The photophoretic velocities of black SiO₂ and olivine dust aggregates as a function the dust-aggregate radius. a) and c) show the results of black SiO₂ for the median velocity and the width of the velocity distribution, respectively. b) and d) show the results of olivine for the median velocity and the width of the velocity distribution, respectively. Theoretical curves, following Eq. (4), are shown for an illumination intensity of 1 W cm^-2, for a gas pressure of 100 Pa (1000   dyn/cm²), and |λ_p/J₁| = 0.1,1,10   W   m^-1   K^-1. Brownian motion can only contribute to velocities below the blue line. An error estimate for the single data points is indicated in figure a).