Effective surface temperatures or upper limits for a number of cooling isolated NSs including XMMU J1732 versus their ages (data points) compared with four theoretical cooling curves for a 1.5 M_⊙ star. The plot contains four CCOs with reliable temperature measurements (labeled with squares). For XMMU J1732, the dashed error bar indicates a conservative range of 10–40 kyr adopted for the NS age. MU refers to a non-superfluid star with a heat blanket made of iron; SF corresponds to a star with strong proton superfluidity in the core and the same heat blanket; MUac and SFac refer, respectively, to the same models as MU and SF but with a fully carbon heat blanketing envelope (see text for details).