We present experimental results on Rayleigh-Bénard convection in ${\mathrm{SF}}_6$ near the gas-liquid critical point. We measured the critical temperature difference for the onset of convection, Δ${\mathit{T}}_0$, as a function of the reduced average temperature τ=(T¯-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$ and found Δ${\mathit{T}}_0$=525×${\mathrm{\tau }}^{1.89}$, which is close to the expected power law behavior. The strong temperature dependence of the physical properties is used to scan the Prandtl number in a wide range. A new, many ‘‘target’’ pattern state, initiated by a defect instability, was observed.

• Received 12 March 1993

DOI:https://doi.org/10.1103/PhysRevLett.70.3888