2021ApJ...919..128R

Understanding common envelope (CE) evolution is an outstanding problem in binary evolution. Although the CE phase is not driven by gravitational wave (GW) emission, the inspiraling binary emits GWs that passively trace CE dynamics. Detecting this GW signal would provide direct insight into gas-driven physics. Even a non-detection might offer invaluable constraints. We investigate the prospects of detection of a Galactic CE by LISA. While the dynamical phase of the CE is likely sufficiently loud for detection, it is short and thus rare. We focus instead on the self-regulated phase that proceeds on a thermal timescale. Based on population-synthesis calculations and the (unknown) signal duration in the LISA band, we expect ∼0.1-100 sources in the Galaxy during the mission duration. We map the GW observable parameter space of frequency f_GW and its derivative {dot}f_GW, remaining agnostic on the specifics of the inspiral and find that signals with signal-to-noise ratios > 10 are possible if the CE stalls at separations such that f_GW >= 2 x 10^–3 Hz. We investigate the possibility of misidentifying the signal with other known sources. If the second derivative \ddotf_GW can also be measured, the signal can be distinguished from other sources using a GW braking index. Alternatively, coupling LISA with electromagnetic observations of peculiar red giant stars and/or infrared and optical transients, might allow for the disentangling of a Galactic CE from other Galactic and extragalactic GW sources.