Common envelope events with low-mass giants: understanding the transition to the slow spiral-in

Abstract

We present a three-dimensional (3D) study of common envelope events (CEEs) to provide a foundation for future one-dimensional (1D) methods to model the self-regulated phase of a CEE. The considered CEEs with a low-mass red giant end with one of three different outcomes -merger, slow spiral-in, or prompt formation of a binary. To understand which physical processes determine different outcomes, and to evaluate how well 1D simulations model the self-regulated phase of a CEE, we introduce tools that map our 3D models to 1D profiles. We discuss the differences in the angular momentum and energy redistribution in 1D and 3D codes. We identified four types of ejection processes: the pre-plunge-in ejection, the outflow during the plunge-in, the outflow driven by recombination, and the ejection triggered by a contraction of the circumbinary envelope. Significant mass is lost in all cases, including the mergers. Therefore a self-regulated spiral-in can start only with a strongly reduced envelope mass. We derive the condition to start a recombination outflow, which can proceed either as a runaway or a stationary outflow. We show that the way the energy of the inspiraling companion is added to the envelope in 1D studies intensifies the envelope's entropy increase, alters the start of the recombination outflow, and leads to different outcomes in 1D and 3D studies. The steady recombination outflow may dispel most of the envelope in all slow spiral-in cases, making the existence of a long-term self-regulated phase debatable, at least for low-mass giant donors.

Episodic mass ejections from common-envelope objects

“…Initial attempts to model CE events were limited to 1-dimensional studies, such as those reported by Taam et al (1978), Meyer & Meyer-Hofmeister (1979), and Delgado (1980). Using such 1-d studies, three phases have been identified in the evolution of CE simulations (Podsiadlowski 2001;Ivanova 2002): first the loss of co-rotation phase, in which the initial embedding of the binary within the envelope occurs. This phase may occur over a long time-scale (depending on the nature of the onset of CE evolution), with the expected initial co-rotation of the system leading to low rates of energy dissipation.…”

Post-common envelope binary systems experiencing helium-shell driven stable mass transfer

“…Several estimates exist for the duration of the CE phase. Podsiadlowski (2001), using a stellar evolution code, predicts that a CE phase may last 100 to 1000 yr. CE 3D hydrodynamic simulations of the dynamical in-fall phase (not including ejection) by Ricker & Taam (2012) estimate it to be longer than about 50 d. Passy et al (2012b) find that most of the in-spiral happens within 200 to 300 d, and Ivanova & Nandez (2016) find this to be a few hundred days. The lack of conclusive observational evidence leaves the CE duration unconstrained and motivates the search for observable signatures of CE evolution which could serve as diagnostic of the instabilities of the spiral-in and the history of the mass loss associated with CE evolution.…”

Planetary Nebulae Embryo after a Common Envelope Event

“…Recombination energy is a "potential" energy in the sense that it is not readily available to drive an envelope expansion at the start of the CE event, but, after the recombination is triggered, it can be useful at the late stages of a CE event. Recombination energy can remove all the remaining bound parts of the envelope if the envelope material starts to recombine after it expands beyond the "recombination" radius (for the definition, see [21]). The discussion on whether the recombination energy can be used for driving the expansion of a CE, or it would be lost as radiation remains active.…”

“…The velocity of the ejected material is not constant across the ejecta-see the typical direction-averaged profile in Figure 1. Ivanova and Nandez [21] identified four kinds of matter ejection processes during a CE event. First is an "initial ejection", before the plunge (for exact definitions of various stages, see [21]).…”