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Modeling the Formation of Thorne-Żytkow Objects: Hydrodynamical Simulations of Neutron Star-Main Sequence Mergers

Published online by Cambridge University Press:  09 November 2026

Lauryn E. WilliamsOpen the ORCID record for Lauryn E. Williams[Opens in a new window],
Philip Chang,
Emily M. Levesque and
Thomas R. Quinn
Lauryn E. Williams*
Affiliation:
University of Washington
Philip Chang
Affiliation:
University of Wisconsin - Milwaukee
Emily M. Levesque
Affiliation:
University of Washington
Thomas R. Quinn
Affiliation:
University of Washington
*
email: laurynwi@uw.edu
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Abstract

Thorne-Żytkow Objects (TŻOs) are theoretical hybrid stars in which a neutron star is at the core of a large, diffuse envelope. It is theorized that TŻOs may be formed when a newly formed neutron star receives a “kick” that leads to a collision with its secondary main-sequence companion. Using a moving-mesh hydrodynamics solver integrated into the parallel-code Charm N-body GrAvity solver (ChaNGa), we conduct a parameter study simulating dynamical mergers between neutron stars and a range of massive main sequence stars. Our hydrodynamical simulations explore the stability of the resulting post-merger outcome and how it varies depending on whether the periastron distance is greater or less than 1 stellar radius. This study further solidifies the viability of TŻOs formed through this pathway, with the relevance of this work significantly progressing our limited understanding regarding TŻO formation and their role as terminal products of massive binary evolution.

Keywords

Stellar MergersHydrodynamical simulations

Information

Type
Poster Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S402: Massive Stars Across Redshifts in the Era of JWST and Large Surveys , January 2025, pp. 348 - 351
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Creative Commons
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

A Thorne-Żytkow Object (TŻO) is a hypothetical class of stars where there is a neutron star (NS) at the core of large, diffuse envelope. These objects are believed to be formed in one of three ways: (1) through a stellar collision within a globular cluster (Benz & Hills (Reference Benz and Hills1992)), (2) through a common envelope phase where there is double-core evolution (Taam et al. (Reference Taam, Bodenheimer and Ostriker1978); Reference Ablimit, Podsiadlowski, Hirai and WickerAblimit et al. (2022)), and (3) a newly formed NS recieves a “kick” that results in a collision with its secondary companion (Leonard et al. (Reference Leonard, Hills and Dewey1994)). Here we refer to scenario (3) as the “impact scenario”. Modeling the formation of these objects is crucial for us to understand the physics of these systems. Recently there have been efforts modeling their formation (Everson et al. (Reference Everson, Hutchinson-Smith, Vigna-Gómez and Ramirez-Ruiz2024); Hutchinson-Smith et al. (Reference Hutchinson-Smith, Everson and Twum2024)). While these are important studies into scenario (2), there are fewer studies investigating the “impact scenario” (Reference Hirai and PodsiadlowskiHirai & Podsiadlowski (2022)). Using a moving-mesh (MM) solver on top of the N-body code ChaNGa, we simulated the mergers of a 1.4 M Subscript circled dot $_{\odot}$ NS and a 5-15 M Subscript circled dot $_{\odot}$ main sequence (MS) star with 3 periastron distances, r Subscript p $r_p$ , for the orbit of the NS ( r Subscript p Baseline equals 0 comma 0.5 comma 1 upper R Subscript star Baseline $r_p = 0, 0.5, 1 R_{\star}$ ).

2. Methods

2.1. Setup and Initial Conditions

The r Subscript p $r_p$ values are chosen to represent three different potential dynamical interactions: a direct collision, envelope disturbance, and a grazing encounter, respectively. We use the MM hydrodynamical solver MANGA (Chang et al. (Reference Chang, Wadsley and Quinn2017); Reference Prust and ChangPrust & Chang (2019)) integrated onto parallel-code Charm N-body GrAvity solver (ChaNGa; Menon et al. (Reference Menon, Wesolowski and Zheng2015); Reference Wadsley, Keller and QuinnWadsley et al. (2017)). For detailed information about ChaNGa and MANGA see Menon et al. (Reference Menon, Wesolowski and Zheng2015); Wadsley et al. (Reference Wadsley, Keller and Quinn2017); Jetley et al. (Reference Jetley, Gioachin, Mendes, Kale and Quinn2008); Chang et al. (Reference Chang, Wadsley and Quinn2017); Prust & Chang (Reference Prust and Chang2019).

We create our initial conditions by constructing a zero-age MS star of a given mass using MESA (Paxton et al. (Reference Paxton, Bildsten and Dotter2011); Paxton et al. (Reference Paxton, Cantiello and Arras2013); Paxton et al. (Reference Paxton, Marchant and Schwab2015); Paxton et al. (Reference Paxton, Schwab and Bauer2018); Paxton et al. (Reference Paxton, Smolec and Schwab2019); Jermyn et al. (Reference Jermyn, Bauer and Schwab2023)), and take the resulting radius and mass to construct a polytropic star with polytropic index normal upper Gamma equals 4 divided by 3 $\Gamma = 4/3$ , matching the mass and radius to within about 1%. We map the constructed temperature and density profile to mesh points to define the simulated star.

We choose MS masses of upper M Subscript star Baseline equals 5 comma 7 comma 9 comma 12 comma 15 upper M Subscript circled dot Baseline $M_{\star} = 5,7,9,12,15 M_{\odot}$ , and the 1.4 upper M Subscript circled dot $M_{\odot}$ NS is modeled as a dark-matter particle, that is, a point mass that feels only gravitational forces and is not affected by hydrodynamics. The NS starts at a distance of 2 upper R Subscript star $R_{\star}$ , to ensure the initial conditions are not significantly different from an isolated star and to be outside the star’s Roche lobe by a factor of 2 in terms of tidal gravity. We give the NS an appropriate velocity vector to achieve the desired r Subscript p $r_p$ , set to 0, 0.5, or 1 upper R Subscript star $R_{\star}$ for a parabolic (energy, upper E equals 0 $E = 0$ ). Each simulation has approximately 265,000 total mesh points ( upper N Subscript t o t $N_{tot}$ ) of which roughly 104,000 mesh points define the initial star, upper N Subscript star $N_{\star}$ .

3. Results

Figure 1 shows a series of density projection plots from our high resolution simulation left parenthesis upper N Subscript star Baseline left parenthesis upper N Subscript t o t Baseline right parenthesis equals 575 upper K left parenthesis 980 upper K right parenthesis right parenthesis $(N_{\star}(N_{tot}) = 575K(980K))$ of a 7 upper M Subscript circled dot $M_{\odot}$ MS star successfully merging with the NS, becoming dynamically stable after 3.6 days. We call this object a dynamical TŻO (dTŻO) to reflect their nature as being dynamically formed TŻOs while not yet assuming all the complex physical processes that define a classical TŻO.

Figure 1.

Frames from our 7 upper M Subscript circled dot $M_\odot$ MS star merging with the NS: the highest density point, representing the core of the MS star, is indicated by the pentagon and the NS is indicated by the cross. The color bar denotes the density of the gas. The NS successfully merges with the MS star at t equals 3 $t = 3$ days.

Heat map showing density distribution over time during a star merger.

4. Conclusion

We found that stable dynamical TŻOs can be robustly formed through MS-NS collisions for small periastron distances, r Subscript p Baseline less than or equals 1 upper R Subscript star $r_p \le 1 R_{\star}$ . This work represents an important first step using a flexible and powerful code capable of capturing their key dynamical features. For further details on the dynamical stability of these objects and a discussion surrounding the resulting simulation parameters, see Williams et al. (Reference Williams, Chang, Levesque and Quinn2025). Further research will include simulating periastron distances larger than 1 upper R Subscript star $1 R_{\star}$ to examine other products of these binary interactions (Reference Hirai and PodsiadlowskiHirai & Podsiadlowski (2022)).

Supplementary material

To view supplementary material for this article, please visit https://doi.org/10.1017/S174392132610550X

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Figure 0

Figure 1.Frames from our 7 M⊙$M_\odot$ MS star merging with the NS: the highest density point, representing the core of the MS star, is indicated by the pentagon and the NS is indicated by the cross. The color bar denotes the density of the gas. The NS successfully merges with the MS star at t=3$t = 3$ days.

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