K2 Observations of SN 2018oh Reveal a Two-Component Rising Light Curve for a Type Ia Supernova

Abstract: We present an exquisite, 30-min cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Pan-STARRS1 and CTIO 4-m DECam observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical SNe Ia. This "flux excess" relative to canonical SN Ia behavior is confirmed in our $i$-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14$\pm0.04$ days after explosion, has a FWHM of 3.12$\pm0.04$ days, a blackbody temperature of $T=17,500^{+11,500}_{-9,000}$ K, a peak luminosity of $4.3\pm0.2\times10^{37}\,{\rm erg\,s^{-1}}$, and a total integrated energy of $1.27\pm0.01\times10^{43}\,{\rm erg}$. We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of $\sim$$2\times10^{12}\,{\rm cm}$ based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system.

Insights into the Early Flux Excess of SN 2018oh from K2 Observations

The research paper by Dimitriadis et al. explores the early photometric evolution of the Type Ia supernova SN 2018oh, leveraging the unique capabilities of the K2 mission for high-cadence observations. This study is significant in that it provides unprecedented insights into the early light curve behavior and possible progenitor scenarios for Type Ia supernovae (SNe Ia).

Observations and Methodology

The authors present a detailed analysis of the K2 light curve for SN 2018oh, complemented by multi-color observations from Pan-STARRS1 and DECam. The K2 observations began prior to the explosion of the supernova and continued past peak brightness. What sets this dataset apart is the discovery of a two-component rising light curve, characterized by an initial steep linear gradient followed by a quadratic rise typical for SNe Ia. This "flux excess" deviates from expected SN Ia dynamics but is clearly evident in both K2 and ground-based $i$-band observations.

Key Findings

1. Two-Component Rise: SN 2018oh exhibited an unusual two-phase rise, with a significant early-time flux excess. The initial linear rise peaked approximately 2.14 days after explosion, followed by conventional SN Ia behavior. The paper meticulously quantifies the properties of this excess, including its peak luminosity and integrated energy.

2. Comparison with Models: The authors compare observations with theoretical models to account for the flux excess. These models include:

   • Companion Interaction: Suggests interaction of the SN ejecta with a non-degenerate companion star, potentially supporting a single-degenerate (SD) progenitor scenario. The estimated orbital separation of $\sim2\times10^{12}\,{\rm cm}$ indicates a subgiant companion.
   • Double Detonation: Involves a helium shell explosion over a sub-Chandrasekhar mass white dwarf, resulting in shallow $^{56}$Ni near the surface.
   • Off-Center Nickel Distribution: Posits a non-standard distribution of $^{56}$Ni within the ejecta, influencing early light curve features.

3. Color Analysis: SN 2018oh demonstrated notably blue colors at early epochs, specifically during the flux excess phase. This spectral characteristic aligns more closely with the companion-interaction model, suggesting a single-degenerate progenitor may be plausible.

Implications

This detailed analysis of SN 2018oh provides critical insights into the early evolution of SNe Ia, highlighting the possibility of complexities such as binary interactions influencing observable characteristics. If confirmed through further analysis, the presence of a binary companion would imply a significant fraction of SNe Ia originate from single-degenerate systems.

Future Directions

The authors propose further spectroscopic and photometric studies of SN 2018oh as it transitions into late-time phases—capable of revealing additional characteristics of ejecta-companion interaction, such as stripped hydrogen or helium lines. Additionally, expanding early-time SN Ia observations across multiple events can refine progenitor models and lead to more comprehensive theoretical frameworks that account for the diversity observed in SNe Ia behavior.

The insights garnered from SN 2018oh inspire both theoretical and observational endeavours in the quest to elucidate the mechanisms underlying Type Ia supernovae, with significant implications for cosmology and astrophysics.