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SN 2013df is a nearby Type IIb supernova that seems to be the spectroscopictwin of the well-known SN 1993J. Previous studies revealed many, but not allinteresting properties of this event. Our goal was to add new understanding ofboth the early and late-time phases of SN 2013df. Our spectral analysis isbased on 6 optical spectra obtained with the 9.2m Hobby-Eberly Telescope duringthe first month after explosion, complemented by a near-infrared spectrum. Weapplied the SYNAPPS spectral synthesis code to constrain the chemicalcomposition and physical properties of the ejecta. A principal result is theidentification 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 A. Thisfinding, together with the lack of clear separation of H and He lines invelocity space, indicates that both H and He features form at the outerenvelope during the early phases. We also obtained ground-based BVRI andg'r'i'z' photometric data up to +45 days and unfiltered measurements with theROTSE-IIIb telescope up to +168 days. From the modelling of the early-timequasi-bolometric light curve, we find $M_{ej} \sim 3.2-4.6 M_{\odot}$ and$E_{kin} \sim 2.6-2.8 \times 10^{51}$ erg for the initial ejecta mass and theinitial kinetic energy, respectively, which agree well with the values derivedfrom the separate modelling of the light-curve tail. Late-time mid-infraredexcess indicates circumstellar interaction starting ~1 year after explosion, inaccordance with previously published optical, X-ray, and radio data.

The Continuing Story of SN IIb 2013df: New Optical and IR Observations and Analysis