Erythrohenosis -- The crimson chronicles of two giants

TL;DR

Erythrohenosis investigates the collision and merger of two red giants in dense stellar environments, proposing an end-to-end multi-stage evolution from grazing encounters to a terminal explosion. By combining three-dimensional SPH simulations with analytical and semi-analytical models (Airy-boundary accretion, BHL-like dynamical friction, Sedov-Taylor blast dynamics, and a data-informed drag framework), the study predicts distinctive observational fingerprints across EM and GW channels, including quasi-periodic luminous precursors, a rotating, centrally filled remnant with morphomnesia, and a rapid, gas-drag-dominated gravitational-wave chirp with a time-varying apparent chirp mass. The work demonstrates robust links between tidal precursors, envelope dynamics, and final explosion morphology, offering concrete diagnostics for multi-messenger surveys. These results provide a physically grounded pathway to identify and interpret luminous red novae-like transients arising from red-giant mergers, and they establish a framework for incorporating complex hydrodynamics into GW-informed astrophysical transients.

Abstract

We investigate erythrohenosis -- the collision and merger of two red giants -- establishing an end-to-end model for this fundamental evolutionary channel in dense stellar environments. Combining three-dimensional SPH simulations of a binary with analytical modeling, we characterize the event from initial encounter to terminal explosion. We demonstrate that grazing encounters induce tidal capture and rapid orbital decay, accompanied by large-amplitude, nonlinear stellar oscillations. The subsequent inspiral spins up the common envelope into a stable, non-spherical equilibrium, powering a luminous precursor with quasi-periodic bursts. The terminal explosion, modeled with angular momentum conservation, produces an intrinsically flattened remnant that preserves a geometric memory, or morphomnesia, of its binary origin. The associated gravitational wave signal features a rapid, drag-dominated frequency evolution, identifiable by a unique time-varying apparent chirp mass. These results define a distinctive multi-stage observational fingerprint -- linking transient optical precursors, asymmetric nebulae, and anomalous gravitational wave chirps -- to guide identification in current and future multi-messenger surveys.

Figures (27)

• Figure 1: Temperature as a function of the density for the initial conditions after $\sim 6.89\times 10^9\,\text{yrs}$. This moment captures a $1\,M_{\odot}$ star firmly in its red giant phase, with a degenerate core, active hydrogen shell burning, and an inflated envelope—all hallmarks of post-main-sequence evolution in low-metallicity environments. The integration beyond this point allows us to map the entire RGB trajectory until numerical constraints halt the simulation.
• Figure 2: Multi-panel visualization of a $1\,M_{\odot}$, $Z=0.002$ star's evolution into a red giant. From left to right, top to bottom: (1) HR diagram ($\log T_{\text{eff}}$ against $\log L$) showing the transition from main sequence to red giant branch; (2) Radius evolution ($R/R_{\odot}$ against time) highlighting envelope expansion; (3) He core mass growth ($M_{\text{He}}$ against time); (4) Nuclear luminosity ($L_{\text{H}}$, $L_{\text{He}}$ against time) tracking shell/core burning; (5) Surface gravity ($\log g$ against time) documenting envelope dilution; (6) Effective temperature ($T_{\text{eff}}$ against time) cooling evolution. Together, these panels capture the star's structural transformation, with the snapshot at $6.89\,$Gyr. See main text for detailed analysis.
• Figure 3: Initial conditions of a representative red giant model. Left: Rendered visualization showing $\log \rho$, showing the density structure of the stellar envelope. Right: Raw SPH particle distribution in the $x$-$y$ plane, with the compact core visible at the centre, as on the left panel.
• Figure 4: Erythrohenosis at its best: Three key stages in the first encounter of the collision of two red giants with different initial properties. The top panel shows the initial configuration, with the larger star (right) and smaller companion (left) prior to interaction. The lower panels depict: (left) the first collision moment, and (right) a subsequent phase of this encounter. All panels display log-density renderings.
• Figure 5: Trajectories of the two compact cores in the $xy$-plane, extracted from the red giant progenitors. The solid blue line (dark to light gradient) shows the primary core's path, while the dashed orange line (dark to light gradient) tracks the secondary core. Both trajectories are normalized by the initial separation $d_0$ and colored by simulation time (darker shades indicate earlier times). The fading color scheme reveals how friction from the surrounding gas envelope causes the cores to spiral inward, with their closest approach occurring in the later, lighter-colored phases. The distinct line styles ($-$ for primary, $--$ for secondary) help identify interactions where the cores exchange momentum. The axes show normalized coordinates to the initial separation ($x/d_0$, $y/d_0$).