High resolution optical spectroscopy of Praesepe white dwarfs

We present the results of a high resolution optical spectroscopic study of nine white dwarf candidate members of Praesepe undertaken with the VLT and UVES. We find, contrary to a number of previous studies, that WD0836+201 (LB390, EG59) and WD0837+199 (LB393, EG61) are magnetic and non-magnetic white dwarfs respectively. Subsequently, we determine the radial velocities for the eight non-magnetic degenerates and provide compelling evidence that WD0837+185 is a radial velocity variable and possibly a double-degenerate system. We also find that our result for WD0837+218, in conjunction with its projected spatial location and position in initial mass-final mass space, argues it is more likely to be a field star than a cluster member. After eliminating these two white dwarfs, and WD0836+199 which has no clean SDSS photometry, we use the remaining 5 stars to substantiate modern theoretical mass-radius relations for white dwarfs. In light of our new results we re-examine the white dwarf members of Praesepe and use them to further constrain the initial mass-final mass relation. We find a a near monotonic IFMR, which can still be adequately represented by simple linear function with only one outlier which may have formed from a blue straggler star.