A triple-induced merger in T Ori C, solves the proplyd lifetime problem and the curiously young shell of M42

Abstract

There has long been a tension between the stellar ages in the Orion Nebula Cluster and the rapid external photoevaporation of its protoplanetary discs. Recently, the shell of the Orion Nebula has similarly been found to be dynamically younger than the stars. We propose a new scenario to resolve both age problems. In our scenario the ONC's most massive star, $θ^1$~Ori~C1, formed from the merger between two binary stars induced by vZLK oscillations due to the present-day binary companion $θ^1$~Ori~C2. This merger, happening between the last episode of star formation and today, resulted in an increased luminosity, leading to stronger external photoevaporation and accelerated expansion of the shell. {We use Monte Carlo simulations to explore both whether a sudden brightening of $θ^1$~Ori~C1 could reconcile the stellar ages and shell expansion, and whether triple progenitors to $θ^1$~Ori~C\, could result in stellar mergers on the relevant timescale. A sudden brightening of $θ^1$~Ori~C1 a few hundred thousand years ago can reproduce today's shell while maintaining a system age close to the stellar age. However, the required increase in stellar wind luminosity is greater than a simple merger can explain. At lower luminosity, the mixing of a relatively greater amount of cool photoevaporative wind material into the hot bubble could result in the required increase, and we identify a lower limit on the required strength of this effect. Additionally, we find that progenitor triples merge readily, on timescales even shorter than the scenario would require. These mergers could be deferred by increased pre-main sequence tidal effects, rotation, and perturbing encounters with other cluster members. Our proposed scenario with a merger driven by von Zeipel-Lidov-Kozai evolution solves two problems; the Orion Nebula's proplyd problem and the shell lifetime problems.