Synthetic spectral time series of our merger model (dark red line) and the observed spectral evolution of iPTF14atg (black line, epochs denote the time since explosion). The light red area indicates the spread due to different viewing angles from the asymmetric merger ejecta.

iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN 2002es, is the first SN Ia for which a strong UV flash was observed in the early-time light curves. This has been interpreted as evidence for a single-degenerate (SD) progenitor system where such a signal is expected from interactions between the SN ejecta and the non-degenerate companion star. In this paper, we compare synthetic observables of multi-dimensional state-of-the-art explosion models for different progenitor scenarios to the light curves and spectra of iPTF14atg. From our models, we have difficulties explaining the spectral evolution of iPTF14atg within the SD progenitor channel. In contrast, we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and 0.76 solar masses, respectively, provides an excellent match to the spectral evolution of iPTF14atg from 10d before to several weeks after maximum light. Our merger model does not naturally explain the initial UV flash of iPTF14atg. We discuss several possibilities like interactions of the SN ejecta with the circus-stellar medium and surface radioactivity from a He ignited merger that may be able to account for the early UV emission in violent merger models.

Paper published in Monthly Notices of the Royal Astronomical Society, full text available at http://arxiv.org/abs/1604.05730