星无法维持热平衡。它们绝热地离开主序，并在失去高熵包层时迅速收缩。（III）低于$0.5-1 M_\odot$（取决于初始质量）时，均匀低熵核心暴露，恒星绝热膨胀。（IV）低于$0.15-0.4 M_\odot$时，热时标和引力波时标变得相当，恒星在维持这一平衡的同时冷却并收缩。总体而言，质量转移导致轨道膨胀，减慢了轨道演化，并导致在较低频率下发射引力波，使引力波信号更难探测。如果位于人马座A*周围，质量转移恒星大部分时间相对接近LISA灵敏度曲线，在阶段II和III之间的过渡期间最大信噪比达到约600。
英文摘要
We consider main-sequence stars orbiting close to a supermassive black hole (SMBH), which are potential gravitational wave (GW) sources for LISA if their orbital periods are of the order of an hour. At such a short orbital period, mass transfer from the star to the SMBH occurs. The evolution of the semi-major axis and GW frequency depends on the evolution of the stellar mass and radius. We use MESA to study stars that transfer mass to SMBHs similar to Sagittarius A* starting on the zero-age main sequence (MS). We identify 4 evolutionary phases. (I) Stars initially with mass $>2 M_\odot$ remain on the MS as their mass and radius decrease. (II) Below $2 M_\odot$, the separation is sufficiently small so that the GW timescale is too short for the stars to maintain thermal equilibrium. They evolve adiabatically off the MS and shrink rapidly as they lose their high-entropy envelope. (III) Below $0.5-1 M_\odot$, depending on the initial mass, the uniform low-entropy core is exposed and stars expand adiabatically. (IV) Below $0.15-0.4 M_\odot$, the thermal and GW timescales become comparable, and stars cool and shrink while maintaining this balance. Overall, mass transfer causes the orbit to expand, slowing down the orbital evolution and leading to GW emission at lower frequencies making the GW signal harder to detect. If located around Sagittarius A*, mass transferring stars spend most of the time relatively close to the LISA sensitivity curve, with maximal SNR reaching around 600 during the transition between stages II and III.
Comments12 pages, 6 figures
