A binary merger product as the direct progenitor of a Type II-P supernova

TL;DR

This study addresses whether Type II-P supernovae can originate from binary evolution by combining direct pre-explosion imaging of SN 2018gj with sophisticated binary-evolution modelling. The progenitor is a red supergiant with $M_{\rm He} \approx 4.6\,M_\odot$ and $M_{\rm H} \approx 3.5\,M_\odot$, yet it exists in an old environment, and the SN shows an unusually short plateau, both of which are inconsistent with a standard single-star history. Bayesian model comparison, together with posydon v2 binary population synthesis, favors a reverse-merger binary scenario in which two near-equal-mass stars coalesce after a complex mass-transfer history, producing the observed core-envelope configuration before explosion. This work provides robust observational support for the binary progenitor channel of SNe II-P and offers a practical framework for uncovering binary-origin II-P progenitors in current and future time-domain surveys.

Abstract

Type II-P supernovae (SNe II-P) are the most common class of core-collapse SNe in the local Universe and play critical roles in many aspects of astrophysics. Since decades ago theorists have predicted that SNe II-P may originate not only from single stars but also from interacting binaries. While ~20 SNII-P progenitors have been directly detected on pre-explosion images, observational evidence still remains scarce for this speculated binary progenitor channel. In this work, we report the discovery of a red supergiant progenitor for the Type II-P SN 2018gj. While the progenitor resembles those of other SNe II-P in terms of effective temperature and luminosity, it is located in a very old environment and SN 2018gj has an abnormally short plateau in the light curve. With state-of-the-art binary evolution simulations, we find these characteristics can only be explained if the progenitor of SN 2018gj is the merger product of a close binary system, which developed a different interior structure and evolved over a longer timescale compared with single-star evolution. This work provides the first compelling evidence for the long-sought binary progenitor channel toward SNe II-P, and our methodology serves as an innovative and pragmatic tool to motivate further investigations into this previously hidden population of SNe II-P from binaries.

Figures (7)

• Figure 1: Direct pre-explosion detection of SN 2018gj's progenitor and the photometric evolution of SN 2018gj. (a) SDSS u/g/r composite image of NGC 6217, where SN 2018gj is located in the far outskirt. (b) HST F555W/F625W/F814W composite image of the local SN environment, which is a very sparse region without any prominent features of recent star formation. (c--l) HST images taken before and after the explosion of SN 2018gj. The crosshairs indicate the SN position. (m) Light curves of SN 2018gj from the literature Teja2023 and late-time photometry from the HST observations. SN 2018gj has a very short plateau length of 70 days in contrast to 100 days for most SNe II-P. In 2021 and 2023, the F814W brightness becomes much fainter than the pre-explosion level, suggesting that the SN progenitor has disappeared after explosion.
• Figure 2: Progenitor properties derived from the direct detections.(a) SED of the SN progenitor and the best-fitting marcsmarcs.ref theoretical spectrum, consistent with being a luminous RSG. (b) Position of SN 2018gj's progenitor on the HRD in comparison with other confirmed SNe II-P progenitors and the bpassbpass.ref single-star evolutionary tracks for solar metallicity.
• Figure 3: Estimation of the stellar age for SN 2018gj's progenitor. (a) Map of stars detected on the HST/WFC3/F814W images in the environment of SN 2018gj with the symbol size reflecting their magnitudes. (b) The stellar age distribution in the SN local environment derived from the CMD fitting. (c,d) CMD of the resolved stellar populations inside the $R_{\rm od}$ region (gray) and inside the $R_{\rm env}$ region but outside the $R_{\rm od}$ region (orange) in comparison with the SN progenitor (red point). The error bars are the photometric uncertainties and the grey-shaded area corresponds to the detection limits. The background color scale shows the normalized probability density distribution for the environmental sources simulated with the bpass binary population models.
• Figure 4: Final He-core mass ($M_{\rm He}$), final H-envelope mass ($M_{\rm H}$) and age of SN 2018gj's progenitor. For comparison, the solid/dotted/dashed black lines show the relations between the parameters for single-star progenitors from binary_c/bpass/posydon and the color scales show the probability density distributions for binary progenitors as predicted by binary_cZapartas2017. The probability densities are on logarithmic scales and the color bars are in units of $M_\odot^{-2}$ (panel a) or $M_\odot^{-1}$ dex$^{-1}$ (panel b).
• Figure 5: Pre-supernova evolution simulated with posydon. (a) Pre-SN evolution of the progenitor system of SN 2018gj on the HRD. (b) Evolution of the total stellar mass and He-core mass of the primary star, secondary star, and merger product before explosion. Episodes of mass transfer are labeled in vertical, which are self-explanatory. The blue dotted region reflects the uncertainty of the total mass of the merger product with upper/lower limit corresponding to a conservative merging/partial CE ejection. Evolution continues up to core carbon depletion, expected to occur less than thousands of years before core collapse, with no significant further change in stellar mass or position on the HRD. (c) A schematic plot of the pre-SN evolution.