We present rotation period measurements for 41 field M-dwarfs , all of which have masses inferred ( from their parallaxes and 2MASS K-band magnitudes ) to be between the hydrogen burning limit and 0.35 { M } _ { \odot } , and thus should remain fully-convective throughout their lifetimes .
We measure a wide range of rotation periods , from 0.28 days to 154 days , with the latter commensurate with the typical sensitivity limit of our observations .
Using kinematics as a proxy for age , we find that the majority of objects likely to be thick disk or halo members ( and hence , on average , older ) rotate very slowly , with a median period of 92 days , compared to 0.7 days for those likely to be thin disk members ( on average , younger ) , although there are still some rapid rotators in the thick disk sample .
When combined with literature measurements for M-dwarfs , these results indicate an increase in spin-down times with decreasing stellar mass , in agreement with previous work , and that the spin-down time becomes comparable to the age of the thick disk sample below the fully-convective boundary .
We additionally infer that the spin-down must remove a substantial amount of angular momentum once it begins in order to produce the slow rotators we observe in the thick disk candidates , suggesting that fully-convective M-dwarfs may still experience strong winds .