Luminous Type II supernovae for their low expansion velocities

Abstract

We present optical and near-infrared data of three Type II supernovae (SNe II), SN 2008bm, SN 2009aj, and SN 2009au. These SNe display the following common characteristics: signs of early interaction of the ejecta with circumstellar material (CSM), blue B − V colours, weakness of metal lines, low expansion velocities, and V-band absolute magnitudes 2–3 mag brighter than those expected for normal SNe II based on their expansion velocities. Two more SNe reported in the literature (SN 1983K and LSQ13fn) share properties similar to our sample. Analysing this set of five SNe II, which are luminous for their low expansion velocities (LLEV), we find that their properties can be reproduced assuming ejecta–CSM interaction that lasts between 4 and 11 weeks post-explosion. The contribution of this interaction to the radiation field seems to be the dominant component determining the observed weakness of metal lines in the spectra rather than the progenitor metallicity. Based on hydrodynamic simulations, we find that the interaction of the ejecta with a CSM of ∼3.6 M⊙ can reproduce the light curves and expansion velocities of SN 2009aj. Using data collected by the Chilean Automatic Supernova Search, we estimate an upper limit for the LLEV SNe II fraction to be 2–4 per cent of all normal SNe II. With the current data set, it is not clear whether the LLEV events are a separated class of SNe II with a different progenitor system, or whether they are the extreme of a continuum mediated by CSM interaction with the rest of the normal SN II population.

Progenitor properties of type II supernovae: fitting to hydrodynamical models using Markov chain Monte Carlo methods

“…Fig. 13 of Rodríguez et al 2020). Our velocity models should therefore be of the same order of magnitude as or display higher velocities than early observations.…”

Type II supernovae from the Carnegie Supernova Project-I. I. Bolometric light curves of 74 SNe II using uBgVriYJH photometry

“…SN 2009aj is found in the upper limit of the M bol,end distribution. This SN is more luminous than typical SNe II probably due to CSM interaction (Rodríguez et al 2020). At the radioactive tail phase, the sample ranges from −12.3 mag 5 However, the effect of uncorrected host-galaxy extinction on the shape of bolometric LCs will not affect the overall analysis.…”

“…12, the temperature curves are colour-coded with their s 2 values. Six objects present temperatures lower than 5000 K and only one, SN 2005lw, is a fast decliner with a s 2 value of 1.53 ± 0.08 mag per 100 days, while the others (SNe 2004dy, 2007sq, 2008bh, 2008bp, and 2009ao) and more luminous than 'normal' SNe II (Rodríguez et al 2020).…”

“…The weighted mean and standard deviation of redshift-independent distances taken from NED 1 were employed for SN 2005kh and SN 2009A, where only Tully-Fisher estimates are available. In the case of SN 2009aj, we used the distance from Rodríguez et al (2020). In Anderson et al (2014b) distances were estimated using cosmic-microwave background corrected recession velocities if this value is higher than 2000 km s −1 , together with an H 0 = 73 km s −1 Mpc −1 , Ω m = 0.27, and Ω Λ = 0.73.…”

“…Explosion epochs were taken from Gutiérrez et al (2017b) with one exception. In the case of SN 2008bm, the new estimate by Rodríguez et al (2020) was used, who include pre-explosion non-detections closer to the SN discovery. It should be noted that SN 2005gk, SN 2005hd, and SN 2005kh do not allow for reliable explosion epoch estimates because of the lack of prediscovery non-detections and/or spectra.…”