The temperature and cooling age of the white dwarf companion to the millisecond pulsar PSR B1855+09

We report on Keck and Hubble Space Telescope observations of the binary mil lisecond pulsar PSR B1855+09. We detect its white dwarf companion and measu re m(F555W) = 25.90 +/- 0.12 and m(F814W) = 24.19 +/- 0.11 (Vega system). F rom the reddening-corrected color, (m(F555W)-m(F814W))(0) = 1.06 +/- 0.21, we infer a temperature T-eff = 4800 +/- 800 K. The white dwarf mass is know n accurately from measurements of the Shapiro delay of the pulsar signal, M -C = 0.258(-0.016)(+0.028) M.. Hence, given a cooling model, one can use th e measured temperature to determine the cooling age. The main uncertainty i n the cooling models for such low-mass white dwarfs is the amount of residu al nuclear burning, which is set by the thickness of the hydrogen layer sur rounding the helium core. From the properties of similar systems, it has be en inferred that helium white dwarfs form with thick hydrogen layers, with mass greater than or similar to 3 x 10(-3) M., which leads to significant a dditional heating. This is consistent with expectations from simple evoluti onary models of the preceding binary evolution. For PSR B1855+09, though, s uch models lead to a cooling age of similar to 10 Gyr, which is twice the s pin-down age of the pulsar. It could be that the spin-down age were incorre ct, which would call the standard vacuum dipole braking model into question . For two other pulsar companions, however, ages well over 10 Gyr are infer red, indicating that the problem may lie with the cooling models. There is no age discrepancy for models in which the white dwarfs are formed with thi nner hydrogen layers (less than or similar to 3 x 10(-4) M.).