Accretion in Binary Systems with Slow Stellar Winds

Abstract

Wind accretion in binary systems is commonly described using the Bondi-Hoyle-Lyttleton (BHL) formalism. However, its standard implementation fails in the slow-wind regime, where the wind velocity of the donor star ($v_\mathrm{w}$) is comparable to or smaller than the orbital velocity of the accretor ($v_\mathrm{o}$). Tejeda & Toalá recently proposed a geometrical correction to the BHL formalism that accounts for the wind aberration caused by the binary's orbital motion, which tilts the accretion cylinder and reduces its effective cross-section. Here we present a suite of smoothed particle hydrodynamic simulations performed with PHANTOM to test wind accretion in binary systems operating in this slow-wind regime. We explore circular configurations and directly measure mass accretion efficiencies from the simulations. Our results confirm that the standard BHL prescription systematically overestimates accretion rates for $v_\mathrm{w}/v_\mathrm{o} < 1$, while the geometrically corrected model reproduces the simulated efficiencies with remarkable accuracy. A key finding is that the velocity relevant for accretion estimates is not the value derived from the unperturbed stellar wind, but the local gas velocity measured upstream of the accretor. The gravitational potential of the accretor perturbs the flow, altering the effective relative velocity and modifying the accretion efficiency, particularly for compact orbits. These results provide strong numerical support for the geometrically corrected framework and establish a physically motivated basis for modeling wind-fed accretion in interacting binaries, including symbiotic systems.

Figures (7)

• Figure 1: Unperturbed wind velocity profile ($v_\mathrm{w,u}$) for the mass-losing donor star ($m_1$) computed by phantom (solid line). This profile remains identical across all simulations in our suite. The wind is launched from the donor surface with an initial velocity $v_\mathrm{ini}=10$ km s$^{-1}$, reaching a terminal speed of $v_{\infty}\lesssim 17.8$ km s$^{-1}$. The dashed vertical line at $r=1$ AU indicates the wind injection radius ($R_1$). Symbols represent the local wind velocity ($v_\mathrm{w}$) measured upstream near the accretor for each individual simulation.
• Figure 2: Density map for simulation A4 in the orbital plane. Left: Global view showing the donor ($m_1$), the accretor ($m_2$), and the system's center of mass (cross). Right : Close-up of the secondary and its associated accretion disk. In both panels, the dashed circle centered on $m_2$ represents the characteristic radius $r_\mathrm{acc}$, marking the location where the upstream wind velocity ($v_\mathrm{w}$) is measured to represent the actual flow conditions encountered by the accretor.
• Figure 3: Velocity profiles for simulation A4, extracted extracted along the dashed axis shown in the left panel of Fig. \ref{['fig:density']}. The perturbed profile (solid line) represents the velocity measured in the direction toward the secondary, whereas the unperturbed profile (dashed) corresponds to the opposite direction. For comparison, the theoretical wind profile ($v_{\mathrm{w,u}}$) from Fig. \ref{['fig:wind1D']} is shown in a dotted line. The vertical dotted line indicates the orbital separation $a=8$ AU.
• Figure 4: Top: Mass accretion efficiency $\eta$ as a function of the wind dimensionless wind parameter $w$. Analytical predictions from the geometrically corrected model ($\eta_\mathrm{TT}$, dashed line) and the standard BHL implementation ($\eta_\mathrm{BHL}$, dotted line) are shown for comparison. Bottom: Fractional errors between the analytical predictions that the numerical results. In both panels, diamonds and bullets represents measurements from the A1--A10 ($q = 2.86$) and B1--B10 ($q=0.15$) simulation families, respectively.
• Figure 5: Fractional difference between the unperturbed wind velocity ($v_{\mathrm{w,u}}$) and the local gas velocity measured from the simulations ($v_{\mathrm{w}}$), defined as $1 - v_{\mathrm{w,u}}/v_{\mathrm{w}}$. Symbols represent results for both simulation families (A and B) as a function of orbital separation.