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The Astronomical Journal


We present spectral types and spectrophotometry for 21 double degenerate (DD) common proper motion binaries and provide estimates of their colors, absolute visual and bolometric magnitudes, and cooling ages. The most typical double degenerate pair contains stars of similar color and spectral type, with an average Mv = 14.20, and average cooling age of ≈3×10⁹ yr, thus typically belonging to the extremely old disk population subcomponent. The oldest pairs in the sample are 9 × 10⁹ yr. The differential cooling ages, Δ log τc, range between 0.01 and 0.84, with the most typical pair having Δ log τc = 0.30, corresponding to a factor of 2 difference in cooling age of its components. One system differs in component cooling age by a factor of 7. The median and mean separations of the DD pairs are 2.63 AU and 2.61 AU, respectively, both smaller than the values for main sequence plus white dwarf pairs (WD + MS). Their distribution of separations N (log a) may be skewed toward smaller bins of log a than the distribution characterizing the MS + WD pairs but is larger than the distribution characterizing the MS + MS pairs. The latter comparison, once the role of selection effects in the two samples are properly evaluated, may be indicating orbital amplification of the DD pairs due to nonexplosive, isotropic, mass loss on a timescale longer than the orbital period, while the product of the total systemic mass and the semimajor axis was conserved. Their smaller separations relative to MS + WD pairs may be attributed to the smaller dynamical effect of mass loss on their orbits, due to their lower initial mass ratios. The DA/non-DA ratio, for the pairs hot enough to show Balmer lines (i.e., Mv < 14.0) is 3:1, higher than the anomalous 1:1 found for the MS + WD pairs. The DD sample, even in the absence of radial velocities, appears to have velocity dispersions, 2-7 times larger than selected samples of single white dwarfs and MS + WD pairs, chosen to be in the same color/absolute magnitude range as the DD components. This property may be a result of the dynamical inflation of their velocity dispersions due to their extremely ancient total stellar ages.



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