Title: The Role of Dredge-up in Double White Dwarf Mergers

Abstract

We present the results of an investigation of the dredge-up and mixing during the merger of two white dwarfs (WDs) with different chemical compositions by conducting hydrodynamic simulations of binary mergers for three representative mass ratios. In all the simulations, the total mass of the two WDs is $$\lesssim$$1.0 M_⊙. Mergers involving a CO and a He WD have been suggested as a possible formation channel for R Coronae Borealis (RCB)–type stars, and we are interested in testing if such mergers lead to conditions and outcomes in agreement with observations. Even if the conditions during the merger and subsequent nucleosynthesis favor the production of ¹⁸O, the merger must avoid dredging up large amounts of ¹⁶O, or else it will be difficult to produce sufficient ¹⁸O to explain the oxygen ratio observed to be of order unity. Here, we performed a total of nine simulations using two different grid-based hydrodynamics codes using fixed and adaptive meshes and one smooth particle hydrodynamics (SPH) code. We find that in most of the simulations, >10^-2 M_⊙ of ¹⁶O is indeed dredged up during the merger. However, in SPH simulations where the accretor is a hybrid He/CO WD with a ~0.1 M_⊙ layer of helium on top, we find that no ¹⁶O is being dredged up, while in the q = 0.8 simulation <10^-4 M_⊙ of ¹⁶O has been brought up, making a WD binary consisting of a hybrid CO/He WD and a companion He WD an excellent candidate for the progenitor of RCB stars.

Authors:

Staff, Jan. E.  ^[1];

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Wiggins, Brandon Kerry  ^[2];

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Marcello, Dominic  ^[3];

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Motl, Patrick M.  ^[4];

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Even, Wesley Paul  ^[5];

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Fryer, Chris Lee  ^[5];

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Raskin, Cody ^[6]; Clayton, Geoffrey C.  ^[3];

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Frank, Juhan ^[3]

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+ Show Author Affiliations

1. Univ. of the Virgin Islands, St. Thomas, VI (United States). College of Science and Math
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Southern Utah Univ., Cedar City, UT (United States)
3. Louisiana State Univ., Baton Rouge, LA (United States). Dept. of Physics and Astronomy
4. Indiana Univ., Kokomo, IN (United States). School of Sciences
5. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
6. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Publication Date:

2018-07-20

Research Org.:

Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA); Australian Research Council (ARC); National Aeronautic and Space Administration (NASA); National Science Foundation (NSF)

OSTI Identifier:

1469529

Report Number(s):

LA-UR-17-25739
Journal ID: ISSN 1538-4357

Grant/Contract Number:  

AC52-06NA25396; NNX10AC72G; NNX15AP95A; AST-1240655; TG-AST090104; GAST110034

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal (Online)

Additional Journal Information:

Journal Name: The Astrophysical Journal (Online); Journal Volume: 862; Journal Issue: 1; Journal ID: ISSN 1538-4357

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; binaries: close; hydrodynamics; methods: numerical; white dwarfs