The effects of star-gas interactions on binary evolution in open clusters

TL;DR

The paper addresses how gas-mediated friction, specifically DF and NDF from star–gas interactions, shapes the binary population and global structure of open clusters. It uses large-scale direct $N$-body simulations with the PeTar code to model a cluster of $N=10^4$ stars over $\sim200$ Myr across ambient gas densities spanning $\rho_0$ from $30$ to $3\times10^{8}\ \,\mathrm{m_p\,cm^{-3}}$, incorporating simplified stellar evolution and binary tracking. The main findings show that NDF, driven by stellar outflows, systematically expels neutron stars and heats the cluster, promoting expansion and increasing survival of wide/soft binaries, while DF tends to harden the core by removing orbital energy and reduces the overall binary survival at high densities; the effects grow with ambient density and vary by binary type, particularly MS--NS systems. These results imply that gas-mediated friction can significantly alter binary fractions, orbital parameter distributions, and possibly the rates of compact-object mergers in clusters, with observable signatures in half-mass versus half-light radii and binary demographics. The work provides a framework to interpret cluster lifetimes and GW source populations in gas-rich environments, while highlighting limitations such as isotropic winds and fixed gas temperature for future refinements.

Abstract

Star-gas interactions can provide gravitational feedback that influences the dynamical evolution of stellar clusters, through processes such as dynamical friction (DF) and its non-dissipative counterpart, negative dynamical friction (NDF). Using the \texttt{PeTar} code, we perform direct $N$-body simulations of an open cluster initially containing $10^4$ stars, evolving within a gaseous medium spanning a range of ambient densities. Our results demonstrate that NDF associated with stellar outflows interacting with the surrounding gas can enhance the rate of cluster expansion, preferentially transporting stars toward the cluster outskirts. This behavior is accompanied by a more rapid decline in the number of binaries composed of a neutron star and a main-sequence star. A statistical analysis of binary orbital parameters further indicates that, compared to DF-dominated evolution, NDF tends to retain systems with larger semi-major axes and lower eccentricities. Outflow-ambient gas interactions can modify the dynamical processing of binaries in star clusters, leading to changes in the survival fraction and composition of the remaining binary population.

Figures (13)

• Figure 1: Mass distribution of the initial stellar population in this work. The blue step line shows the sampled initial stellar mass distribution, while the black dashed line indicates the 2002Sci...295...82K IMF (Eq. \ref{['eq:IMF_kroupa']}), shown in units of $\mathrm{d}f/\mathrm{d} \lg M$.
• Figure 2: $I_{\rm DF}(\mathcal{M})$ for $\ln{\Lambda}=10$. The black curve shows the analytic expression for $I_{\rm DF}$ given by Equation \ref{['eq:I_DF']}, while the orange and green curves show the polynomial approximations adopted in our simulations to ensure numerical smoothness.
• Figure 3: $I_{\rm NDF}(\mathcal{M})$ for $\mathcal{M}_{\rm{crit}}=10$, where $\mathcal{M}_{\rm crit}$ is the Mach number at which $p_0=p_s$. The line styles and colors follow those in Figure \ref{['fig:a_DF']}, with the black curve corresponding to the analytic expression and the colored curves showing the polynomial approximations.
• Figure 4: Radial trajectories of stars that transition from the MS phase to the NS phase, shown as a function of time (horizontal axis) and distance from the cluster center (vertical axis). Solid lines represent stellar trajectories during the MS phase, while dashed lines indicate the subsequent trajectories after the stars have become NSs. Only 10 randomly selected trajectories are shown for each simulation for clarity. Breaks in the curves mark objects that leave the simulation volume. Rows correspond to increasing ambient gas density from $30~m_p~\mathrm{cm^{-3}}$ to $3\times10^{3}~m_p~\mathrm{cm^{-3}}$. Columns compare simulations including only DF (left) and those including NDF (right).
• Figure 5: Radial trajectories of stars that transition from the MS phase to the NS phase in simulations with other ambient gas densities than those shown in Figure \ref{['fig:track']}. The layout and line styles follow the same conventions as in Figure \ref{['fig:track']}. Panels labeled NONE show gas-free reference models, while panels labeled NDF-D3E4, NDF-D3E6, and NDF-D3E8 illustrate the effects of NDF in progressively denser environments.