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

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.