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This paper examines the properties of flows around objects embedded withincommon envelopes in the simplified context of a "wind tunnel." We establishcharacteristic relationships between key common envelope flow parameters likethe Mach number and density scale height. Our wind tunnel is athree-dimensional, cartesian geometry hydrodynamic simulation setup thatincludes the gravity of the primary and secondary stars and allows us to studythe coefficients of drag and accretion experienced by the embedded object.Accretion and drag lead to a transformation of an embedded object and its orbitduring a common envelope phase. We present two suites of simulations spanning arange of density gradients and Mach numbers -- relevant for flow near the limbof a stellar envelope to the deep interior. In one suite, we adopt an ideal gasadiabatic exponent of $\gamma=5/3$, in the other, $\gamma=4/3$. We find thatcoefficients of drag rise in flows with steeper density gradients and thatcoefficients of drag and accretion are consistently higher in the morecompressible, $\gamma=4/3$ flow. We illustrate the impact of these newlyderived coefficients by integrating the inspiral of a secondary object throughthe envelopes of $3M_\odot$ ($\gamma\approx5/3$) and $80M_\odot$($\gamma\approx4/3$) giants. In these examples, we find a relatively rapidinitial inspiral because, near the stellar limb, dynamical friction drag isgenerated mainly from dense gas focussed from deeper within the primary-star'senvelope. This rapid initial inspiral timescale carries potential implicationsfor the timescale of transients from early common envelope interaction.

astrophysical journal/the astrophysical journal

Common Envelope Wind Tunnel: Coefficients of Drag and Accretion in a  Simplified Context for Studying Flows Around Objects Embedded Within Stellar  Envelopes