Explosion sites of SN 1994W-like transients

¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
² Turku Collegium for Science, Medicine and Technology, University of Turku, 20014 Turku, Finland
³ Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Finland
⁴ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
⁵ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁶ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati, 33, 00078 Monte Porzio Catone (RM), Italy
⁹ SSDC-ASI, Via del Politecnico, snc, 00133 Roma, Italy

^⋆ Corresponding author: erkki.kankare@utu.fi

Received: 27 March 2025
Accepted: 22 May 2025

Abstract

We study a sample of narrow-line transients that share characteristics with the Type IIn classified supernova (SN) 1994W, a prototypical member of this class of events, via an investigation of their explosion sites and spectrophotometric data. The normalised cumulative rank (NCR) method was used to compare the explosion sites of ten events to the star formation distributions of their host galaxies, and to the sites of different evolved massive stars. The resulting sample mean value of NCR_Hα = 0.170±0.076 is low, while the NCR_NUV distribution is flat with a mean value of 0.488±0.084. The NCR distribution of SN 1994W-like events is consistent with relatively low-mass red supergiants (RSGs) and, despite the small sample size, inconsistent with high-mass stars such as luminous blue variables. To explain the nature of SN 1994W-like transients, an interaction between an expanding ejecta and a relatively massive circumstellar medium is likely required, with the latter possibly having been produced by a H envelope ejection via a nuclear flash event, or a luminous red nova (LRN) from a stellar merger; both channels are consistent with the low-mass RSGs suggested by the NCR results. In this context, we find the early −26 d spectrum from light curve maximum of SN 2003G to have similarities to those of F8-type supergiant stars and LRNe. Finally, based on late-time HST imaging, we set the deepest limits for the surviving precursor of SN 2011ht to M_F438W>−3.8 and M_F555W>−4.0 mag. This would exclude most supergiants as a non-terminal progenitor, assuming that such a star is not completely obscured by newly formed dust.