The formation of low-mass helium white dwarfs orbiting pulsars

Abstract

Context. Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) that have been spun up to high rotation rates via accretion of matter from a companion star in a low-mass X-ray binary (LMXB). This scenario has been strongly supported by various pieces of observational evidence. However, many details of this recycling scenario remain to be understood. Aims. Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods P orb 2−9 h. In particular, we examine i) if the observed systems can be reproduced by theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (M WD , P orb ) is valid for systems with P orb < 2 days. Methods. Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ∼400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period, and the so-called γ-index of magnetic braking. Subsequently, we followed the orbital and the interior evolution of the detached low-mass (proto) He WDs, including stages with residual shell hydrogen burning. Results. We find that severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass <0.20 M , which suggests that something needs to be modified or is missing in the standard input physics of LMXB modelling. Results from previous independent studies support this conclusion. We demonstrate that the theoretically calculated (M WD , P orb )-relation is in general also valid for systems with P orb < 2 days, although with a large scatter in He WD masses between 0.15−0.20 M . The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).

Ultra-stripped supernovae: progenitors and fate

“…For recent applications of BEC to X-ray binaries and further description of Case BB RLO, see e.g. Tauris et al (2011Tauris et al ( , 2012Tauris et al ( , 2013; Lazarus et al (2014); Istrate et al (2014).…”

The Formation of EL CVn-type Binaries

“…For Z = 0.004 (see the lower panel of Fig.11), the bulge appears when MWD 0.25M⊙ (or Mc 0.22M⊙) due to the higher He abundance in the core at a low Z. We see a similar bulge (appeared at the proto-He WD mass of 0.21M⊙) in Fig.13 of Istrate et al (2014a). Accompanying the bulges, the MWD −P relation has a certain degree of dispersion, consistent with non-degenerate cores of the donors in this mass range.…”

OPTICAL IDENTIFICATION OF He WHITE DWARFS ORBITING FOUR MILLISECOND PULSARS IN THE GLOBULAR CLUSTER 47 TUCANAE

“…This value, combined with the surface magnetic field (B ≈ 3.145 × 10 8 G) and assuming an NS with a radius of 10 km and a canonical mass of 1.4 M e , can be used to infer the typical accretion rate that reaccelerated the NS during the low-mass X-ray binary phase. By using Equation (8) of van den Heuvel (2009), we find that the system past accretion history likely proceeded at a subEddington rate M M 0.02 , EDD (˙˙) as expected from the typical evolution of close binary systems with light donor stars (Tauris & Savonije 1999;Istrate et al 2014). Although the mass accretion rate strongly depends on the NS radius, the general result does not change assuming different radii or even different NS masses.…”

“…However, this model is valid for binary systems with 0.18 M e < M WD < 0.45 M e , thus only marginally representative of our sample, where most of the companions appear to be less massive than 0.18 M e . More updated models (from Istrate et al 2014; gray points in Figure 6) are in better agreement, although they are the results of simulations of donor stars with metallicity Z = 0.02, larger than that of 47 Tucanae (i.e., Z = 0.008, Lapenna et al 2015).…”