M_1^2 V_7^{-3} \rho_{24} ~{\rm g~s^{-1}}\\
& \sim & 3.5 \times 10^{-18} M_1^2 V_7^{-3} \rho_{24}~M_\odot~{\rm yr^{-1}},
\end{array}$$ where $\rho_{24}=\rho/(10^{-24}{\rm g~cm^{-3}})$. By comparing a sample of pulsar proper motion data with their Monte Carlo simulations, Hansen & Phinney[@hp97] found that the mean speed of pulsars at birth is $V\sim (2.5 - 3.0)\times 10^7 {\rm cm~s^{-1}}$. Diamond et al.[@djp95] investigated the local interstellar medium (ISM) distribution with ROSAT all-sky EUV survey, and found that the ISM mean density is $\sim 10^{-25}
-10^{-24}$ g cm$^{-3}$. Thus the minimum rotation period for accretion of a typical new born strange star in a typical ISM is $\sim 10^4$ s. In supernova remnants, the medium density may be denser so that $\rho\sim 10^{-22}$ g cm$^{-3}$. In this case, the critical period for possible accretion is still $\sim 10^3$s. Since the longest period of the present known pulsars is only 8.5s[@ymj99], we conclude that accretion is almost impossible for most isolated radio pulsars in their lifetimes, unless they are born with much less-than-average proper motion speed or they are in very dense medium. Another reason against the possible accretion is that when a pulsar is active, relativistic particles are believed to flow out from the open field line 