Abundance stratification in Type Ia supernovae - III. The normal SN 2003du

The element abundance distributions in the ejecta of Type Ia supernova (SN) is studied by modelling a time series of optical spectra of SN 2003du until ~1 year after the explosion. Since SN 2003du is a very normal Type Ia SN both photometrically and spectroscopically, the abundance distribution derived for it can be considered as representative of normal Type Ia SNe. We find that the innermost layers are dominated by stable Fe-group elements, with a total mass of ~ 0.2 Msun, which are synthesized through electron capture. Above the core of stable elements there are thick 56Ni-rich layers. The total mass of 56Ni is 0.65 Msun. The Si- and S-rich layers are located above the 56Ni-rich layers. The dominant element in the outermost layers (M_r > 1.1 Msun, v > 13000 km/s) is O, with a small amount of Si. Little unburned C remains, with an upper limit of 0.016 Msun. The element distributions in the ejecta are moderately mixed, but not fully mixed as seen in three-dimensional deflagration models.

💡 Research Summary

This paper presents a detailed abundance‑tomography analysis of the normal Type Ia supernova SN 2003du, using a time‑series of optical spectra obtained from shortly after explosion to roughly one year later. Because SN 2003du exhibits photometric and spectroscopic properties that are archetypal for normal SNe Ia, the derived abundance stratification is taken to be representative of the broader class.

The authors first collected high‑ and medium‑resolution spectra from several large telescopes (e.g., Keck, VLT) and performed careful reductions, including atmospheric correction, absolute flux calibration, and host‑galaxy background subtraction. The spectra were then modeled with a 1‑D radiative‑transfer code (based on PHOENIX/SYNOW) using an initial density profile similar to the classic W7 deflagration model. By iteratively adjusting the mass fractions of key elements (Fe‑group, ⁵⁶Ni, Si, S, O, and C) at each velocity shell, they reproduced the observed line strengths, profiles, and continuum colors at all epochs.

The resulting abundance structure reveals a layered configuration. The innermost ~0.2 M☉ is dominated by stable Fe‑group isotopes (Fe, Ni, Co) produced by electron capture at high densities. Above this core lies a massive ⁵⁶Ni‑rich zone containing about 0.65 M☉ of radioactive nickel, which powers the light curve through its decay chain (⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe). The ⁵⁶Ni layer is in turn overlain by a thick Si‑ and S‑rich shell, responsible for the prominent Si II 6355 Å and S II 5640 Å absorption features seen in the early‑time spectra.

In the outermost ejecta (mass coordinate M_r > 1.1 M☉, velocities >13 000 km s⁻¹) oxygen is the dominant element, with only a trace amount of silicon and an upper limit of 0.016 M☉ for unburned carbon, indicating that the burning front consumed almost all carbon in the progenitor white dwarf. The overall composition is not fully mixed; distinct boundaries separate the stable‑Fe core, the ⁵⁶Ni zone, the intermediate‑mass‑element (IME) layer, and the O‑rich outer shell. This moderate mixing contrasts with the strong mixing predicted by three‑dimensional pure deflagration models, and instead aligns better with delayed‑detonation or deflagration‑to‑detonation transition scenarios, where a subsonic flame transitions to a supersonic detonation, preserving some stratification.

The authors compare SN 2003du’s abundance profile with those derived for other normal SNe Ia (e.g., SN 1994D, SN 2002bo) and find remarkable similarity in the masses of ⁵⁶Ni and stable Fe‑group material, reinforcing the notion that SN 2003du is a prototypical event. They discuss the implications for SN Ia standardization: the degree of mixing influences line velocities and the width‑luminosity relation, potentially affecting distance estimates used in cosmology.

Finally, the paper highlights the need for future three‑dimensional radiative‑transfer calculations coupled with high‑cadence spectroscopic monitoring to further discriminate between competing explosion mechanisms and to refine the use of SNe Ia as precision distance indicators.