  We investigate theoretically the bifurcation scenario for colloidal
suspensions subject to a vertical temperature gradient taking into account the
effect of sedimentation. In contrast to molecular binary mixtures, here the
thermal relaxation time is much shorter than that for concentration
fluctuations. This allows for differently prepared ground states, where a
concentration profile due to sedimentation and/or the Soret effect has been
established or not. This gives rise to different linear instability behaviors,
which are manifest in the temporal evolution into the final, generally
stationary convective state. In a certain range above a rather high barometric
number there is a coexistence between the quiescent state and the stationary
convective one, allowing for a hysteretic scenario.
