The helium common-envelope wind scenario for SN 2020eyj

TL;DR

SN 2020eyj presents a large helium-rich CSM around a SN Ia, challenging standard SD and CD channels. The authors develop a helium common-envelope wind (HeCEW) framework by simulating WD+He-star binaries, showing that a helium CE with $M_{\rm CE}>0.3\,M_{\odot}$ can form and be ejected prior to explosion, with $M_{\rm CE}$ reaching up to $0.78\,M_{\odot}$ for favorable initial conditions. This HeCEW channel can produce a delayed interaction consistent with observations, explain the dim peak brightness and low ejecta velocity, and predict a rare SN Ia-like population of order $0.01\% - 0.1\%$ of SNe Ia. The paper discusses Merger-to-Explosion Delay (MED) and how a dynamical CE ejection could create the required gap between the progenitor and the CSM, contrasting with OTW-based SD scenarios, and outlines implications for future population studies and observations.

Abstract

SN 2020eyj is the first type Ia supernova (SN Ia) showing the signature of a compact helium-rich circumstellar material (CSM). Such a large CSM is difficult to explain in a single-degenerate scenario where the donor star is a helium star. Here we show that, under certain conditions, it is possible that the transfer of helium leads to a common envelope (CE) engulfing the system, similar to the common-envelope wind model proposed by Meng \& Podsiadlowski (2017). If in such a helium common-envelope wind (HeCEW) model the initial white dwarf (WD) mass is larger than 1.1 $M_{\rm \odot}$ and the helium star more massive than 1.8 $M_{\rm \odot}$, the mass of a helium CE can be larger than 0.3 $M_{\rm \odot}$ prior to supernova explosion. The CE mass heavily depends on the initial parameters of the binary system. A dynamical CE ejection event could occur shortly before the supernova, and then our model may naturally explain the properties of SN 2020eyj, specifically the massive He-rich CSM, its dim peak brightness, low ejecta velocity and low birth rate.

Figures (4)

• Figure 2: Illustrative binary evolution calculation in the HeCEW model. The evolution of various parameters is shown, including the WD mass, $M_{\rm WD}$, the secondary mass, $M_{\rm 2}$, the mass-transfer rate, $\dot{M}_{\rm 2}$, the mass-growth rate of the WD, $\dot{M}_{\rm WD}$, the mass of the CE, $M_{\rm CE}$, the mass-loss rate from the system, $\dot{M}_{\rm loss}$, the frictional luminosity, $L_{\rm f}$, and the merger timescale for the binary system, $-I/\dot{I}$, as labeled in each panel. The evolutionary track of the donor star and the evolution of the orbital period are shown as solid and dashed curves in panel (1), respectively. Dotted vertical lines in all panels and asterisks in panel (1) indicate the position where the WD is expected to explode as a SN Ia. The initial and the final binary parameters are given in panel (1).
• Figure 3: The CE mass and the companion mass when $M_{\rm WD}=1.378$$M_{\rm \odot}$ for different initial WD masses. The vertical dotted line shows the lower limit for the amount of CSM around the progenitor of SN 2020eyj.
• Figure 4: The CE mass as a function of the initial companion mass when $M_{\rm WD}=1.378$$M_{\rm \odot}$ for $M^{\rm i}_{\rm WD}=1.3$$M_{\rm \odot}$. The horizontal dotted line shows the lower limit for the amount of CSM required around the progenitor of SN 2020eyj.
• Figure 5: The wind mass-loss rate and the companion mass when $M_{\rm WD}=1.378$$M_{\rm \odot}$ for different initial WD masses. The vertical dotted line shows the lower limit of the wind mass-loss rate for the progenitor of SN 2020eyj if the OTW model were adopted.