SN2003bg: a broad-lined Type IIb Supernova with Hydrogen

Models for the spectra and the light curve, in the photospheric as well as in the late nebular phase, are used to infer the properties of the very radio-bright, broad-lined type IIb Supernova 2003bg. Consistent fits to the light curve and the spectral evolution are obtained with an explosion that ejected ~ 4 M_sun of material with a kinetic energy of ~ 5 10^51 erg. A thin layer of hydrogen, comprising ~ 0.05 M_sun, is inferred to be present in the ejecta at the highest velocities (v >~ 9000 km/s), while a thicker helium layer, comprising ~ 1.25 M_sun, was ejected at velocities between 6500 and 9000 km/s. At lower velocities, heavier elements are present, including ~ 0.2 M_sun of 56Ni that shape the light curve and the late-time nebular spectra. These values suggest that the progenitor star had a mass of ~ 20-25 M_sun (comparable to, but maybe somewhat smaller than that of the progenitor of the XRF/SN 2008D). The rather broad-lined early spectra are the result of the presence of a small amount of material (~ 0.03 M_sun) at velocities > 0.1 c, which carries ~ 10 % of the explosion kinetic energy. No clear signatures of a highly aspherical explosion are detected.

💡 Research Summary

The paper presents a comprehensive modeling study of the radio‑bright, broad‑lined Type IIb supernova SN 2003bg, using both its photometric light curve and spectroscopic evolution from the early photospheric phase through the late nebular stage. By fitting a series of radiative‑transfer simulations to the observed data, the authors derive a self‑consistent set of explosion parameters. The ejecta mass is estimated at roughly 4 M⊙, and the kinetic energy of the explosion is about 5 × 10⁵¹ erg, which is significantly higher than the canonical 10⁵¹ erg typical of ordinary core‑collapse supernovae.

A thin hydrogen layer of approximately 0.05 M⊙ is present at the highest velocities (v ≳ 9,000 km s⁻¹). This outermost shell produces the weak H α absorption seen in the early spectra and confirms the classification as a IIb event, where most of the original hydrogen envelope has been stripped away. Beneath this lies a more massive helium shell, containing about 1.25 M⊙ of He, extending from roughly 6,500 to 9,000 km s⁻¹. The He I lines dominate the spectral appearance during the photospheric phase and shape the light‑curve decline.

In the inner ejecta, the authors infer the presence of ~0.2 M⊙ of radioactive ⁵⁶Ni. The decay of ⁵⁶Ni → ⁵⁶Co → ⁵⁶Fe supplies the bulk of the luminosity around peak and powers the nebular emission lines observed at later times. The nebular‑phase spectral synthesis reproduces the observed Fe II and Co II features, confirming the Ni mass estimate and indicating a relatively well‑mixed core.

A notable feature of SN 2003bg is the presence of a small amount of material (~0.03 M⊙) moving at velocities exceeding 0.1 c (≈30,000 km s⁻¹). Although this high‑velocity component contains only a few percent of the total ejecta mass, it carries about 10 % of the kinetic energy and is responsible for the unusually broad absorption lines seen in the earliest spectra. The authors argue that this component does not require a strongly aspherical explosion; the line profiles remain largely symmetric, and no significant polarization is detected.

From the derived ejecta mass, kinetic energy, and composition, the progenitor star is estimated to have had an initial mass of roughly 20–25 M⊙. This places SN 2003bg in a similar mass regime to the X‑ray flash‑associated SN 2008D, though perhaps slightly less massive. The progenitor likely experienced extensive mass loss, either through strong stellar winds or binary interaction, stripping away most of its hydrogen envelope while leaving a thin residual layer.

Overall, the study demonstrates that SN 2003bg combines the hallmark traits of Type IIb supernovae—thin outer hydrogen, a substantial helium shell, and a ⁵⁶Ni‑rich core—with a higher-than‑average explosion energy and a modest high‑velocity tail. These findings provide valuable constraints on the diversity of core‑collapse explosions, the role of progenitor mass loss, and the mechanisms that can generate broad‑lined spectra without invoking extreme asymmetry.