Evolution of Low-Mass X-ray Binaries Below the Bifurcation Period
Argelander-Institute for Astronomy, University of Bonn
Millisecond pulsars(MSPs) are generally believed to be old neutron stars (NSs) which have been spun up to high rotation rates via accretion of matter from a companion in a low-mass X-ray binary (LMXB). We investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarfs companions (HeWDs) in tight binaries with orbital periods, Porb = 2 − 9 hr. In particular, we examine: i) if such observed systems can be reproduced from theoretical modelling using standard prescriptions of orbital angular momentum losses, ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, iii) if the correlation between WD mass and orbital period (MWD, Porb) is valid for systems with Porb ≤ 2 d. Moreover, we analyse the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass.
We find that a severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with HeWD companions of mass < 0.20 Msun. We demonstrate that the theoretically calculated (MWD, Porb)-relation is in general also valid for systems with orbital periods less than 2 days, although with a large scatter in HeWD masses between 0.15 − 0.2 Msun. Finally, we find that the time between Roche-lobe detachment until the low-mass proto-HeWD reaches the WD cooling track is typically between 1 − 2 Gyr, depending systematically on the WD mass and therefore its luminosity. This can explain a number of recent observations, including MSP systems hosting HeWD companions with very small surface gravities and high effective temperatures.
Date: Wednesday, 20 August 2014 Time: 13:30 Where: All Bishops University, Johnson Physics Building, (Room 120) Contact: Lorne Nelson