Exoplanets & Stellar Populations Group

ω Cen HST Large MultiCycle Program (GO-14118, P.I. Bedin)

The white dwarf cooling sequence (WD CS) lies in the least-explored region of the color-magnitude diagram of old stellar populations. Deep imaging with Hubble Space Telescope (HST) has reached for the first time the end of the WD CS in three “classical” old globular clusters, M4, NGC6397, and 47 Tuc. While each of these clusters hosts multiple populations, they do not show a large range of initial He abundances, and their WD CSs are consistent with what predicted for single-population systems.
In this Large Program, for which 66 orbits have been allocated during HST's Cycle 23, we propose to reach the end of the WD CS in a quite different type of cluster. Omega Centauri has long been known to host multiple stellar populations, with a large spread also in [Fe/H] and, most importantly, a large range of initial He, as can be deduced from its main sequence. It has already been established that the upper part of Omega Centauri's WD CS is bifurcated into two sequences. A previous analysis has hypothesised that the bright WD CS consists of a standard CO-WD sequence, and a redder sequence of low-mass WDs with both CO and He-cores. The bluer of two WD CS is populated by the evolved stars of the He-normal component, while the redder WD CS hosts the end products of the He-rich population. This points to a clear connection between GCs with enhanced He populations, and the morphology of their WD CS.
Omega Centauri is close enough that its entire WD CS is within the reach of HST. Observing the whole WD CS of this cluster down to its termination will provide a clear solution for the origin of the multiple WD CS of this cluster, and answer some important key questions about stellar evolution for He-enriched populations.