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White Dwarfs

Prepared by G. Marconi


The fraction of total stellar mass stored in faint white dwarf (WD) is important both for the dynamical evolution of star clusters and potentially for the mass of the galactic disk and halo. In the clusters, which are presumed to be the source of field WDs, the WDs fraction must depend on the cluster age and kinematical evolution (Vesperini and Heggie 1997). In the last few years a number of studies have been devoted to study the properties of WDs in open and globular clusters. These studies have been motivated by the independent information available from cluster WDs on cluster distances (Renzini et al. 1996), cluster ages (von Hippel et al. 1995), and constraints on stellar evolution (Richer et. al 1997). Most of these cluster measurements have been made using HST capability to detect and separate the faint WDs from background galaxies. On the other hand the performances of HST in detecting WDs in open clusters are limitated by two factors: a) the limited field of view; b) the blooming from very bright stars (e.g. von Hippel et al. 1995). The new generation of EEV CCDs installed at LBT prime focus (LBTPF) will settle in principle both the problems. Example: in a old open cluster like NGC 2477 (von Hippel et al. 1995) 4 WDs have been discovered down to 25 magnitudes using an huge amount of HST-WFPC2 (F.O.V 3' 44'') time (10000 sec). Using LBT PF we can completely cover the cluster ($\simeq$ 120 squared arcmin), and then we reasonably expect to find out up to 120 WDs in a reasonable amount of time (3600 sec.).

What use will be made of this WD sample ?

A) The mean mass of the WDs is 0.66$\pm$0.05 M$\odot$ assuming a pure carbon core for the WD (although a carbon-oxygen mixture makes little difference to the mass). We thus know along which cooling curve the WDs ought to be located. With larger sample of WDs it will then be possible to test in detail the current best evolutionary models for cooling WDs. The Wood (1995) models together with the atmospheres of Bergeron (Bergeron P., Wesemael F. $\&$ Beauchamp A. 1995, PASP, 107, 1047) B$\ddot{o}$hm-Vitense, E. 1993, AJ, 106, 1113(1995) or those of the Chabrier (1997) group can be adopted.

B) The ages of stellar systems are important in a wide variety of applications. Among the most interesting of these are the ages of the oldest known star clusters, the globular clusters. As the most ancient datable systems these objects provide an important lower limit to the age of the Universe which can be compared with ages derived from other techniques (eg the expansion age). The usual technique of deriving globular cluster ages is to fit theoretical isochrones to the cluster turnoffs or otherwise use the properties of the brightest portions of the cluster CMD (eg VandenBerg et al 1996). A method which is independent of this technique is to determine the age of the coolest white dwarf in the cluster. This WD presumably formed from a star with a very short main sequence lifetime, so that it has spent almost its entire existence as a WD. Its cooling age is then a very good approximation to the age of the cluster itself. This is a project that we are planning to pursue with the Advanced camera on HST and with LBTPF like instrument. With new photometry and an augmented sample of WDs in the open clusters we will be in a position to compare the isochrone age with the derived age of the coolest WDs. Agreement between these two ages will strengthen the use of WDs in dating stellar systems. Any difference might require a serious reanalysis of age dating techniques, particularly if the WDs produced an age which is larger than the turnoff age.

C) The WD population in a cluster can be used to extend the cluster mass function beyond its turnoff point. The cluster WDs, having evolved from stars more massive than the current cluster turnoff, provide the potential of extending the mass function to heavier objects: the older the WD, the more massive its progenitor. An open clusters like NGC 2477, is close enough (m-M)= 10.50 so that WDs that evolved from massive cluster stars will still be easily observed. Then we will be able to extend the cluster mass function all the way up to its termination point for massive stars (or at least to the maximum mass of main sequence stars that can produce a WD). Thus simultaneously we will learn what this maximum mass was in the cluster.

D) As a final point, since we will have a complete sample of WDs in an open cluster, it will be possible to determine the mass fraction of WDs in this typical cluster. This quantity is important in understanding the dynamical evolution of star clusters and the mass of both the Galactic disk and halo (see further discussion in von Hippel 1998). Other open clusters have extremely small sample of known WDs (typical is one on two!), so that only instrument like LBTPF has the potential of providing the most accurate estimate of the WD fraction in an intermediate age cluster.


next up previous contents
Next: Near Earth Objects Up: The Scientific Cases for Previous: Transient Optical Phenomena
Guido Buscema
1999-01-29