Eccentric millisecond pulsar + subdwarf B star from rotationally delayed accretion-induced-collapse scenario

Abstract

Eccentric millisecond pulsar + helium white dwarf (MSP + He WD) systems have attracted increasing attention, with the rotationally delayed accretion-induced collapse (RD-AIC) scenario proposed as a possible formation channel. Given the similarity between the formation channels of He WDs and subdwarf B (sdB) stars, eccentric MSP + sdB binaries could also exist in the Galaxy, though none have been detected so far. Theoretical predictions of their properties would greatly aid in their discovery. Here, within the RD-AIC framework, I present predictions for their orbital parameters, including MSP mass, secondary mass, eccentricity and orbital period. Based on two detailed binary population synthesis calculations, I estimate their Galactic birth rate to be $(0.67-1.5)\times10^{\rm -4}~{\rm yr^{\rm -1}}$. Then, a very conservative upper limit for their total number in the Galaxy is 15000, implying that the most optimistic fraction of eccentric MSP + sdB systems among all MSP + sdB populations could reach up to 55\%. These systems are relatively young, with ages on the order of a few hundred Myr, and should therefore be found in relatively young environments. Furthermore, most MSPs in such eccentric binaries have masses below 1.5 $M_{\odot}$. I also briefly discuss their potential future applications in various astrophysical context.