Rotational disruption of dust grains by mechanical torques for high-velocity gas-grain collisions

Abstract: Dust grains moving at hypersonic velocities of $v_{d}\gtrsim 100\rm km~s^{-1}$ through an ambient gas are known to be destroyed by nonthermal sputtering. Yet, previous studies of nonthermal sputtering disregarded the fact that dust grains can be spun-up to suprathermal rotation by stochastic mechanical torques from gas-grain collisions. In this paper, we show that such grain suprathermal rotation can disrupt a small grain into small fragments because induced centrifugal stress exceeds the maximum tensile strength of grain material, $S_{\rm max}$. We term this mechanism {\it MEchanical Torque Disruption} (METD). We find that METD is more efficient than nonthermal sputtering in destroying smallest grains ($a<10$ nm) of nonideal structures moving with velocities of $v_{d}<500 \rm km~s^{-1}$. The ratio of rotational disruption to sputtering time is $\tau_{\rm disr}/\tau_{\rm sp}\sim 0.7(S_{\rm max}/10^{9}\rm erg~cm^{-3})(\bar{A}_{\rm sp}/12)(Y_{\rm sp}/0.1)(a/0.01\mu m)^{3}(300\rm km~s^{-1}/v_{d})^{2}$ where $a$ is the radius of spherical grains, and $Y_{\rm sp}$ is sputtering yield. We also consider the high-energy regime and find that the rate of METD is reduced and becomes less efficient than sputtering for $v_{d}>500\rm km~s^{-1}$ because impinging particles only transfer part of their momentum to the grain. We finally discuss implications of the METD mechanism for the destruction of hypersonic grains accelerated by radiation pressure as well as grains in fast shocks. Our results suggest that the destruction of small grains by METD in fast shocks of supernova remnants may be more efficient than previously predicted by nonthermal sputtering, depending on grain internal structures.