Entering the Wind Roche Lobe Overflow realm in Symbiotic Systems

TL;DR

The paper combines stellar evolution with N-body dynamics to study wind accretion, WRLO, drag, and tidal effects in compact symbiotic binaries (2–7 AU). It shows that systems can transition between standard wind accretion and WRLO during high donor mass-loss phases, and that wind drag and tides drive progressively more compact orbits, with tidal decay best matching observed compact systems. Only a minority of cases reach the Chandrasekhar limit, primarily when WRLO operates in high-mass WD and donor configurations; many systems terminate at Roche-lobe overflow, which is not hydrodynamically modeled here. The results underscore tidal forces as a key ingredient in the evolution of compact symbiotics and suggest that observations reflect ongoing tidal orbital decay in these binaries.

Abstract

We present a suite of dynamical simulations designed to explore the orbital and accretion properties of compact (2$-$7 AU) symbiotic systems, focusing on wind accretion, drag forces, and tidal interactions. Using three levels of physical complexity, we model systems of accreting white dwarfs (WDs) with masses of 0.7, 1.0, and 1.2 M$_\odot$ orbiting evolving Solar-like stars with 1, 2, and 3 M$_\odot$. We show that systems alternate between standard wind accretion and Wind Roche Lobe Overflow (WRLO) regimes during periods of high mass-loss rate experienced by the donor star (the peak of red giant phase and/or thermal pulses). For some configurations, the standard wind accretion has mass accretion efficiencies similar to those obtained by WRLO regime. Tidal forces play a key role in compact systems, leading to orbital shrinkage and enhanced accretion efficiency. We find that systems with high-mass WDs ($\geq 1$ M$_\odot$) and massive donors (2$-$3 M$_\odot$) are the only ones to reach the Chandrasekhar limit. Interestingly, the majority of our simulations reach the Roche lobe overflow condition that is not further simulated given the need of more complex hydrodynamical simulations. Our analysis shows that increasing physical realism, by including drag and tides, systematically leads to more compact final orbital configurations. Comparison with compact known symbiotic systems seems to suggest that they are very likely experiencing orbital decay produced by tidal forces.

Figures (9)

• Figure 1: Density profiles of the donor star with $m_{1,0}=2$ M$_\odot$ as a function of stellar radius at selected evolutionary stages. The profiles correspond to (1) the beginning of the RG phase, (2) the maximum expansion at the RGB phase, and (3) the largest expansion achieved during the TPAGB phase.
• Figure 2: Semimajor axis evolution (left panels) and accretion efficiency over time (right panels) for a symbiotic binary system with initial masses $m_{1,0}=1$$M_\odot$, $m_{2,0}=1.2$ M$_\odot$ and an initial separation $a_0=6$ AU. The top panels show the entire duration of the integration, while the bottom panels focus on the TPAGB phase. Different colours correspond to simulations with varying levels of physical complexity, as described in Section \ref{['sec:2.3']}. In the left panels, the dotted lines trace the evolution of the donor star's Roche lobe radius. The light gray shaded region denotes the dust condensation radius ($R_\mathrm{cond}$), and the darker gray area marks the stellar surface radius ($R_1$).
• Figure 3: Similar to Figure \ref{['fig:a_eta']} but illustrating the evolution of a symbiotic system during the TPAGB phase with $m_{1,0}=2$ M$_\odot$ (top panels) and $m_{3,0}=3$ M$_\odot$ (bottom panels).
• Figure 4: Evolution of the accretion efficiency ($\eta$ - top panel) and dimensionless mass ratio ($q=m_2/(m_1+m_2)$ - bottom panel) as a function of the dimensionless velocity parameter ($\varw=\varv_\mathrm{w}/\varv_\mathrm{o}$) for models with $m_{1,0}= 3.0$ M$_\odot$, $m_{2,0} = 1.0$ M$_\odot$, and $a_0=6$ AU. The dashed line represents the accretion efficiency predicted by TejedaToala2025 for the adopted initial configuration of the systems.
• Figure 5: Mass accretion rate $\dot{M}_\mathrm{acc}$ as a function of the accretor's mass for simulations with $m_{1,0}= 3.0$ M$_\odot$, $m_{2,0} = 1.2$ M$_\odot$, and $a=6$ AU. Different colours refer to levels of physical complexity in the simulations. This figure was adapted from Chomiuk2021 which adapted it from results presented in Wolf2013. Dotted lines show constant nova recurrence times. The dashed lines show the limits between the three accretion regimes (nova recurrence, steady burning, and radiation-driven wind).