THE EXTREMELY LOW LUMINOSITY WHITE DWARF ESO 439-26

摘要

We present a detailed analysis of the extremely low luminosity white dwarf ESO 439-26. The measured trigonometric parallax of π = 0.0237″ ± 0.0030″, together with the observed V magnitude of 20.52, yields an absolute visual magnitude of M_v = 17.4 ± 0.3. ESO 439—26 thus lies more than 1 mag faintward of the observed cutoff of the white-dwarf luminosity function in the Galactic disk and could therefore be the coolest and hence oldest white dwarf ever detected. However, we show from a detailed analysis of the optical energy distribution that the intrinsic low luminosity of ESO 439—26 is the result of its small radius, or large mass. Our best solution is reached with T_(eff)= 4560 ± 100 K, log g = 9.04 ± 0.14, and a pure-helium atmospheric composition. Evolutionary models are used to derive a mass of M = 1.21 ± 0.07 solar mass and a cooling age of 6.4 ± 0.3 Gyr, a value that is smaller than the current estimates of the age of the Galactic disk.