The evolution of coeval stellar hierarchical triple systems

Abstract

We study the evolution of coeval stellar hierarchical triple systems with intermediate mass (initial primary mass 1.0 < m_1 /M_sun < 6.5) and relatively wide inner binary systems (a_1 [1 − e_1^2 ] > 12 AU, where a_1 and e_1 are the initial inner orbit semi-major axis and eccentricity respectively). For these triple systems the inner binary stars do not interact in the absence of the third star (tertiary). Special attention is paid to the channels in which the tertiary affects the inner binary system through high-amplitude eccentricity cycles or dynamical instability of the triple system such that potentially a compact object merger is triggered in the inner binary system. To model hierarchical triple systems, we have developed a new algorithm which couples an existing rapid binary population synthesis code with a newly written module that computes the secular gravitational three-body dynamics. With this algorithm, we perform a population synthesis study of triples and we present the main channels. We find that in ∼ 9% of systems of the computed sample, Kozai cycles with tidal friction (KCTF) are responsible for significantly shrinking the inner binary orbit to a1 < 12 AU, possibly leading to common-envelope (CE) evolution and/or an inner binary merger. In ∼ 5% of all systems the eccentricity driven by Kozai cycles is high enough for an orbital collision (eccentric merger) and ∼ 10% of the triple systems become dynamically unstable. In the latter possibility, previous N-body simulations show that in ∼ 10% of such cases the destabilization eventually leads to a collision. The latter two channels of eccentric mergers and dynamical instability are unique channels in the evolution of hierarchical triple systems and the associated type of mergers are otherwise expected to occur only in dense stellar systems. Several channels found in the population synthesis study involve mergers of CO WDs and therefore potentially lead to type Ia supernovae (SNe Ia). We estimate the expected rate of SNe Ia and compare the delay time distribution to a binary population synthesis study and observations. We find that the expected triple-induced SNe Ia rate is one to three orders of magnitude lower than the binary population synthesis rate. The former represents a lower limit, however, due to fact that only initially wide-orbit inner binary systems are considered in this work. Further study is needed in which triple systems are included with initially tighter inner orbits.