The enhancement factors for low orbital eccentricities as a function of x, where the dimensionless parameter x is determined by Eq. (9). Solid line represents Eqs. (14) and (15) with a use of m0min = m0max and h(e1) = 1. Circles indicate the factor numerically calculated by equations (68) and (69) of Kobayashi et al. (2009) around the sun for spherical dust listed in Table 1.