Abstract
Abstract As a candidate ‘super-Chandrasekhar’ or 09dc-like Type Ia supernova (SN Ia), SN 2012dn shares many characteristics with other members of this remarkable class of objects but lacks their extraordinary luminosity. Here, we present and discuss the most comprehensive optical data set of this SN to date, comprised of a densely sampled series of early-time spectra obtained within the Nearby Supernova Factory project, plus photometry and spectroscopy obtained at the VLT about 1 yr after the explosion. The light curves, colour curves, spectral time series and ejecta velocities of SN 2012dn are compared with those of other 09dc-like and normal SNe Ia, the overall variety within the class of 09dc-like SNe Ia is discussed, and new criteria for 09dc-likeness are proposed. Particular attention is directed to additional insight that the late-phase data provide. The nebular spectra show forbidden lines of oxygen and calcium, elements that are usually not seen in late-time spectra of SNe Ia, while the ionisation state of the emitting iron plasma is low, pointing to low ejecta temperatures and high densities. The optical light curves are characterised by an enhanced fading starting ∼60 d after maximum and very low luminosities in the nebular phase, which is most readily explained by unusually early formation of clumpy dust in the ejecta. Taken together, these effects suggest a strongly perturbed ejecta density profile, which might lend support to the idea that 09dc-like characteristics arise from a brief episode of interaction with a hydrogen-deficient envelope during the first hours or days after the explosion.
Highlights
When the first ‘super-Chandrasekhar’ Type Ia supernova (SN Ia) was identified (SN 2003fg; Howell et al 2006; Branch 2006), the outstanding characteristics were a luminosity twice as high as in ordinary SNe Ia, and comparatively low ejecta velocities
We present and discuss the most comprehensive optical data set of this SN to date, comprised of a densely sampled series of early-time spectra obtained within the Nearby Supernova Factory project, plus photometry and spectroscopy obtained at the Very Large Telescope about 1 yr after the explosion
It was readily realised that, within the paradigm of radioactivity-driven light curves, SN 2003fg could not be consistent with a total ejecta mass limited to 1.4 M, the Chandrasekhar-mass (MCh) stability limit of non-rotating white dwarfs (WDs), since the required 56Ni mass alone would be close to that number
Summary
When the first ‘super-Chandrasekhar’ Type Ia supernova (SN Ia) was identified (SN 2003fg; Howell et al 2006; Branch 2006), the outstanding characteristics were a luminosity twice as high as in ordinary SNe Ia, and comparatively low ejecta velocities. Scalzo et al (2012, 2014) studied a group of objects with strong spectroscopic similarities to SN 2007if at early phases (e.g., strong Fe III absorption lines), but much less extreme photometric properties, and suggested them to result from the tamped-detonation scenario (i.e., the detonation of a WD inside an extended envelope; Khokhlov et al 1993; Hoflich & Khokhlov 1996). Most of these objects are classical 91T-like SNe. We will show in Section 5.4 that 91T-like SNe differ in several respects from 09dc-like events as defined here.
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