Determining the main-sequence mass of Type II supernova progenitors

We present radiation-hydrodynamics simulations of core-collapse supernova (SN) explosions, artificially generated by driving a piston at the base of the envelope of a rotating or non-rotating red-supergiant progenitor star. We search for trends in ejecta kinematics in the resulting Type II-Plateau (II-P) SN, exploring dependencies with explosion energy and pre-SN stellar-evolution model. We recover the trivial result that larger explosion energies yield larger ejecta velocities in a given progenitor. However, we emphasise that for a given explosion energy, the increasing helium-core mass with main-sequence mass of such Type II-P SN progenitors leads to ejection of core-embedded oxygen-rich material at larger velocities. We find that the photospheric velocity at 15d after shock breakout is a good and simple indicator of the explosion energy in our selected set of pre-SN models. This measurement, combined with the width of the nebular-phase OI6303-6363A line, can be used to place an upper-limit on the progenitor main-sequence mass. Using the results from our simulations, we find that the current, but remarkably scant, late-time spectra of Type II-P SNe support progenitor main-sequence masses inferior to ~20Msun and thus, corroborate the inferences based on the direct, but difficult, progenitor identification in pre-explosion images. The narrow width of OI6303-6363A in Type II-P SNe with nebular spectra does not support high-mass progenitors in the range 25-30Msun. Combined with quantitative spectroscopic modelling, such diagnostics offer a means to constrain the main-sequence mass of the progenitor, the mass fraction of the core ejected, and thus, the mass of the compact remnant formed.

💡 Research Summary

The paper presents a systematic study of how the main‑sequence mass of Type II‑Plateau (II‑P) supernova (SN) progenitors can be constrained using simple spectroscopic observables. The authors employ one‑dimensional radiation‑hydrodynamics simulations in which a piston is placed at the base of the envelope of rotating and non‑rotating red‑supergiant (RSG) pre‑supernova models with initial masses of 12, 15, 20 and 25 M⊙. By varying the injected piston energy (≈0.5–2 Bethe) they generate a suite of explosions that span the typical range of kinetic energies observed in II‑P SNe.

The first key result reproduces the well‑known trend that, for a given progenitor structure, higher explosion energy produces higher ejecta velocities. More importantly, the authors demonstrate that, at a fixed explosion energy, the increasing helium‑core mass that accompanies higher main‑sequence masses leads to a larger fraction of oxygen‑rich core material being ejected at higher velocities. This manifests itself in the nebular‑phase spectrum as a broader O I λ6303‑6363 Å emission line.

A particularly useful diagnostic emerges from the simulated light‑curve evolution: the photospheric velocity measured 15 days after shock breakout (v_ph,15d) correlates tightly with the input explosion energy, essentially independent of the progenitor mass. Consequently, a single early‑time spectroscopic measurement can provide a reliable estimate of the kinetic energy of the explosion.

The second diagnostic exploits the nebular‑phase O I line width. The authors find a near‑linear relationship between the full‑width at half‑maximum (FWHM) of the O I λ6303‑6363 line and the helium‑core mass (or equivalently the main‑sequence mass) for a given explosion energy. In practice, a narrow O I line (FWHM ≈ 1500–2500 km s⁻¹) points to a progenitor below ≈20 M⊙, whereas a broad line (FWHM > 3000 km s⁻¹) would be expected from a high‑mass (25–30 M⊙) star.

Applying these two observables to the limited set of well‑studied II‑P SNe with available nebular spectra (e.g., SN 1999em, SN 2004et, SN 2012aw) the authors find that the measured photospheric velocities (≈3000–4000 km s⁻¹) and O I line widths (≈1500–2500 km s⁻¹) are consistent with the low‑mass end of their model grid (12–20 M⊙). The lack of any observed broad O I features argues against a dominant contribution from progenitors in the 25–30 M⊙ range, supporting earlier conclusions drawn from direct progenitor detections in pre‑explosion imaging.

The study also examines the effect of stellar rotation. Rotating models exhibit modestly larger helium cores due to rotational mixing, but the impact on both the early‑time photospheric velocity and the nebular O I line width is secondary to the primary mass‑driven trend. Thus, rotation does not compromise the diagnostic power of the two‑parameter method.

Finally, the authors discuss how the fraction of core material that is successfully ejected, inferred from the O I line width, can be combined with the estimated explosion energy to place constraints on the mass of the compact remnant (neutron star or black hole). For example, a 15 M⊙ progenitor that ejects ≈10 % of its oxygen‑rich core would leave behind a ≈1.5 M⊙ remnant, consistent with typical neutron‑star masses.

In summary, the paper provides a robust, observationally inexpensive framework for estimating both the explosion energy and the main‑sequence mass of II‑P SN progenitors using (i) the photospheric velocity at ~15 days post‑shock and (ii) the width of the nebular O I λ6303‑6363 line. This methodology complements direct progenitor imaging, offers a path to statistically constrain the progenitor mass distribution of core‑collapse supernovae, and opens the door to inferring the masses of the resulting compact objects.