Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

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Abstract
We present spectroscopic observations of four previously unidentified candidate white dwarfs (WDs) in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional WDs in Praesepe leaves the total known WD population of the cluster at five, well below the number expected from commonly assumed initial mass functions, although several undiscovered cluster WDs may lie in the outer regions of the cluster. All known Praesepe member WDs are concentrated within 06 of the cluster center, and the radial profile of cluster WDs is quite similar to the profile of massive cluster stars. This profile is mildly inconsistent with that of ~1 M⊙ cluster stars and suggests that the WDs did not receive a velocity kick during the progenitor star's mass-loss phase. If complete, the observed Praesepe WD population is consistent with a steeper high-end initial mass function than is commonly assumed, although the calculated slopes are inconsistent with the present-day mass function of Praesepe. Searches for WDs outside the core of Praesepe and further study of the WD populations of additional open clusters are necessary to constrain further the underlying cause of the WD deficit.
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