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We present the results of numerically solving the rate equations for thefirst 31 rotational states of CS in the ground vibrational state to determinethe conditions under which the J=1-0, J=2-1 and J=3-2 transitions are invertedto produce maser emission. The essence of our results is that the CS($v=0$)masers are collisionally pumped and that, depending on the spectral energydistribution, dust emission can suppress the masers. Apart from the J=1-0 andJ=2-1 masers the calculations also show that the J=3-2 transition can beinverted to produce maser emission. It is found that beaming is necessary toexplain the observed brightness temperatures of the recently discovered CSmasers in W51 e2e. The model calculations suggest that a CS abundance of a fewtimes $10^{-5}$ and CS($v=0$) column densities of the order$10^{16}\,\mathrm{cm^{-2}}$ are required for these masers. The rarity of the CSmasers in high mass star forming regions might be the result of a required highCS abundance as well as due to attenuation of the maser emission inside as wellas outside of the hot core.

On the pumping of the CS($v=0$) masers in W51 e2e