Supernova spectra below strong circumstellar interaction

Abstract

We construct spectra of supernovae (SNe) interacting strongly with a circumstellar medium (CSM) by adding SN templates, a blackbody continuum, and an emission-line spectrum. In a Monte Carlo simulation we vary a large number of parameters, such as the SN type, brightness and phase, the strength of the CSM interaction, the extinction, and the signal to noise ratio (S/N) of the observed spectrum. We generate more than 800 spectra, distribute them to ten different human classifiers, and study how the different simulation parameters affect the appearance of the spectra and their classification. The SNe IIn showing some structure over the continuum were characterized as "SNe IInS" to allow for a better quantification. We demonstrate that the flux ratio of the underlying SN to the continuum f V is the single most important parameter determining whether a spectrum can be classified correctly. Other parameters, such as extinction, S/N, and the width and strength of the emission lines, do not play a significant role. Thermonuclear SNe get progressively classified as Ia-CSM, IInS, and IIn as f V decreases. The transition between Ia-CSM and IInS occurs at f V ∼ 0.2−0.3. It is therefore possible to determine that SNe Ia-CSM are found at the (un-extincted) magnitude range −19.5 > M > −21.6, in very good agreement with observations, and that the faintest SN IIn that can hide a SN Ia has M = −20.1. The literature sample of SNe Ia-CSM shows an association with 91T-like SNe Ia. Our experiment does not support that this association can be attributed to a luminosity bias (91T-like being brighter than normal events). We therefore conclude that this association has real physical origins and we propose that 91T-like explosions result from single degenerate progenitors that are responsible for the CSM. Despite the spectroscopic similarities between SNe Ibc and SNe Ia, the number of misclassifications between these types was very small in our simulation and mostly at low S/N. Combined with the SN luminosity function needed to reproduce the observed SN Ia-CSM luminosities, it is unlikely that SNe Ibc constitute an important contaminant within this sample. We show how Type II spectra transition to IIn and how the Hα profiles vary with f V . SNe IIn fainter than M = −17.2 are unable to mask SNe IIP brighter than M = −15. A more advanced simulation, including radiative transfer, shows that our simplified model is a good first order approximation. The spectra obtained are in good agreement with real data.

Type IIn supernova light-curve properties measured from an untargeted survey sample

“…These SNe Ia-CSM have been proposed to happen when white dwarfs explode inside the CSM of its nondegenerate binary companion (see Silverman et al 2013, and references therein). In simulations by Leloudas et al (2015) it was shown that SNe Ia-CSM can be mistaken for genuine SNe IIn under some circumstances. Leloudas et al (2015) shows that a SN Ia (of typical peak magnitude M ≈ −19) can be hidden under an apparent SN IIn spectrum if the CSI is strong and the peak magnitude of the SN IIn is M ≈ −20.9.…”

Physical conditions and element abundances in supernova and γ-ray burst host galaxies at different redshifts

“…SLSNII are H rich. Leloudas et al (2015) refer to Extreme Emission Line Galaxies (EELG) observations. For spectroscopy they used FORS2 and X shooter on the VLT, OSIRIS on the GTC and IMACS on Magellan.…”

“…Narrow lines originate from the host galaxy, downstream of the shock produced by collision of the fast moving ejecta with the CSM (see Leloudas et al 2015 and references therein). We report in Table 2 the data presented by Leloudas et al (2015) in their table B1. For each galaxy the observed line ratios are followed in the two next rows by the best fitting results calculated by shock dominated (SD) models (ml1-ml26) and radiation dominated (RD) models (mll1-mll26) which account for the photoionization flux from the SB + shocks, respectively.…”

Metallicity at the explosion sites of interacting transients

“…These events are interpreted as thermonuclear SNe interacting with H-rich CSM (Hamuy et al 2003;Aldering et al 2006;Dilday et al 2012;Taddia et al 2012;Silverman et al 2013;Fox et al 2015), although a CC origin has also been proposed (Benetti et al 2006;Inserra et al 2014). Their spectra are well represented by the sum of narrow Balmer emission lines and SN Ia spectra diluted by a blue continuum (Leloudas et al 2015b).…”