temperatures of isolated neutron stars compared to theoretical cooling curves. (a) Cooling curve for a nonsuperfluid $1.3\,{\mbox{$M_\odot$}}$ star. Any other panel shows 88 curves from $M=1.1$ to $1.97\,{\mbox{$M_\odot$}}$ with step $0.01 {\mbox{$M_\odot$}}$: (b) No superfluidity, sharp direct Urca threshold; (c) proton superfluidity 1p and (d) 2p which broaden the threshold. []{data-label="cool4"}](yakfig2.ps){width="\textwidth"}

Neutrino emissivity profiles in a stellar core at $T=3\times 10^8$ K are shown in Fig. \[tcrho\]b. The vertical dotted line shows the direct Urca threshold, $\rho_{\rm D}$. The thick line (NoSF) is for nonsuperfluid matter. At $\rho<\rho_{\rm D}$ the neutrino emission is determined by the modified Urca process. When $\rho$ increases above $\rho_\mathrm{D}$, the emissivity jumps by 7 orders of magnitude due to the direct Urca onset.

Fig. \[cool4\]a presents the cooling curve for a $1.3{\mbox{$M_\odot$}}$ nonsuperfluid star compared with the data. The curve is a typical example of slow cooling via the modified Urca process. It is actually universal, being almost the same for all stars with $1.1\,{\mbox{$M_\odot$}}\leq M < M_{\rm D}$, and for a wide class of EOSs [@pa92]. This universal curve goes through the scatter of observational points but cannot explain the sources which are hottest and coolest for their age. These sources seem to have 