Precession and polar alignment of accretion discs in triple (or multiple) stellar systems

We investigate the mechanism of polar alignment for accretion discs in hierarchical systems (HSs) with more than two stars. In eccentric binary systems, low mass that are sufficiently tilted to orbit align a configuration respect plane by aligning their angular momentum eccentricity vector. HSs, secular evolution orbital parameters makes vector system precess time. This precession undermines stability hosted HSs. analytically show criteria derived literature necessary but not sufficient conditions Then, we derive an analytical criterion general, find orbiting innermost level HS can go polar. Conversely, radially extended outer levels cannot polarly and evolve as around circular binary. confirm our findings through detailed numerical simulations. Also, results compatible observed distribution disc-orbit mutual inclination. Finally, compare disc inclinations populations. Binaries host mainly coplanar discs, while HSs wide range inclinations. suggest wider from oscillation (such Kozai-Lidov oscillations), rather different initial condition or between binaries.