Visualizing the mass transfer flow in direct-impact accretion

TL;DR

The paper tackles how to visualize and analyze complex 3D mass-transfer flows during direct-impact mergers of close double white dwarf binaries, with particular relevance to R Coronae Borealis star formation. It combines high-resolution Octo-Tiger simulations of a representative $q = 0.7$ system with a robust visualization workflow, including Cartesian resampling, a dynamic co-rotating frame, surface LIC, inflow/outflow metrics, streamlines/pathlines, and FTLE/LCS analysis. The authors reveal a pronounced accretion belt and a shear layer at the belt–accretor interface, and they demonstrate how LCS ridges delineate the main flow boundaries and mixing that lead to dredge-up. The work provides a transferable toolkit for astrophysical hydrodynamics visualization, offering insights into mass-transfer physics that could apply to AM CVn systems, CVs, and Type Ia SN progenitors, while acknowledging physical simplifications such as the EOS and radiation transport.

Abstract

We use a variety of visualization techniques to display the interior and surface flows in a double white dwarf binary undergoing direct-impact mass transfer and evolving dynamically to a merger. The structure of the flow can be interpreted in terms of standard dynamical, cyclostrophic and geostrophic arguments. We describe and showcase some visualization and analysis techniques of potential interest for astrophysical hydrodynamics. In the context of R Coronae Borealis stars, we find that mixing of accretor material with donor material at the shear layer between the fast accretion belt and the slower rotating accretor body will always result in some dredge-up. We also discuss briefly some potential applications to other types of binaries.

Figures (14)

• Figure 1: Surfaces at different isovalues for time step $t=86.83 ~\text{s}$ in $~\text{g/cm}^3$: $\rho=0.1$ (), $\rho=1$ (), $\rho=5$ (), $\rho=20$ (), $\rho=100$ (), $\rho=500$ () as revealed by slicing through the binary at the orbital plane and viewing it from the side of the removed material. The binary rotates counter-clockwise. The figure shows an exploded view of the internal structure of the non-degenerate layers of a double white dwarf binary during the mass transfer phase that precedes the eventual merger of the two stars into a single object. The star on the right (the donor) is transferring mass through a stream striking the upper layers of the more massive white dwarf on the left (the accretor). Several pathlines show how the stream impacts the accretor, with most of the mass flowing around the accretor while some fraction of the stream spreads in all directions around the point of impact. A few "vacuum" particles are swept up in the leading side of the donor, and become part of the stream. Similarly a few particles in the trailing side of the donor are swept into the wake and join the surface flow on the donor.
• Figure 2: Angular frequency for the rotation of the stars plotted over time for rcb11. Note the drastic change close to the merger.
• Figure 3: Two views of the binary for rcb11 at a time $t=100.44 ~\text{s}$ (left) and rcb12 at $t=113.62 ~\text{s}$. Velocity magnitudes are mapped logarithmically to color () in the range $[1 \cdot 10^6,~1 \cdot 10^9] ~\text{cm/s}$. The flow lines shown are the result of LIC on the isopycnic surface $\rho =100 ~\text{g/cm}^3$. In this and subsequent figures, the donor is on the right and the accretor on the left, unless noted otherwise. The triad of axes indicates the viewing direction: \ref{['fig:4views-iso-z']} and \ref{['fig:4views-rcb12-iso-z']} are viewed down the rotation axis, \ref{['fig:4views-iso+y']} and \ref{['fig:4views-rcb12-iso+y']} are viewed along the $y$-axis, i.e. from the trailing side of the donor. The common center of mass is shown as black or white dot in a circle.
• Figure 4: Four views of the binary for rcb11 at a time $t=2759.89 ~\text{s}$ (left) and rcb12 at $t=4306.21 ~\text{s}$, approximately one orbital period before tidal disruption of the donor. Velocity magnitudes are mapped logarithmically to color () in the range $[1 \cdot 10^6,~1 \cdot 10^9] ~\text{cm/s}$. The flow lines shown are the result of LIC on the isopycnic surface $\rho =100 ~\text{g/cm}^3$. Note how some overflow through $L_2$ on the extreme right is beginning to take place. The bulging on the left of the accretor in \ref{['fig:4views-predisruption-iso+y']} and \ref{['fig:4views-rcb12-predisruption-iso+y']} is due to the accretion belt. The center of mass of the system is buried inside the flow, near the center of the images.
• Figure 5: Two views of the binary for rcb 11 at a time $t=2873.81 ~\text{s}$ (left column) and for rcb 12 at a time $t=4419.79 ~\text{s}$ (right column), just as tidal disruption of the donor is occurring. In \ref{['fig:4views-postdisruption-iso-z']} and \ref{['fig:4views-rcb12-postdisruption-iso-z']} we see some overflow through $L_3$ on the left of the accretor. \ref{['fig:4views-postdisruption-iso-z']},\ref{['fig:4views-rcb12-postdisruption-iso-z']} Surface at $\rho=100 ~\text{g/cm}^3$ viewed from $z>0$. \ref{['fig:4views-postdisruption-iso-y']},\ref{['fig:4views-rcb12-postdisruption-iso-y']} Surface at $\rho=100 ~\text{g/cm}^3$ viewed from $y>0$. Projected surface velocity magnitudes are mapped logarithmically to color () in the range $[1 \cdot 10^6,~1 \cdot 10^9] ~\text{cm/s}$. The flow lines shown are the result of LIC on the isopycnic surface $\rho =100 ~\text{g/cm}^3$. The view in \ref{['fig:4views-postdisruption-iso-y']} and \ref{['fig:4views-rcb12-postdisruption-iso-y']} puts the accretor on the right, showing the incipient overflow through $L_3$ turning toward the observer. The center of mass is invisible, being located inside the fluid.