On The Progenitors Of Am Cvn Stars As Lisa Sources: The Evolved Donor Star Channel

The space gravitational wave (GW) detector Laser Interferometer Space Antenna (LISA) that is planned to be launched in the early 2030s will detect the low-frequency GW signals in the Galaxy. AM CVn stars were generally thought to be important low-frequency GW sources. Employing the MESA code, in this work we calculate the evolution of a great number of binary systems consisting of a white dwarf (WD) and a main sequence (MS) star, and diagnose whether their descendant-AM CVn stars will be visible with LISA. The simulated results show that the progenitors of these LISA sources, within a distance of 1 kpc, are WD-MS binaries with a donor star of 1.0-1.4 M (for initial WD mass of 0.5 M) or 1.0-2.0 M (for initial WD mass of 0.7 M), and an initial orbital period slightly smaller than the bifurcation period. Our simulations also indicate that 10 verification AM CVn sources can be reproduced by the standard magnetic braking model, and are potential LISA sources. Based on the birth rate of AM CVn stars simulated by the population synthesis, the birth rate of AM CVn-LISA sources evolving from the evolved donor star channel within a distance of 1 kpc can be estimated to be (0.6-1.4) x 10(-6) yr(-1), and the predicted number of AM CVn-LISA sources is about 340-810. Therefore, the evolved donor star channel plays an important role in forming AM CVn-LISA sources in the Galaxy.