The Double-Peaked Calcium-Strong SN 2025coe: Progenitor Constraints from Early Interaction and Ejecta Asymmetries

Abstract

Supernova (SN) 2025coe at a distance of $\sim$25 Mpc is the second-closest calcium-strong (CaST) transient. It was discovered at a large projected offset of $\sim$34 kpc from its potential host galaxy NGC 3277. Multiband photometry of SN 2025coe indicates the presence of two peaks at day $\sim$2 and day $\sim$11 after explosion. Modeling the bolometric light curve, we find that the first peak can be reproduced either by shock cooling of a compact envelope ($R_{\mathrm{env}}$ $\approx$6-40 $R_{\odot }$; $M_{\mathrm{env}}$ $\approx$0.1-0.2 $M_{\odot }$) or by interaction with close-in circumstellar material (CSM; $R_{\mathrm{CSM}}\lesssim 6\times 10^{14}$ cm), or a combination of both. The second peak is dominated by radioactive decay of ${}^{56}$Ni ($M_{\mathrm{ej}}\approx$0.4-0.5 $M_{\odot }$; $M_{{}^{56}\mathrm{Ni}}\approx 1.4\times 10^{-2}$ $M_{\odot }$). SN 2025coe rapidly evolves from the photospheric phase dominated by He I P-Cygni profiles to nebular phase spectra dominated by strong [Ca II] $\hat{I}»\hat{I}»$7291, 7323 and weak [O I] $\hat{I}»\hat{I}»$6300, 6364 emission lines. Simultaneous line profile modeling of [Ca II] and [O I] at nebular phases shows that an asymmetric core-collapse explosion of a low-mass ($\lesssim$3.3 $M_{\odot }$) He-core progenitor can explain the observed line profiles. Alternatively, lack of local star formation at the site of the SN explosion combined with a low ejecta mass is also consistent with a thermonuclear explosion due to a low-mass hybrid He-C/O white dwarf + C/O white dwarf merger.

28 pages, 17 figures, 3 tables; Accepted for publication in ApJ