A double white-dwarf cooling sequence in ω centauri

Abstract

We have applied our empirical-point-spread-function-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of ω Cen, and found a well-defined split in the bright part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of ∼2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component (∼0.55 M CO-core DA objects), while the red WDCS hosts the end products of the He-rich population (∼0.46 M objects, and ∼10% CO-core and ∼90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch. © 2013. The American Astronomical Society. All rights reserved.