Peak `nebular' emission and early flux excesses in Ca-strong supernovae

October 2026 • 2026A&A...714A.262T

Authors • Touchard-Paxton, C.-G. • Maguire, K. • Frohmaier, C. • Jacobson-Galán, W. V. • Pursiainen, M. • Angus, C. • Sollerman, J. • Polin, A. • Terwel, J. H. • Seth, R. • O'Donnell, C. • Smartt, S. J. • Srivastav, S. • Bochenek, A. • de Boer, T. • Das, K. K. • Fremling, C. • Gordon, A. C. • Graham, M. J. • Huber, M. E. • Lin, C.-C. • Liu, C. • Masci, F. J. • Minguez, P. • Paek, G. S. H. • Purdum, J. • Rusholme, B. • Smith, K. • Smith, R. • Wainscoat, R.

Abstract • Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transients characterised by rapid photometric evolution (rise times of <=16 d) and nebular-phase spectra dominated by strong [Ca II] λλ7291,7324 emission, compared to the strength of [O I] λλ6300,6364 emission. Despite their unique spectroscopic signatures and association with remote environments at large physical offsets from their host galaxy, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compiled a sample of 46 Ca-strong SNe, including 8 newly classified objects -- the largest sample presented to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. This includes Gaussian decomposition of the [Ca II] and [O I] nebular emission lines to measure bulk velocities and line profile evolution from peak brightness to deep nebular-phase epochs. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

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• POSTSCRIPT https://www.aanda.org/10.1051/0004-6361/202660845/postscript