Exclusion of a direct progenitor detection for the Type Ic SN 2017ein based on late-time observations

TL;DR

This work tackles the progenitor question for SN 2017ein, a unique Type Ic supernova with a candidate progenitor in pre-explosion data. Using late-time HST imaging at $2.4$–$3.8$ years after peak brightness and differential astrometry, the authors test whether the pre-explosion source disappeared as a true progenitor. They find Source A persists with near-identical brightness and color, indicating it is not the genuine progenitor nor a binary companion, and the F336W limit disfavors a star cluster, consistent with a chance-alignment scenario (probability ≈ 4.8%). Incorporating a low ejecta mass of $\\sim 1\,M_\\odot$, they argue the progenitor was likely a moderately massive star with $M_{ m ini} \\sim 8$–$20\,M_\\odot$ stripped by binary interaction, rather than a very massive Wolf-Rayet star. This reframes Type Ic progenitor channels toward binary evolution and demonstrates the power of late-time high-resolution imaging to confirm or refute progenitor identifications.

Abstract

To date, SN 2017ein is the only Type Ic supernova with a directly identified progenitor candidate. This candidate points to a very massive ($>$45 $M_\odot$) Wolf-Rayet progenitor, but its disappearance after the explosion of SN 2017ein remains unconfirmed. In this work, we revisit SN 2017ein in late-time images acquired by the Hubble Space Telescope (HST) at 2.4--3.8 yrs after peak brightness. We find this source has not disappeared and its brightness and color remain almost the same as in the pre-explosion images. Thus, we conclude that the pre-explosion source is not the genuine progenitor of SN 2017ein. We exclude the possibility that it is a companion star of the progenitor, since it has a much lower extinction than SN 2017ein; its color is also inconsistent with a star cluster, indicated by the newly added magnitude limit in F336W, apart from F555W and F814W. We suggest, therefore, this source is an unrelated star in chance alignment with SN 2017ein. Based on the low ejecta mass, we propose that SN 2017ein is most likely originated from a moderately massive star with $M_{\rm ini}$ $\sim$ 8--20 $M_\odot$, stripped by binary interaction, rather than a very massive Wolf-Rayet progenitor.

Figures (5)

• Figure 1: Pre- and post-explosion HST images of the site of SN 2017ein (F336W is not included here). The SN position is denoted by the white crosshair. Each image has a size of $\sim$ 160$\times$160 pc and is oriented with North up and East to the left.
• Figure 2: Brightness and color evolution of Source A at different epochs. The pre-explosion magnitudes measured by kilpatrick2018potential, Xiang2019ob and van2018sn are also plotted for comparison. The dashed lines represent the inverse-variance weighted averages of the pre-explosion and the last epoch, and the green-shaded bands indicate the errors. All the magnitudes have been corrected for systematic bias between the different epochs.
• Figure 3: F336W$-$F555W v.s. F555W$-$F814W color-color diagrams for Source A (red circle) and model stars (bottom panel, solid lines) and star clusters (top panel, solid lines). The arrows in the upper-left corner show the reddening vector under a standard extinction law with $R_V$ = 3.1.
• Figure A1: Comparison of photometry as reported by dolphot for observations at different epochs. The black dashed line shows the one-to-one relation while the orange dash-dotted line corresponds to systematic shifts as calculated by inverse-variance weighted averages.
• Figure A2: Histograms of magnitude differences with respect to the epoch of t = 3.8 yrs. The solid lines are Gaussian fit curves and the dashed lines represent the means of the Gaussian fit. Normalization has been applied to each graph.