The ¹²C(,)¹⁶O Nuclear Reaction Rate from Asteroseismology of the DBV White Dwarf CBS 114

We have identified 7 independent pulsation modes in the helium-atmosphere variable (DBV) white dwarf star CBS 114, based on 65 hours of time-resolved CCD photometry from the 0.75-m telescope at SAAO. We interpret these pulsations as non-radial g-modes with the same spherical degree =1, as suggested by the mean period spacing of 37.10.7 s. Although we did not observe multiplet structure, we demonstrate that if the rotation period is ~1 day then the consequences of assuming =0 are not serious for the purposes of model-fitting. We use a genetic-algorithm-based fitting method to find the globally optimal model parameters, including the stellar mass (M_*=0.73 M), the effective temperature (T_eff=21,000 K), the mass of the atmospheric helium layer (log [M_He/M_*]=-6.66), and the central oxygen mass fraction (X_O=0.61). The latter value can provide an independent measurement of the astrophysically-important but experimentally uncertain rate for the ¹²C(,)¹⁶O nuclear reaction. A model of the internal chemical profile with the same mass as our fit to CBS 114 requires a reaction rate of S₃₀₀=1805 keV b to match the derived central oxygen mass fraction. This value is consistent with-but much more precise than-the rate derived from recent high-energy laboratory measurements (S^lab₃₀₀=16550 keV b). By contrast, the rate previously derived from a similar treatment of the white dwarf GD 358 was significantly higher (S₃₀₀=37040 keV b). This suggests either that presently unknown sources of systematic uncertainty in our models must affect the analysis of GD 358 and CBS 114 in different ways, or that the two stars have distinct evolutionary origins, or both. An asteroseismological determination of the central oxygen mass fraction for additional DBV white dwarfs will help us to decide which of these scenarios is most likely to be correct.

 
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