SNhunt151: an explosive event inside a dense cocoon
Drake, A. J.; Djorgovski, S. G.; Geier, S.; Filippenko, A. V.; Howell, D. A.; Kankare, E.; Ochner, P.; Turatto, M.; Cappellaro, E.; Benetti, S.; Ciabattari, F.; Pignata, G.; Valenti, S.; Tartaglia, L.; Elias-Rosa, N.; Tomasella, L.; Terreran, G.; Isern, J.; Pastorello, A.; Kangas, T.; Morales-Garoffolo, A.; Howerton, S. C.; Leonini, S.
Italy, Spain, United States, United Kingdom, Finland, Chile
Abstract
SNhunt151 was initially classified as a supernova (SN) impostor (nonterminal outburst of a massive star). It exhibited a slow increase in luminosity, lasting about 450 d, followed by a major brightening that reaches MV ≈ -18 mag. No source is detected to MV ≳ -13 mag in archival images at the position of SNhunt151 before the slow rise. Low-to-mid-resolution optical spectra obtained during the pronounced brightening show very little evolution, being dominated at all times by multicomponent Balmer emission lines, a signature of interaction between the material ejected in the new outburst and the pre-existing circumstellar medium. We also analysed mid-infrared images from the Spitzer Space Telescope, detecting a source at the transient position in 2014 and 2015. Overall, SNhunt151 is spectroscopically a Type IIn SN, somewhat similar to SN 2009ip. However, there are also some differences, such as a slow pre-discovery rise, a relatively broad light-curve peak showing a longer rise time (∼50 d), and a slower decline, along with a negligible change in the temperature around the peak (T ≤ 104 K). We suggest that SNhunt151 is the result of an outburst, or an SN explosion, within a dense circumstellar nebula, similar to those embedding some luminous blue variables like η Carinae and originating from past mass-loss events.