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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 binarymillisecond pulsar PSR B1855+09. We detect its white dwarf companion andmeasure mF555W=25.90+/-0.12 andmF814W=24.19+/-0.11 (Vega system). From thereddening-corrected color,(mF555W-mF814W)0=1.06+/-0.21, we infera temperature Teff=4800+/-800 K. The white dwarf mass isknown accurately from measurements of the Shapiro delay of the pulsarsignal, MC=0.258+0.028-0.016Msolar. Hence, given a cooling model, one can use themeasured temperature to determine the cooling age. The main uncertaintyin the cooling models for such low-mass white dwarfs is the amount ofresidual nuclear burning, which is set by the thickness of the hydrogenlayer surrounding the helium core. From the properties of similarsystems, it has been inferred that helium white dwarfs form with thickhydrogen layers, with mass >~3x10-3 Msolar,which leads to significant additional heating. This is consistent withexpectations from simple evolutionary models of the preceding binaryevolution. For PSR B1855+09, though, such models lead to a cooling ageof ~10 Gyr, which is twice the spin-down age of the pulsar. It could bethat the spin-down age were incorrect, which would call the standardvacuum dipole braking model into question. For two other pulsarcompanions, however, ages well over 10 Gyr are inferred, indicating thatthe problem may lie with the cooling models. There is no age discrepancyfor models in which the white dwarfs are formed with thinner hydrogenlayers (<~3x10-4 Msolar).
| A photometric study of wide visual double stars. IV - UVBY photometry of wide visual double stars with G-type primaries
We performed uvby aperture photometry of wide visual double stars withG-type primaries. The minimum angular separation of the components is 10arcsecs and mainly less than 20 arcsecs. As both components have oftensimilar photometric and astrophysical parameters, we infer that most ofthese double stars have components of common origin.
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