Evolutionary Properties of Helium‐rich, Degenerate Dwarfs in Binaries Containing Compact Companions

Abstract

The evolution of binaries containing low-mass donors ( 2 M ) and degenerate accretors (i.e., white dwarfs [0.7 M ] or neutron stars [1.4 M ]) is systematically investigated over a wide expanse of parameter space. The donors are assumed to have metallicities in the range of 0:0001 Z 0:02 and could be either unevolved (zeroage main sequence) or ascending the red giant branch at the onset of mass transfer [corresponding to 5 P P orb (hr) P 2 ; 10 3 ]. The evolutionary tracks form part of a very detailed grid of nearly 200 sequences, each corresponding to a different set of initial conditions. During the initial phases of their evolution, the systems resemble either low-mass X-ray binaries or cataclysmic variables. However, as has been pointed out by several authors (e.g., Podsiadlowski et al.), the evolutionary tracks can exhibit a wide variety of behaviors and outcomes. We focus on systems that evolve to produce helium-rich degenerate dwarfs (HeDDs) that are either detached or still losing mass after a Hubble time. Of the systems that avoid dynamical instability, some evolve to ultrashort values of P orb (<60 minutes), while others that lie above the bifurcation limit (which we show to be quite sensitive to the assumed metallicity) produce wide binary millisecond pulsars (BMSPs) containing HeDD companions that have P orb values of P2 ; 10 4 hr. We also show that the mass and composition of the envelopes of HeDDs that are about to descend the cooling branch are particularly sensitive to the input physics and this in turn dictates whether they experience extremely vigorous hydrogen shell flashes. Our computed values of the envelope mass, m env (corresponding to the mass of those layers below the surface containing some hydrogen), typically have a dependence on the final donor mass (m f ) and metallicity (Z ) that can be expressed as m env / m À2:8 f Z À0:2 . Furthermore, for systems that do not evolve too close to the bifurcation limit, we find that the final orbital period is given by P orb; f (hr) ffi 2:5 Z 0:3 ; 10 10:7ðm f =M Þ. Finally, we use our results to comment on the properties of the optical companions to binary millisecond pulsars and compare their inferred ages with the spindown times of the pulsars. The structure of young HeDDs should be particularly useful as starting models for more sophisticated cooling calculations that take into account important long-term physical processes and thus we are making our models available for retrieval.

New evolutionary sequences for extremely low-mass white dwarfs

“…It should be mentioned that in this paper we have not considered different values of the initial mass of the secondary (mass-losing) star. Although the envelope mass of the resulting white dwarf, a key factor in dictating the cooling times, is expected to be only weakly dependent on the initial mass of the mass-losing star (Nelson et al 2004), different orbital angular-momentum loss prescriptions due to mass loss could have an impact on the final envelope mass. This issue has not been considered in this paper.…”

“…A total of 14 initial He-core white dwarf models with stellar masses between 0.155 and 0.435 M were derived for initial orbital periods at the beginning of the Roche lobe phase in the range 0.9 to 300 d. In particular, nine sequences span the range of masses corresponding to ELM white dwarfs (M 0.20 M ). At this point, it is worth mentioning that the envelope mass of the resulting white dwarf, a key factor in dictating the cooling times, is only weakly dependent on the initial mass of the secondary (mass-losing) star (Nelson et al 2004). However, different angular-momentum loss prescriptions due to mass loss, which could have an impact on the final envelope mass, have not been explored in this paper.…”

“…Because of the very low stellar mass values that characterize these ELM white dwarfs (lower than about 0.20 M 1 ), they are believed to be the result of compact binary evolution, during which the envelope of a red giant star is removed before the core reaches enough mass to ignite helium. The evolution of He-core white dwarfs has been studied by Driebe et al (1998), Sarna et al (2000), Althaus et al (2001), Serenelli et al (2002), Nelson et al (2004), Benvenuto & De Vito (2005), Panei et al (2007), and more recently by Gautschy (2013).…”

Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors

“…The resulting ages are thus much greater than those resulting from considering only the decrease in the thermal content of the ions. More recently, Nelson et al (2004) have also found that hydrogen burning strongly delays the cooling process, especially for very low-mass white dwarfs. They found flashes taking place for masses in the range 0.21 M < ∼ M WD < ∼ 0.28 M , in good agreement with the predictions of Driebe et al (1998).…”

The double degenerate system NLTT 11748

“…The constraints placed on their total ages help retrace the prior evolution of these systems (see Tauris & Savonije 1999;Nelemans et al 2001;Nelson et al 2004).…”