Abstract

Formation of low-mass helium white dwarfs in close binaries

Alina Istrate (Argelander Institut fur Astronomie, Bonn)

T.M Tauris-Argelander Institute for Astronomy, Max-Planck-Institute for Radioastronomy; N.Langer- Argelander Institute for Astronomy ; J. Antoniadis - Max-Planck Institute for Radioastronomy

Recently, a large number of low mass (< 0.20 Msun) helium white dwarfs (He WDs) also known as extremely low-mass white dwarfs (ELM WDs) have been discovered as a result of several surveys as the WASP Project, the ELM Survey or the Keppler mission. The far majority of them have as companion another compact object (a WD or millisecond pulsar). There appears to be discrepancies between current theoretical modeling of low-mass He WDs and a number of key observational cases, indicating that some details of their formation scenario remains to be understood. Numerical calculations with a detailed stellar evolution code were used to trace the evolution from the zero age main sequence in approximately 400 low-mass X-ray binary systems with different initial values of donor mass, neutron star mass, orbital period and strength of magnetic braking. Our prime focus is to examine the thermal evolution and the radial contraction phase towards settling on the WD cooling track, and investigate how this evolution depends on the WD mass and the amount of hydrogen left after the mass transfer ceased. We find that the timescale from Roche lobe detachment until the low-mass proto He WD reaches the WD cooling track is typically 0.5-2 Gyr, depending systematically on the WD mass and the initial donor mass. This long time scale of low-mass proto He WD evolution can explain a number of recent puzzling observations such as those WDs which have small surface gravities and high effective temperature.

Mode of presentation: oral