Host: Davide Gerosa
Speaker: Aleksandra Olejak (MPA Garching)
Title: Supermassive black holes stripping a subgiant star down to its helium core: a new type of multi-messenger source for LISA
Abstract : Some stars orbiting supermassive black holes (SMBHs) may gradually spiral inward due to gravitational-wave (GW) emission and eventually begin transferring mass on nearly circular orbits. Yet, the nature of this phase: how long it lasts and whether it's stable, remains an open question.In this talk, I’ll present our work exploring such a scenario, focusing on a low-mass subgiant star with a radiative envelope as it begins to transfer mass to an SMBH. We find that even if none of the angular momentum of the transferred material returns to the orbit to counterbalance the GW-driven decay, the system can still sustain a long-lived, stable mass-transfer phase. In the presented example, a 2-solar-mass subgiant star loses nearly its entire hydrogen envelope before entering the LISA band, leaving behind a helium core that continues its GW-driven inspiral, becoming a loud, long-lived LISA source. In the case of a Galactic event, such a system could be detectable for hundreds of thousands of years, eventually reaching an extreme signal-to-noise ratio of up to a million.
Interestingly, hydrogen shell flashes in the residual envelope can temporarily puff up the star, triggering a few short-lived episodes of rapid mass transfer, with orbital periods around 20–30 hours. Ultimately, the helium core may be partially disrupted by the SMBH when the orbital period shrinks to about 10 minutes, corresponding to a GW frequency of a few millihertz. 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, for example, sources in the Abell clusters.