The Evolution of Helium White Dwarfs: Applications to Millisecond Pulsars

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We present a grid of evolutionary tracks for low-mass white dwarfs with helium cores in the mass range from 0.179 to 0.414 Mₛun. The lower mass limit is well suited for comparison with white dwarf companions of millisecond pulsars (MSP). The derived cooling ages are of the order of 10⁹ yrs due to residual nuclear burning. The cooling ages are consistent with age estimations of MSP systems based on the pulsars' spin-down. For example, for the system PSR 1012+5307 we derived a white dwarf cooling age of 6 +/-1 Gyr in good agreement with the spin-down age of 7 Gyr. For the companion mass we found M=0.19 +/- 0.02 Mₛun. We studied other MSP systems as well selecting only systems with well given ages and/or masses, and determined the effective temperatures and surface gravities of the companion white dwarfs with the present evolutionary models.