The end Stage of Stellar Evolution: White Dwarfs, Hot Subdwarfs and Nuclei of Planetary Nebulae

Abstract

The region of the H—R diagram to the left and below the main sequence (see Figure 1) is occupied by stars whose nuclear-burning phases are either ending or essentially ended. These objects for the most part include the degenerate stellar cores from the prior asymptotic giant branch (AGB) or double shell source burning phase, after the outer stellar envelope has been depleted by shell burning and extensive mass loss. Some remnants may evolve directly from the region of the horizontal branch (HB) after the core helium burning phase. After a brief phase dominated by gravitational contraction and residual shell burning, these dying stars enter the white dwarf cooling sequence, which was well defined more than twenty five years before the launch of the IUE satellite (Mestel, 1952). Recent results indicate that single stars with initial masses as large as about 8 M ⊙ manage to lose enough envelope mass primarily during their AGB and red giant phases that the stellar cores do not exceed the Chandrasekhar limit, permitting nonviolent evolution as a white dwarf (Reimers and Koester, 1982, Weidemann and Koester, 1983). Thus, an important task for IUE has been to search for and obtain spectra of pre-white dwarfs and white dwarfs from the young disk, old disk and halo populations.