Taken from my thesis: Time it takes for dust at a stellocentric radial distance to spiral into the star due to P-R drag. the model for these curves are based on the derivations of Burns et al. (1979): t_pr = 400 * (rd^2/Mstar)/beta. This relations can be derived from Equation 1.2 in that paper. Large values of beta represent smaller particles and vice-versa. I have plotted the curves for different values of beta and for different spectral types (stellar masses).

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