Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Type of Multimessenger Source for LISA

Aleksandra Olejak, Jakob Stegmann, Selma E. de Mink, Ruggero Valli, Re’em Sari, Stephen Justham and Taeho Ryu
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Abstract

Some stars orbiting supermassive black holes (SMBHs) are expected to undergo a gravitational wave (GW)–driven inspiral and initiate mass transfer on nearly circular orbits. However, the stability and duration of such phases remain unexplored. In this work, we focus on the evolution of a low-mass, radiative-envelope subgiant star being stripped by an SMBH. We find that such systems can undergo a long-lasting, stable mass transfer phase, even if none of the angular momentum of the transferred material returns to the orbit to counterbalance the GW-driven decay. We show an example where a 2 M⊙ subgiant is stripped before entering the Laser Interferometer Space Antenna (LISA) band and loses almost its entire hydrogen envelope. The remaining helium core undergoes a prolonged GW-driven inspiral, becoming a loud LISA source. If formed in our Galaxy, the system would be detectable for several hundred thousand years, ultimately reaching extreme signal-to-noise ratios of a million. Hydrogen shell flashes in the residual envelope cause temporary radial expansions of the stripped star. As a result, a few additional phases of rapid mass transfer occur at orbital periods of 20–30 hr. Eventually, the core possibly undergoes circular partial tidal disruption at an orbital period of ∼10 minutes, corresponding to a GW emission frequency of a few mHz. We estimate a chance of about 1% that such a detectable LISA source exists in our own Galactic center. The loud final GW transient may lead to a few detections reaching as far as ∼1 Gpc, including, e.g., the Abell clusters.
超大质量黑洞将一颗次巨星剥离至其氦核:LISA的一种新型多信使源
一些围绕超大质量黑洞(SMBHs)运行的恒星预计会经历引力波(GW)驱动的激励,并在接近圆形的轨道上开始传质。然而,这些阶段的稳定性和持续时间仍未得到探索。在这项工作中,我们关注的是被SMBH剥离的低质量辐射包层亚巨星的演化。我们发现这样的系统可以经历一个持久的,稳定的质量传递阶段,即使没有转移物质的角动量返回到轨道来抵消gw驱动的衰变。我们展示了一个例子,其中一个2 M⊙的亚巨星在进入激光干涉仪空间天线(LISA)波段之前被剥离,几乎失去了整个氢包层。剩余的氦核经历了长时间的gw驱动的激励,成为一个响亮的LISA源。如果在我们的银河系中形成,这个系统将在几十万年内被探测到,最终达到100万的极端信噪比。残余包层中的氢壳闪光引起被剥离恒星的暂时径向膨胀。因此,在20-30小时的轨道周期内会出现一些额外的快速传质阶段。最终,地核可能在轨道周期约10分钟内经历圆形部分潮汐破坏,对应于几兆赫兹的吉瓦发射频率。我们估计在银河系中心存在这样一个可探测到的LISA源的可能性约为1%。响亮的最终GW瞬态可能导致一些检测达到~ 1 Gpc,包括Abell星系团等。
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