Since the first appearance of stars in the early Universe, not-too-massive stars share the final fate of cooling down as electron-degenerate white dwarfs (WDs). However, cooling times are so long that they can exceed the current estimates of the Hubble time, and less massive WD are never expected to reach their final equilibrium temperature.
On theoretical grounds, the luminosity of a WD can be calibrated in terms of cooling times and, on this basis, the cutoff of the WD luminosity function in the solar neighborhood has already been used to constrain the age of the galactic stellar disk (see, e.g., Wood 1992; Oswalt et al. 1995). Field WDs are expected to come from progenitors born at all stages of the life of the disk population and have therefore mixed ages and mixed cooling times. As a result, the observed population depends both on the history of star formation, following the appearance of the galactic stellar disk, and on the Initial Mass Function (IMF), which characterizes the ongoing formation of stellar structures.
This is not the case for WDs populating stellar clusters, as coming from progenitors with the same age and chemical composition (Simple Stellar Population), thus making theoretical predictions much easier. Moreover, cluster WDs have negligible relative distances, so that the WD sequence can be more easily distinguised from field stars. Unfortunately, the intrinsically low luminosity of WDs has prevented for a long time the observation of the cluster WD population from the ground. Only recently, cluster WDs have been detected in the galactic cluster M67 (Richer et al. 1998) as well as in some galactic globulars (Paresce et al. 1995; Richer et al. 1995; Renzini et al. 1996; Cool et al. 1996), thanks to the unprecedented sensitivity of the Hubble Space Telescope.
The theoretical scenario for cluster WD is already well understood. WDs are expected to enter their cooling phase after a time determined by the original mass of the progenitor, more massive progenitors reaching this stage earlier. Consequently, the WD sequence should be populated according to two parameters only: the cluster age and the original cluster IMF. This paper is devoted to substantiating such predictions. We discuss theoretical simulations concerning the expected distribution of cluster WD in connection with the two parameters mentioned above and within the context of observations accessible with HST.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999