The Masses of White Dwarfs in the Praesepe Open Cluster

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Abstract
We present results from an in-depth photometric and spectroscopic study of white dwarfs in the Praesepe open cluster. From high signal-to-noise ratio spectra, we have estimated log g and Teff for six DA white dwarfs using model atmosphere fits to the Balmer lines. Evolutionary models are then used to determine masses, radii, and cooling times. Good agreement is found with masses determined using gravitational redshifts primarily by Reid and with masses determined from the cluster's distance. Included in these comparisons are white dwarfs analyzed in a similar way in the Hyades and Pleiades clusters. The cooling times and cluster ages are then used to determine initial masses (Mi) for each white dwarf. A monotonic initial-final mass relation (Mi-Mf) is determined for these and two other well-studied clusters, although there are indications that more envelope mass is lost in the asymptotic red giant branch thermal pulse phase than current theory predicts. One Praesepe object—LB 5893 (0836+197)—is anomalous in the sense that it has the highest Mf but the highest Teff and the smallest inferred Mi. LB 393 = EG 61 (0837+199) shows Zeeman-split lines indicative of a strong magnetic field, the first such white dwarf found in a star cluster.

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