Neutron Star-Main Sequence Collisions Robustly Form Dynamically Stable Thorne-Żytkow Objects

TL;DR

The paper investigates whether Thorne-Żytkow Objects can form dynamically stable via the impact scenario where a newly born neutron star collides with its massive main-sequence companion. The authors employ a moving-mesh hydrodynamics scheme (MANGA) embedded in the ChaNGa N-body code to simulate NS-MS mergers at periastron distances $r_p = 0, 0.5, 1\,R_{\star}$, using MS masses in the range $M_\star = 5$–$15\,M_\odot$ and treating the NS as a point mass with gravitational softening. They find robust formation of dynamically stable TŻO remnants (dTŻOs) for $r_p \le R_{\star}$; in particular, a $7\,M_\odot$ MS star merges with the NS to form a dTŻO by about $t \simeq 3.6$ days, with most of the initial mass remaining bound ($f_e \lesssim 0.1$) and interiors showing convective mixing as indicated by flat entropy profiles. The core temperatures exceed $10^9$ K, underscoring the need to include nuclear burning in future modeling. Overall, the results establish the impact scenario as a viable channel for TŻO formation and provide dynamic, self-consistent initial models for subsequent stellar evolution and population-synthesis studies.

Abstract

Thorne-Żytkow Objects (TŻOs) are hypothetical hybrid stars with a neutron star at the core of a large, diffuse envelope. (TŻOs) may be formed when a newly formed neutron star that is kicked by its supernova collides with its main-sequence companion. Using a moving-mesh hydrodynamics solver integrated into the parallel-code Charm N-body GrAvity solver, we demonstrate that these ``impact scenario'' formation processes robustly form (TŻOs) for periastron distances less than one stellar radius. These (TŻOs) are dynamically stable and they can serve as initial models for further evolutionary studies.

Figures (3)

• Figure 1: Frames from our 7 $M_\odot$ 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 $t = 3$ days (Frame e).
• Figure 2: Separation between the center of the MS star and the NS, r (in units of $R_{\odot})$, over simulation time is plotted in the left column, with $\rho_{c}$ of our dTŻOs plotted in the right column. Each line is colored by the progenitor mass, ranging from 5 $M_{\odot}$ to 15 $M_{\odot}$, and each row corresponds to a different type of encounter: direct-collision ($r_{p} = 0 \ R_{\star}$,top), envelope-disturbance ($r_{p} = 0.5 \ R_{\star}$, middle), and grazing-encounter ($r_{p} = 1 \ R_{\star}$, bottom).
• Figure 3: Radial temperature and entropy profiles of our dTŻOs. The top row shows the temperature profiles, and the bottom row shows the entropy profiles, extending from the center of the dTŻO to approximately $20 \ R_{\odot}$ limited by resolution. The flat entropy profile indicates that the dTŻO is fully mixed or has a convective core. The three columns correspond to one of the three encounter types that we modeled: direct-collision, envelope-disturbance, and grazing-encounter($r_{p} = 0, 0.5, 1 \ R_{\star}$).