Massive stars exploding in a He-rich circumstellar medium – V. Observations of the slow-evolving SN Ibn OGLE-2012-SN-006

Abstract

We present optical observations of the peculiar Type Ibn supernova (SN Ibn) OGLE-2012-SN-006, discovered and monitored by the OGLE − IV survey, and spectroscopically followed by P ESST O at late phases. Stringent pre-discovery limits constrain the explosion epoch with fair precision to JD = 2456203.8 ± 4.0. The rise time to the I-band light curve maximum is about two weeks. The object reaches the peak absolute magnitude M I = −19.65 ± 0.19 on JD = 2456218.1 ± 1.8. After maximum, the light curve declines for about 25 days with a rate of 4 mag 100d −1 . The symmetric I-band peak resembles that of canonical Type Ib/c supernovae (SNe), whereas SNe Ibn usually exhibit asymmetric and narrower early-time light curves. Since 25 days past maximum, the light curve flattens with a decline rate slower than that of the 56 Co to 56 Fe decay, although at very late phases it steepens to approach that rate. However, other observables suggest that the match with the 56 Co decay rate is a mere coincidence, and the radioactive decay is not the main mechanism powering the light curve of OGLE-2012-SN-006. An early-time spectrum is dominated by a blue continuum, with only a marginal evidence for the presence of He I lines marking this SN Type. This spectrum shows broad absorptions bluewards than 5000Å, likely O II lines, which are similar to spectral features observed in super-luminous SNe at early epochs. The object has been spectroscopically monitored by P ESST O from 90 to 180 days after peak, and these spectra show the typical features observed in a number of SN 2006jc-like events, including a blue spectral energy distribution and prominent and narrow (v F W HM ≈ 1900 km s −1 ) He I emission lines. This suggests that the ejecta are interacting with He-rich circumstellar material. The detection of broad (10 4 km s −1 ) O I and Ca II features likely produced in the SN ejecta (including the [O I] λλ6300,6364 doublet in the latest spectra) lends support to the interpretation of OGLE-2012-SN-006 as a core-collapse event.

Spectra of Hydrogen-poor Superluminous Supernovae from the Palomar Transient Factory

“…The O II ion has not been detected in the spectra of any normal-luminosity SNe Ic. Aside from SLSNe-I, only the peculiar SN Ib SN 2008D (Soderberg et al 2008;Mazzali et al 2008;Modjaz et al 2009) and the SN Ibn OGLE-2012-SN-006 (Pastorello et al 2015) have shown spectroscopic evidence for this ion. For O II to be present, oxygen must be excited to a relatively high energy level .…”

“…1), and (2) owing to ionization and possibly composition differences, there are likely features in the SLSN-I spectra that are not present in normal-luminosity SNe, and these may blend with or totally dominate the normal features. The latter is certainly true for the O II ion, which dominates the optical spectra of most young SLSNe-I (Quimby et al 2011) but which is not typically seen in lower-luminosity events; two notable exceptions, SN 2008D (Soderberg et al 2008;Mazzali et al 2008;Modjaz et al 2009) and OGLE-2012-SN-006 (Pastorello et al 2015), are discussed below. These features offer the only means to extract velocity information from the spectra in some cases -but, unfortunately, these features are the product of many blended lines , and a simple method for extracting velocity information from them has yet to be developed.…”

LSQ13ddu: a rapidly evolving stripped-envelope supernova with early circumstellar interaction signatures

“…This rise is then usually followed by a decline over the course of ∼1-2 months. A small subset of SNe Ibn have been observed to have much slower postmaximum decline rates, with OGLE-2012-SN-006 being notable for entering an extended plateau-like phase (Pastorello et al 2015b). Additionally one SN Ibn (OGLE-2014-SN-131) was observed to have an extended rise to peak (Karamehmetoglu et al 2017), though such events form a small proportion of the overall class.…”

OGLE-2014-SN-131: A long-rising Type Ibn supernova from a massive progenitor

“…Other than the above criteria used to classify them as Type Ibn, the SNe of this class display a variety of observational characteristics. They are most often fast evolving with a short rise to peak brightness ( 2 weeks) and a subsequent fast decline, but a slowly evolving example, with a post-peak plateau phase, has also been found (OGLE-2012-SN-006;Pastorello et al 2015d). Their spectra can show some amount of hydrogen in emission (see e.g., Foley et al 2007;Pastorello et al 2007) or He features in absorption as broad as a normal Type Ib (Pastorello et al 2015c), which has led to the suggestion that SNe Ibn form part of a continuum from SNe Ib to SNe IIn (Pastorello et al 2015a).…”