Empirical Evidence for Crystallization Sequences of DA Type White Dwarfs

Abstract

The determination of the distribution of white dwarfs in the HR diagram is marred by two observational difficulties: The recent discoveries of degenerate stars with unusual spectral characteristics and the controversy over the existence of crystallization sequences. Observationally, the origin of the controversy can be traced to the single band displayed on the MVvs. (b-y) diagram by several white dwarfs with accurate parallax (Graham 1972). The discrepancy between this single band and the two bands evident in the MVvs. (U-V) diagram (Eggen and Greenstein 1965) is far too great to be attributed to observational errors (Eggen 1969). Theoretically, the number of sequences in the HR diagram is directly related to the cooling properties of degenerate dwarfs, for which crystallization is expected on rather general grounds. On one hand, Van Horn (1968) considered a discontinuous transition to the solid phase with the release of latent heat and the addition of it to the total internal energy. Since the amount of latent heat depends upon the chemical composition, bright and faint crystallization sequences should be observed according to the different stages of evolution reached by the parent stars. On the other hand, Mestel and Ruderman (1967) proposed a gradual transition to the solid phase, and consequently the formation of a single cooling sequence. The purpose of this contribution is to report observational results indicating the existence of two crystallization sequences of DA type white dwarfs.