SN 2025coe: A Multiple-Peaked Calcium-Strong Transient from A White-Dwarf Progenitor

Abstract

SN 2025coe is a calcium-strong transient located at an extremely large projected offset $\sim$39.3 kpc from the center of its host, the nearby early-type galaxy NGC 3277 at a distance of $\sim$25.5 Mpc. In this paper, we present multi-band photometric and spectroscopic observations spanning $\sim$100 days post-discovery. Its multi-band light curves display {multiple} distinct peaks: (1) an initial peak at $t\approx 1.6$ days attributed to shock cooling emission, (2) a secondary peak of $M_{R,peak}\approx$ $-$15.8 mag at $t\approx 10.2$ days powered by radioactive decay, and (3) a {possible} late-time bump at $t\approx 42.8$ days likely caused by ejecta-circumstellar material/clump interaction. Spectral evolution of SN 2025coe reveals a fast transition to the nebular phase within 2 months, where it exhibits an exceptionally high [Ca II]/[O I] ratio larger than 6. Modeling of the bolometric light curve suggests an ejecta mass of $M_{\mathrm{ej}}=0.29_{-0.15}^{+0.14}{M}_{\odot }$, a ${}^{56}$Ni mass of $M_{{}^{56}\mathrm{Ni}}=2.4_{-0.05}^{+0.06}\times 10^{-2}{M}_{\odot }$, and a progenitor envelope with mass $M_e=1.4_{-1.2}^{+6.9}\times 10^{-3}{M}_{\odot }$ and radius $R_e=13.5_{-11.1}^{+64.1}{R}_{\odot }$. The tidal disruption of a hybrid HeCO white dwarf (WD) by a low-mass CO WD provides a natural explanation for the low ejecta mass, the small fraction of ${}^{56}$Ni, and the presence of an extended, low-mass envelope.

12 pages, 9 figures, accepted for publication in ApJ