Traces of Helium Detected in Type Ic Supernova 2014L

Abstract

The absence of helium features in optical spectra is one of the classification criteria for Type Ic supernovae (SNe Ic). However, it is highly debated whether helium is truly absent in ejecta or spectroscopically undetectable in the optical region. The near-infrared (NIR) region contains cleaner He lines that are less blended with other common ions in SNe Ic ejecta. We perform full spectral modeling on the near-peak-light optical and NIR spectra of the SN Ic 2014L to quantitatively constrain helium and other outer-ejecta properties, using the radiative transfer code TARDIS. We employ a deep-learning emulator for SNe Ic spectra that serves as a fast surrogate for TARDIS simulations. We then integrate the emulator within the Bayesian inference framework to infer the ejecta properties. The emulator achieves a mean fractional error of 1% between the emulated and TARDIS fluxes across all wavelengths and all samples in the test dataset. We constrain 0.018 to 0.020 M_sun (16% to 84% posterior percentile) of He above the photosphere near peak light in SN 2014L, inferred from the observed spectra covering 3500A to 24000A. A Bayesian statistical test shows that the observed spectra are inconsistent with no helium. Furthermore, the posterior favors a power-law density exponent of -7.04 to -6.88 (16% to 84% credible interval), consistent with theoretical calculations of radiation-dominated explosions. This work demonstrates that Bayesian radiative-transfer inference over a wide wavelength range provides a powerful path toward systematic constraints on He in SNe Ic.

Figures (6)

• Figure 1: Flowchart of the spectral inference methodology applied in this work, adapted from obrien_probabilistic_2021 and obrien_1991t-like_2024. The corresponding section of each component is labeled.
• Figure 2: Top: The spectral comparison of the observed spectra and the maximum-a-posteriori (MAP) spectra. The observed spectra are plotted as black points with $1\sigma$ error bars. The normalized and continuum-matched emulator spectrum evaluated with the MAP parameter set is plotted in blue and orange for the optical and NIR spectrum, respectively. Note that the two inferred spectra share the same parameters except the luminosity and time, which are locked in a ratio based on observation. The dark and light shaded regions around the MAP indicate the 68.3% and 99.7% credible posterior probability intervals, respectively. We mark the masked-out telluric regions in the NIR with the $\oplus$ symbol. Bottom: We demonstrate that the emulator spectra (black solid line) closely resemble the tardis simulation (blue dotted line) evaluated with the MAP parameter set. The elemental decomposition plot of the tardis simulation is overlaid, representing the interaction type or ion contribution of the energy packets during the last interaction in the simualtion.
• Figure 3: The corner plot of the marginalized posterior parameters over $f_\sigma$. The elemental masses presented in this plot are the elemental mass above $v_\textrm{inner}$ in the ejecta. We group up C and O as the unburnt elements (UBE); Na, Mg, Si, S, and Ca as the intermediate-mass elements (IME); and Ti, Cr, Fe, and $^{56}$Ni as the iron-group elements (IGE). The contour lines denote the 50, 68, 95, and 97$\%$ quantiles and are color-matched to the vertical quantile markers in the diagonal histograms.
• Figure 4: The SDEC plot shows the spectral-energy decomposition for the tardis spectrum evaluated using the MAP parameter set (top row) compared with an inference run in which we remove He (bottom row). The optical region (left column) remains nearly identical between the two models, with no major differences in the strong line profiles. In contrast, the NIR region (right column) demonstrates that the 1 $\mu m$ feature cannot be reproduced without He. For reference, we also plot the observed spectra; however, these are not continuum-matched to the synthetic spectra.
• Figure 5: The last line interaction velocity distribution of the tardis simulation ran with the MAP parameter set. Each line represents the velocity distribution of the packets that line interacted with a specific element as their last interaction before crossing the outermost simulation boundary.