Two-Dimensional Radiation-Hydrodynamic Simulations of Luminous Red Novae

TL;DR

This paper addresses how luminous red novae (LRNe) attain their high luminosities and long plateau phases during binary mergers by testing a shock-powered scenario where dynamically ejected material interacts with preexisting equatorial circumbinary material (CBM).The authors perform axisymmetric, two-dimensional moving-mesh radiation-hydrodynamic simulations with RJET, incorporating hydrogen and helium recombination and realistic opacities to model the coupled hydrodynamics and radiative transport of the ejecta–CBM system.They find that a fast, embedded shock within the ejecta interacting with the CBM can produce a first bright peak ($\gtrsim10^{41}$ erg s$^{-1}$) within a few days, followed by a 100–200 day plateau at $L_{\rm bol}\sim10^{40}$–$10^{41}$ erg s$^{-1}$, with the plateau’s duration and luminosity depending on CBM distribution and viewing angle.The results broadly reproduce properties of bright extragalactic LRNe and offer a framework linking observed diversity to pre-merger mass loss and CBM geometry, while highlighting the need for further work in 3D and with more physically motivated CBM profiles.

Abstract

Luminous Red Novae (LRNe) are transients associated with mass ejection during stellar mergers and common envelope evolution (CEE). LRNe have the potential to illuminate the poorly understood phases of binary evolution leading up to the CEE, during the mass ejection phase, and in the immediate aftermath. However, the mechanism responsible for powering LRN light curves and the origin of their observed diversity remain open questions. Here, we perform two-dimensional moving-mesh radiation-hydrodynamic simulations of LRNe that take into account hydrogen and helium recombination and relevant opacities. We study a typical high-mass stellar merger, which dynamically ejects 2 $M_\odot$ with a characteristic velocity of 410 km/s. This ejecta collides with 2.7 $M_\odot$ of equatorially concentrated circumbinary material (CBM) left behind from a prior phase of non-conservative runaway mass transfer. We find that the resulting light curve is composed of a short, blue peak followed by a redder, predominantly shock-powered plateau with luminosities reaching up to $10^{41}$ erg/s and durations up to 200 days. These luminosities are significantly higher, and the durations much longer, than those produced by a simple spherical ejection of the same mass. They also depend in a complex way on the radial distribution of the CBM and the viewing angle. The shock is embedded in the ejecta and its observational signatures during the optically-thick phase are largely hidden. Our results are broadly compatible with observations of the brightest extragalactic LRNe and pave the way for the transformation of LRNe into powerful probes of binary evolution.

Figures (19)

• Figure 1: Structure and evolution of the simulation d0.3hiE ($R_\text{CBM}=0.3 \times 10^{15}$ cm, $T_\text{ej,in} = 5\times 10^5$ K) at four representative epochs. Each quadrant of each panel shows different quantity: density $\rho$ (upper left), gas temperature $T$ (upper right), tangential velocity $v_\theta$ (lower left), and ionization structure (lower right). Explanation of the color scale scale is given in the legend at the bottom. Tangential velocity is positive for motions toward the plane of symmetry. Ionization structure is based on the evaluation of Saha-like equations for each species with more detailed explanation shown in Figure \ref{['fig:eos']}. Hatched circles show regions where $a_\text{r}T^4/3$ exceeds gas pressure, but the dynamical effect of radiation depends on the local value of the flux-limiter (Eq. [\ref{['eq:app_mom']}]). White line shows the location of the $\tau=2/3$ surface measured along radial rays with solid and dashed lines showing results based on Rosseland and Planck mean opacities, respectively.
• Figure 2: Evolution of radiative flux for simulation d0.3hiE with $T_\text{ej,in}=5 \times 10^5$ K and $R_\text{CBM} = 2.0 \times 10^{15}$ cm. Background color shows the absolute value of the radiative flux while the arrows show its direction. Solid white line shows the position of the $\tau =2/3$ surface measured along radial rays using Rosseland-mean opacity.
• Figure 3: Bolometric light curves (upper row) and photospheric temperatures (lower row) of our simulations for the two different initial ejecta temperatures (left and right columns). In the upper row, colored solid lines are for different values of $R_\text{CBM}$ while solid black lines show simulations without CBM. Colored dotted lines indicate shock power for the appropriate ejecta and CBM combination calculated using semi-analytic model for the same ejecta and CBM properties metzger14Metzger2017pejcha22. In the bottom row, solid lines indicate gas temperatures at Rosseland photosphere and dashed line at Planck photosphere. Photospheric temperatures are shown only for times when the entire photosphere is located on the computational grid. Gray shaded areas indicate regions where the results are significantly affected by the choice of initial conditions.
• Figure 4: Raytracing of our simulation runs d0.1hiE (left column) and d2.0hiE (right column). Upper row shows evolution of luminosities calculated using raytracing for nine orientation angles $\theta$ (thin lines with colors transitioning from black to orange), angular average of raytracing (thick red line), and the angular average of radiative flux in the simulations (thick blue line, same as in Fig. \ref{['fig:lc']}). Lower part shows raytraced images for four selected orientation angles and at five different epochs indicated by thin gray vertical lines in the top row.
• Figure 5: Effect of irradiation from the central source. Left panel shows the luminosities (blue) and photospheric temperatures (red) for the fiducial (dashed, s0.1loE) and irradiated (solid, s0.1loEirr) models. Middle and right panels show the density and gas temperature structure of the irradiated model at one selected epoch indicated by vertical gray dotted line in the left panel.