The continuing story of SN IIb 2013df: new optical and IR observations and analysis

SN 2013df is a nearby Type IIb supernova that seems to be the spectroscopic twin of the well-known SN 1993J. Previous studies revealed many, but not all interesting properties of this event. Our goal was to add new understanding of both the early- and late-time phases of SN 2013df. Our spectral analysis is based on six optical spectra obtained with the 9.2 m Hobby-Eberly Telescope during the first month after explosion, complemented by a near-infrared spectrum. We applied the SYNAPPS spectral synthesis code to constrain the chemical composition and physical properties of the ejecta. A principal result is the identification of `high-velocity' He I lines in the early spectra of SN 2013df, manifest as the blue component of the double-troughed profile at ∼5650 Å. This finding, together with the lack of clear separation of H and He lines in velocity space, indicates that both H and He features form at the outer envelope during the early phases. We also obtained ground-based BVRI and g'r 'I'z' photometric data up to +45 d and unfiltered measurements with the ROTSE-IIIb telescope up to +168 d. From the modelling of the early-time quasi-bolometric light curve, we find M_ej ∼ 3.2-4.6 M_⊙ and E_kin ∼ 2.6-2.8 × 10⁵¹ erg for the initial ejecta mass and the initial kinetic energy, respectively, which agree well with the values derived from the separate modelling of the light-curve tail. Late-time mid-infrared excess indicates circumstellar interaction starting ∼1 yr after explosion, in accordance with previously published optical, X-ray, and radio data.