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

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.

Analysis of Type Ia Supernova 2018oh: Insights from Photometry and Spectroscopy with $Kepler$ 2 Observations

The study of SN 2018oh, a Type Ia supernova within the observational field of the $Kepler$ spacecraft, presents a unique opportunity to examine the photometric and spectroscopic characteristics of this class of supernovae in the greatest detail. Collected data includes observations spanning optical, ultraviolet, and near-infrared wavelengths. The analysis covers early light curve features, bolometric luminosity, and spectral evolution, making noteworthy contributions to the discourse on supernova progenitors and explosion mechanics.

Early Excess Emission and Light Curve Analysis

SN 2018oh was first detected at $z \sim 0.0109$, displaying normal photometric characteristics typical of Type Ia supernovae with a rise time of 18.3 ± 0.3 days and a decline rate $\Delta m_{15}(B)$ of 0.96 ± 0.03 mag. Its distinguishable feature is the early excess emission observed in the light curve recorded by $Kepler$. This early emission implies possible scenarios such as interaction of supernova ejecta with circumstellar material (CSM) or a non-degenerate binary companion star. The presence of excess flux is supported by a calculated delay of about +3.85 days before the radioactivity-driven luminosity commences, hinting at the presence of peripheral influences such as $^{56}$Ni mixing into the outer layers or ejecta-companion interaction.

Bolometric Analysis and Progenitor Constraints

In estimating the bolometric light curve, the peak luminosity was derived as 1.49 x 10$^{43}$ erg s$^{-1}$, translating to a $^{56}$Ni mass of 0.55 ± 0.04 M$_\odot$. The pattern of the light curve supports classical models of radioactive decay, moderated by specific delays inferred from early-time observations. These insights link the progenitor system dynamics directly to models indicating compact progenitors and emphasize the role of the explosion mechanism, fueled initially by the decay of nickel isotopes.

Spectroscopic Evolution and Carbon Features

The spectroscopic data reveal a persistent carbon signature, observable from the early to more evolved phases until about three weeks post-maximum light. The unusual persistence of carbon absorption, specifically the C II features, indicates a greater retention of unburnt carbon within the outer layers of the ejecta than is typically observed in standard Type Ia supernovae. This observation raises important questions regarding the progenitor metallicity and explosion symmetry, as these C II lines suggest incomplete carbon burning or asymmetries in explosive nucleosynthesis.

Moreover, the research highlights the velocity profiles of various significant lines such as C II, Si II, and Ca II. The evolution of these velocities illuminates the dynamics of the supernova ejecta, illustrating various interacting forces and confirming high-velocity features early in the supernova's spectral line development. This offers prospects for tight constraints on progenitor scenarios, potentially eliminating high-metallicity progenitors due to reduced metal signatures.

Implications and Theoretical Considerations

The aforementioned discoveries for SN 2018oh bear significant implications in the cosmic context. On a theoretical level, they challenge the existing consensuses about supernova explosions, suggesting that a portion of Type Ia supernovae may derive from systems with atypically low-metallicity progenitors or might involve more complex interactions, such as binary dynamics not thoroughly accounted for by the common scenarios like double-degenerate or single-degenerate pathways. The observations suggest potential for a mixed model where interactions prompt complex early-time dynamics.

Future theoretical models and simulations must consider these nuances to account for the persistence of early emission excess and unburnt carbon factors. Expanding observational campaigns to a wider range of Type Ia SNe caught at early stages could help refine these models, further solidifying their role as standard candles for cosmological distance measurements.

Conclusion

The study of SN 2018oh through the extensive observational regimen afforded by the $Kepler$ mission provides profound insights into the characteristics and mechanisms of Type Ia supernovae. It compels the scientific community to revisit and re-evaluate the theoretical underpinnings of progenitor systems and supernova explosion dynamics, addressing the ramifications of early light curve deviations and the spectroscopic persistence of elemental features.

Research will benefit from a concerted effort to integrate new observational data with refined theoretical modeling, paving the way for an enhanced understanding of these cosmic events and their utility in probing the deeper cosmos. This approach could broaden the ramifications for cosmological models reliant on these phenomena as pivotal probes in the quest for understanding dark energy and universal expansion.