Implementing tidal and gravitational wave energy losses in few-body codes: A fast and easy drag force model

We present a drag force model for evolving chaotic few-body interactions with the inclusion of orbital energy losses, such as tidal dissipation and gravitational wave (GW) emission. The main effect from such losses is the formation of two-body captures that for compact objects result in GW mergers and for stars lead to either compact binaries, mergers, or disruptions. Studying the inclusion of energy loss terms in few-body interactions is therefore likely to be important for modelling and understanding the variety of transients that soon will be observed by current and upcoming surveys. However, including especially tides in few-body codes has been shown to be technically difficult and computationally heavy, which has led to very few systematic tidal studies. In this paper we derive a drag force term that can be used to model the effects from tidal, as well as other energy losses in few-body interactions, if the two-body orbit averaged energy loss is known a priori. This drag force model is very fast to evolve and gives results in agreement with other approaches, including the impulsive and affine tide approximations.