Key Parameters in Common Envelope Evolution

Common Envelope basics Binary systems can be comprised of any combination of astrophysical objects, from stars and planets to neutron stars and black holes. In a post-LIGO-detection age, we endeavor to explain the existence of close binaries that are able to merge on a timescale less than the age of the universe, as well as the rate of such mergers. Though the start-to-finish evolution of these systems in general is of interest, it is practical to investigate phases of evolution that offer the possibility of being observed; the traditional formation channel for close binaries, in which a common envelope (CE) phase dramatically reduces the separation of a pre-existing binary system, offers that possibility.

The onset of the CE phase occurs when the expanding envelope of one of the stars in the binary extends to the orbit of the other star (or planet, black hole, etc.) and engulfs it (a → b). Due to gravitational interactions between the embedded object and the envelope material, a drag force acts on the former and it eventually plunges in toward the core of the expanding star (b). Depending upon the amount of energy the embedded object can deposit into the envelope material and the timescales on which this energy can be transported throughout the envelope, the CE phase may end in either the ejection of the envelope and a surviving binary comprised of the embedded object and the remaining core at greatly reduced separation (c), or a merger of the two with envelope intact (d).

My work focuses on key physical parameters that describe the CE phase for a range of initial systems, giving insight on the inspiral phase and the final system configuration. This includes looking at the impact of envelope structure on drag forces and expected accretion, as well as inspiral and energy transport timescales.

Common Envelope Wind Tunnel: Range of Applicability and Self-Similarity in Realistic Stellar Envelopes
Rosa Wallace Everson, Morgan MacLeod, Soumi De, Phillip Macias, & Enrico Ramirez-Ruiz
ABSTRACT: Common envelope evolution, the key orbital tightening phase of the traditional formation channel for close binaries, is a multistage process that presents many challenges to the establishment of a fully descriptive, predictive theoretical framework. In an approach complementary to global 3D hydrodynamical modeling, we explore the range of applicability for a simplified drag formalism that incorporates the results of local hydrodynamic "wind tunnel" simulations into a semi-analytical framework in the treatment of the common envelope dynamical inspiral phase using a library of realistic giant branch stellar models across the low, intermediate, and high mass regimes. In terms of a small number of key dimensionless parameters, we characterize a wide range of common envelope events, revealing the broad range of applicability of the drag formalism as well its self-similar nature across mass regimes and ages. Limitations arising from global binary properties and local structural quantities are discussed together with the opportunity for a general prescriptive application for this formalism.

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Common Envelope Evolution and LIGO Source Populations
Rosa Wallace Everson, Morgan MacLeod, Soumi De, & Enrico Ramirez-Ruiz
ABSTRACT: With confirmed gravitational wave detections of binary neutron star (BNS) and binary black hole (BBH) mergers, the channel through which both populations form remains an open question. Common envelope (CE) evolution plays a role in shaping these populations as one of the few formation channels in which the separation of a field binary may be reduced such that the resulting stellar remnants can merge in a Hubble time. CE evolution may include several different inspiral stages from onset to completion, including a quick dynamical phase and a gradual self-regulated phase of orbital decay, the length and characteristics of which impact whether the binary will merge during CE, become a gravitational wave source progenitor, or remain at wide separation. Recent work has shown that CE evolution depends upon more than initial conditions: the structure of the envelope impacts the duration of inspiral and the post-CE properties of the embedded compact object. We explore the implications of including envelope structure in both the BNS and BBH progenitor cases, with new considerations for how the dynamical phase of CE inspiral should be approached, and how these affect the types of systems that we will observe in the future with LIGO.

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Effects of the Common Envelope Phase on Binary Black Hole Evolution
Rosa Wallace Everson, Phillip Macias, Morgan MacLeod, Andrea Antoni, & Enrico Ramirez-Ruiz
ABSTRACT: The detection of gravitational wave signals from binary black hole (BBH) mergers in recent years has raised pressing questions about the formation and characteristics of these systems. In order for BBHs produced in the traditional formation channel to merge in a Hubble time, the pair must undergo a common envelope (CE) phase to dramatically reduce the separation distance of the progenitors prior to CE ejection. Recent work on the CE phase has shown that density gradients in the envelope material produce a significant departure from drag and accretion rates of the embedded compact object as predicted by Hoyle- Lyttleton accretion (HLA) formalism; these effects, in turn, have implications for mass and angular momentum transfer between the donor star and compact object. Using a range of simplified progenitor systems in which a massive, stellar-mass black hole (BH) dynamically inspirals through the envelope of a giant stellar companion, we examine these CE effects.

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Undergraduate Research

An Observationally Constrained 3D Potential-field Source-surface Model for the Evolution of Longitude-dependent Coronal Structures
Rosa Wallace Everson & Mausumi Dikpati
An exploration of using morphological data to constrain models of the magnetic field in the solar corona

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