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

TL;DR

SN 2025coe is a nearby CaST displaying a rare double-peaked light curve and rapid spectral evolution. The study combines high-cadence optical/UV photometry, extensive spectroscopy, bolometric light-curve modeling, and nebular-line synthesis to constrain progenitor scenarios. The results favor an asymmetric, low-mass core-collapse explosion of a He-core star with a compact envelope or close-in CSM, though thermonuclear WD merger channels can explain parts of the observations; no single channel fully accounts for all features. The work highlights the pivotal role of early-time interaction and late-time nebular diagnostics in disentangling CaST progenitor diversity and demonstrates the value of coordinated multiwavelength follow-up.

Abstract

Supernova (SN) 2025coe at a distance of $\sim$25 Mpc is the third-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; $R_{\mathrm{CSM}} \lesssim 8 \times10^{14}$ 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] $λλ$7291, 7323 and weak [O I] $λλ$6300, 6364 emission lines. Simultaneous line profile modeling of [Ca II] and [O I] at nebular phases shows that an asymmetric core-collapse explosion of a low-mass ($\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.

Figures (17)

• Figure 1: Three-color RGB image of the field near SN 2025coe using Las Cumbres Observatory $g$, $r$, and $i$ filter images taken on 2025-03-21. The field of view shows the early-type spiral host galaxy NGC 3277 and SN 2025coe at a significant projected offset of $\sim$34 kpc ($\sim 5'$ from the host center). The image orientation and scale are marked.
• Figure 2: Left: Multiband extinction-corrected photometry of SN 2025coe from Las Cumbres Observatory, ZTF, ATLAS, TNOT, TNT, and Swift with respect to the epoch of explosion ($t_{0}$) and second peak ($t_{o, \mathrm{max}}$). Right: A zoom-in view of the optical light curves around the first peak (marked by a dashed line). The latest available nondetection from ATLAS is marked by an orange downward arrow and the estimated explosion epoch is marked by an orange dotted line.
• Figure 3: Extinction-corrected $r/R$ photometry comparison between SN 2025coe and all other identified CaSTs with a double peak in their optical light curves. The first and second peaks in SN 2025coe are marked in the zoomed-in left panel of the plot. All double-peaked CaSTs decline faster than the fast SN Ic 1994I, and also SNe Ib 2007Y and 2008D. All double-peaked CaSTs fade faster than the expected luminosity decline from $^{56}$Co decay (dashed gray line), indicating incomplete trapping of $\gamma$-ray photons. References for data: SN 2021inl -- Jacobson-Galan22; SN 2021gno -- Jacobson-Galan22; SN 2019ehk -- Jacobson-Galan20b; SN 2018lqo -- De20; iPTF16hgs -- De18; SN 2008D -- Modjaz09; SN 2007Y -- Stritzinger09; SN 1994I --Richmond96.
• Figure 4: Extinction-corrected $g-r$ color comparison between double-peaked CaSTs: SN 2025coe, SN 2021inl, and SN 2021gno Jacobson-Galan22; SN 2019ehk Jacobson-Galan20b, SN 2018lqo De20, iPTF16hgs De18, and other CaSTs with $gr$ photometry near peak luminosity. Data for other CaSTs are adapted from the literature Sullivan11Kasliwal12Valenti14Lunnan17De20. The other CaSTs are marked by their membership in spectroscopic subclasses of Ca-Ib/c Green, Ca-Ib/c Red, and Ca-Ia as described by De20. The $g-r$ colors of SN 2025coe and several other double-peaked CaSTs around peak match better with those of the Ca-Ib/c Green subclass. The Carnegie Supernova Project (CSP) $g-r$ color template presented by Stritzinger18 for SNe Ib is shown for comparison. Shaded region corresponds to the uncertainty in the color template values.
• Figure 5: Top: Bolometric luminosity from blackbody fits to the observed SED for SN 2025coe, SN 2021gno, and SN 2019ehk. Optical and UV photometry for SN 2021gno and SN 2019ehk from Jacobson-Galan22 and Jacobson-Galan20b, respectively. Dashed and dotted lines respectively represent the first and second peaks (estimated from optical photometry) of SN 2025coe. Bottom: Blackbody temperature and radius associated with the SED fit at each epoch of SN 2025coe. As the earliest epoch lacks UV observations, the estimated radius and temperatures are upper and lower limits, respectively.