Helium superluminous SN 2021bnw : an explosion of a massive star with a pre-outburst

Technical Summary: Helium Superluminous SN 2021bnw

Problem Statement
Superluminous supernovae (SLSNe) are transient events with peak absolute magnitudes exceeding −20 mag. While many are powered by magnetar spin-down or radioactive decay, the specific mechanisms driving helium-rich SLSNe (He-SLSNe) remain debated. SN 2021bnw is a nearby He-SLSN characterized by a broad light curve (LC) spanning 140 days and a peak magnitude of −20.5 mag. Pure radioactive powering via $^{56}$Ni decay would require an unphysically large nickel mass (14 $M_\odot$), implying a pair-instability supernova (PISN) origin. However, the observed LC declines rapidly after the peak and lacks the slow-evolving, high-luminosity tail characteristic of PISNe. Conversely, a pulsational pair-instability supernova (PPISN) origin is challenged by the specific energy and mass constraints derived from the data. The study aims to determine the powering mechanism and progenitor nature of SN 2021bnw by modeling its pseudo-bolometric light curve.

Methodology
The authors employed hydrodynamic radiative-transfer simulations using the STELLA code to model the interaction between supernova ejecta and circumstellar matter (CSM), supplemented by radioactive heating.

• Ejecta Models: The ejecta profiles were scaled from a hydrodynamical template of a 90 $M_\odot$ helium star (He90 model by Heger & Woosley 2002). The authors tested ejecta masses ranging from 15 to 90 $M_\odot$. The composition was primarily oxygen, helium, carbon, silicon, and sulfur.
• CSM Models: The ejecta were surrounded by a wind-like CSM ($\rho \propto r^{-2}$) with a helium-rich composition (He:C:O ratio of 5:3:2), motivated by NIR spectral features observed 87 days post-peak. CSM masses were varied between 1 and 22.2 $M_\odot$.
• Energy Sources: The simulations included kinetic energy from the explosion, interaction energy between ejecta and CSM, and radioactive heating from $^{56}$Ni and $^{56}$Co decay. No additional energy injection was simulated during the main explosion phase.
• Early Epoch Analysis: To explain early data points (approx. 38 days before peak) that exceeded the luminosity of the progenitor star, the authors tested models involving a compact CSM and an expanding shell representing a pre-supernova outburst.

Key Results

1. Best-Fit Explosion Parameters: The observed light curve is best reproduced by a combination of CSM interaction and radioactive decay. The optimal models require:

   • Ejecta Mass: 15–22.5 $M_\odot$.
   • CSM Mass: ~7 $M_\odot$.
   • Kinetic Energy: 4–5 foe ($10^{51}$ erg).
   • Radioactive Nickel: 1.7 $M_\odot$ of $^{56}$Ni.
   • Interaction: The collision of the 15–22.5 $M_\odot$ ejecta with the 7 $M_\odot$ CSM powers the first ~80 days of the light curve. The 1.7 $M_\odot$ of $^{56}$Ni is required to sustain the luminosity and extend the light curve to day 140.

2. Early Light Curve Anomaly: The initial data points (pre-peak) cannot be explained by the main ejecta-CSM interaction models. The authors propose these points represent a pre-outburst occurring ~18 days before the terminal explosion. This is modeled as an expanding shell with a mass of 0.5–1 $M_\odot$ and kinetic energy of 0.7–1 foe, undergoing adiabatic cooling.

3. Progenitor Constraints:

   • The total mass of ejecta plus CSM is estimated at 22–30 $M_\odot$.
   • Using stellar evolution relations (Heger & Woosley 2002) and accounting for mass loss, the progenitor's initial Zero-Age Main Sequence (ZAMS) mass is estimated to be not less than 61 $M_\odot$ (and likely below 120 $M_\odot$).
   • The explosion energy (4–5 foe) and high $^{56}$Ni yield (1.7 $M_\odot$) are consistent with a magnetorotational core-collapse (MRCC) scenario, potentially involving a collapsar mechanism, rather than standard neutrino-driven collapse.

Exclusion of PPISN
The authors argue against a PPISN origin for SN 2021bnw based on several factors:

• Mass Limit: The derived total mass (ejecta + CSM) falls below the typical lower limit for PPISN progenitors (34–48.5 $M_\odot$ helium cores).
• Remnant Physics: Standard PPISN models in the relevant mass range typically leave behind a massive black hole (30–40 $M_\odot$) without significant mass ejection or $^{56}$Ni production.
• Energy Requirements: Achieving the required kinetic energy (4–5 foe) and ejecta mass in a realistic core-collapse scenario with a massive remnant is problematic for standard PPISN models.
• Nickel Production: The necessity of 1.7 $M_\odot$ of $^{56}$Ni contradicts PPISN scenarios which generally do not produce such large amounts of radioactive nickel.

Significance and Claims
The paper concludes that SN 2021bnw is most likely the result of a core-collapse explosion of a massive star (ZAMS $\ge$ 61 $M_\odot$) aided by magnetorotational effects. The event was powered by the interaction of the ejecta with a massive, helium-rich CSM shell and a substantial amount of synthesized $^{56}$Ni. The study highlights that while current MRCC simulations typically yield lower $^{56}$Ni masses (0.5–1.14 $M_\odot$), future high-resolution 3D simulations and improved neutrino transport schemes may bridge this gap. The authors explicitly state that while the MRCC scenario is the most plausible explanation, the specific dynamics required to produce 1.7 $M_\odot$ of $^{56}$Ni in such a high-mass progenitor remain an open question for future explosive nucleosynthesis studies. The paper does not claim to definitively prove the MRCC mechanism but presents it as the preferred model over PISN or PPISN alternatives given the current data and modeling constraints.