= Solution
At <periapsis> the release velocity is tangential. A grain with $\beta=1$ feels no net stellar inverse-square force and therefore moves on the tangent line. A grain with $\beta<1$ retains an inward acceleration and bends toward the star; a grain with $\beta>1$ feels a net outward acceleration and bends away from it. At the common observation time, joining these positions in increasing $\beta$ gives the <synchrone> shown below. It starts near the low-$\beta$ orbital trajectory, passes through the force-free $\beta=1$ position, and extends outward through the repelled high-$\beta$ grains.
\Image[../../../paper-316-dust-loci.svg]
{title=Synchrones and syndynes for zero-speed dust release from a parabolic comet}
{description=The left panel joins grains released together at pericentre with different radiation-pressure coefficients. The right panel joins grains of one coefficient released at different comet true anomalies.}
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