Gravitational waves from in-spirals of compact objects in binary common-envelope evolution

Detection of gravitational-wave (GW) sources enables the characterization of binary compact objects (COs) and of their in-spiral. However, other dissipative processes can affect the in-spiral. Here, we show that the in-spiral of COs through a gaseous common envelope (CE) arising from an evolved stellar companion produces a novel type of GW sources, whose evolution is dominated by the dissipative gas dynamical friction effects from the CE, rather than the GW emission itself. The evolution and properties of the GW signals differ from those of isolated gas-poor mergers significantly. We find characteristic strains of ∼10⁻²³–10⁻²¹ (10 kpc/D) for such sources – observable by next-generation space-based GW detectors (at rates of once per a few centuries for LISA, and about once a year for BBO). The evolution of the GW signal can serve as a probe of the interior regions of the evolved star, and the final stages of CE evolution, otherwise inaccessible through other observational means. Moreover, such CE mergers are frequently followed by observable explosive electromagnetic counterparts and/or the formation of exotic stars.