Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

TL;DR

This paper proposes that the Hills mechanism can explain the periodic outbursts of ASASSN-14ko by capturing stars into tight orbits that undergo repeated partial tidal disruptions, providing a new insight into the origin of such transients.

Contribution

The study demonstrates through analytic and numerical methods that the Hills mechanism can produce stars on tightly bound orbits causing periodic transients like ASASSN-14ko, a novel explanation for these phenomena.

Findings

Hills mechanism can produce stars on 114-day orbits around black holes.

Captured stars can undergo repeated partial tidal disruptions.

Gravitational-wave emission likely has a minor effect on orbit decay in this system.

Abstract

Periodic nuclear transients have been detected with increasing frequency, with one such system -- ASASSN-14ko -- exhibiting highly regular outbursts on a timescale of $114 \pm 1$ days. It has been postulated that the outbursts from this source are generated by the repeated partial disruption of a star, but how the star was placed onto such a tightly bound orbit about the supermassive black hole remains unclear. Here we use analytic arguments and three-body integrations to demonstrate that the Hills mechanism, where a binary system is destroyed by the tides of the black hole, can lead to the capture of a star on a $\sim 114$ day orbit and with a pericenter distance that is comparable to the tidal radius of one of the stars within the binary. Thus, Hills capture can produce stars on tightly bound orbits that undergo repeated partial disruption, leading to a viable mechanism for generating not only the outbursts detected from ASASSN-14ko, but for periodic nuclear transients in general. We also show that the rate of change of the period of the captured star due to gravitational-wave emission is likely too small to produce the observed value for ASASSN-14ko, indicating that in this system there must be additional effects that contribute to the decay of the orbit. In general, however, gravitational-wave emission can be important for limiting the lifetimes of these systems, and could produce observable period decay rates in future events.