Photometric and Spectroscopic Properties of Type Ia Supernova 2018oh with Early Excess Emission from the Kepler 2 Observations

Li, W. and Wang, X. and Vinkó, József and Mo, J. and Hosseinzadeh, G. and Sárneczky, Krisztián and Pál, András and Bódi, Attila and Sódorné Bognár, Zsófia and Csák, Balázs and Cseh, Borbála and Kalup, Csilla and Könyves-Tóth, Réka and Kriskovics, Levente and Sódor, Ádám and Szabó, Róbert and Szakáts, Róbert and Zsidi, Gabriella (2019) Photometric and Spectroscopic Properties of Type Ia Supernova 2018oh with Early Excess Emission from the Kepler 2 Observations. ASTROPHYSICAL JOURNAL, 870 (1). ISSN 1538-4357

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Abstract

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically- confirmed type Ia supernova (SN Ia) observed in the $Kepler$ field. The $Kepler$ data revealed an excess emission in its early light curve, allowing to place interesting constraints on its progenitor system (Dimitriadis et al. 2018, Shappee et al. 2018b). Here, we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3$\\pm$0.3 days and $\\Delta$m$_{15}(B)=0.96\\pm$0.03 mag, but it seems to have bluer $B - V$ colors. We construct the "uvoir" bolometric light curve having peak luminosity as 1.49$\\times$10$^{43}$erg s$^{-1}$, from which we derive a nickel mass as 0.55$\\pm$0.04M$_{\\odot}$ by fitting radiation diffusion models powered by centrally located $^{56}$Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of $^{56}$Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a non-degenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia, but is characterized by prominent and persistent carbon absorption features. The C II features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in a SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers.

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