Breakthrough Discovery: Unveiling the Binary Roots of Type II-P Supernovae

In a landmark achievement for astrophysics, an international team led by researchers from China's University of Chinese Academy of Sciences (UCAS) and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has provided the first direct evidence that some Type II-P supernovae—the most common type of core-collapse stellar explosions—originate from the merger of binary star systems. Published in Science Bulletin, the study centers on supernova SN 2018gj, whose progenitor star's unusual characteristics point to a dramatic binary history rather than solitary evolution.

Type II-P supernovae, which comprise over 50% of core-collapse supernovae in the local universe, have long puzzled scientists. Traditional models favored single red supergiant stars as progenitors, but theoretical predictions suggested binaries could play a significant role, potentially accounting for 30-50% of cases. This discovery bridges observation and theory, highlighting the pivotal influence of binary interactions on massive star fates.

Understanding Type II-P Supernovae and Their Progenitors

Core-collapse supernovae mark the explosive deaths of massive stars (typically 8-20 solar masses), forging heavy elements and compact remnants like neutron stars or black holes. Type II-P supernovae are distinguished by hydrogen-rich spectra and a characteristic 'plateau' in their light curves, lasting about 100 days as the expanding hydrogen envelope traps radiation.

Pre-explosion imaging has identified around 20 Type II-P progenitors, mostly red supergiants (RSGs) with luminosities of 10^4 to 10^5 solar luminosities and radii exceeding 500 solar radii. However, discrepancies persist: some progenitors appear in environments too old for single-star models, and light curves vary. Binary scenarios—mass transfer, common envelope ejection, or mergers—offer explanations for these outliers.

The Team Behind the Discovery: Chinese Leadership in Global Astrophysics

Led by co-first authors Sun Ningchen, an associate professor at UCAS and NAOC, and Niu Zexi, a special research assistant at UCAS, the team spans China, Greece, the UK, US, Ireland, and Switzerland. Key contributors include Emmanouil Zapartas from Greece's Institute of Astrophysics and Philipp Podsiadlowski from the University of Oxford.

UCAS and NAOC's role underscores China's rising prowess in astronomical research. With facilities like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and growing international collaborations, Chinese institutions are at the forefront of transient astronomy, training the next generation of astronomers through programs at UCAS.

Observational Clues from SN 2018gj

SN 2018gj exploded on April 10, 2018, in the outskirts of galaxy NGC 6217, 24.2 Mpc away. Hubble Space Telescope (HST) pre-explosion images from 2009 reveal a point source at the site: a luminous RSG with effective temperature log(T_eff/K) = 3.54 ± 0.01, luminosity log(L/L_⊙) = 5.0 ± 0.1, and radius 875 ± 140 R_⊙.

Post-explosion HST visits (2019, 2021, 2023) show the source fading and vanishing, confirming its destruction. The local environment is sparse and old (log age ≥ 7.60 ± 0.05 years), inconsistent with young clusters expected for single 13-16 M_⊙ stars. The light curve's plateau lasted just 70 ± 2 days—among the shortest recorded—hinting at a thinner hydrogen envelope (M_H ≈ 3.5 M_⊙).

These traits mismatch single-star evolution but align with binary merger products. For deeper insights, read the full study on arXiv.

Photo by Hoi An and Da Nang Photographer on Unsplash

Binary Evolution Simulations: Reconstructing the Progenitor's Path

Using advanced binary population synthesis (binary_c, BPASS, POSYDON), the team modeled the progenitor as a merger remnant. A close binary with primary 8.3-9.3 M_⊙, mass ratio 0.85-0.97, and period 3-6.5 days undergoes stable mass transfer, common envelope, and reverse merger, yielding a He-core mass M_He = 4.6 ± 0.7 M_⊙.

This merger product evolves slower (log age 7.1-7.3 years), matching the old field location, with a compact H-envelope explaining the brief plateau. Single-star models fail: they predict younger ages and longer plateaus. Binary probability exceeds 99% under realistic priors (40% binaries among RSGs).

Hydrodynamic Simulations and Light Curve Modeling

Hydrodynamic models confirm the short plateau arises from the merger's stripped envelope. The explosion ejects material with velocities matching observations, and late-time spectra show blueshifted Hα, consistent with binary stripping.

• Merger increases core mass, accelerating evolution post-merger.
• Envelope mass loss via transfer/merger reduces recombination time, shortening plateau.
• Simulations reproduce SN 2018gj's bolometric light curve and velocity evolution.

Implications for Massive Star Evolution and Supernova Diversity

This finding validates binary channels for ~40% of observed supernovae, reshaping models of compact object formation. Merger progenitors may produce neutron stars with higher kicks or black holes via fallback. It explains 'missing' RSGs in surveys and diverse light curves.

Details in Science Bulletin highlight methodological advances for identifying hidden binaries.

China's Growing Influence in Transient Astronomy

UCAS and NAOC exemplify China's investment in astronomy: UCAS trains PhD students in transients, while NAOC operates world-class telescopes. Collaborations with Oxford and FORTH amplify impact. Sun Ningchen noted, "This work exemplifies deep collaboration between observational and theoretical astronomers."

Podsiadlowski added, "We have identified a long-predicted merger progenitor." Zapartas emphasized simulations uncovering mass transfer history.

Photo by Bhong Bahala on Unsplash

Future Directions: Hunting More Binary Progenitors

Upcoming surveys like JWST transients and Rubin Observatory will detect fainter progenitors. Applying this HST+simulation method to ~20 known RSGs could reveal more mergers. Chinese-led projects, including FAST for radio counterparts, promise further breakthroughs.

Prospects include linking mergers to gamma-ray bursts or kilonovae, advancing multi-messenger astronomy.

Stakeholder Perspectives and Broader Impacts

Astrophysicists hail it as a paradigm shift. Chinese higher education benefits: UCAS's interdisciplinary programs foster such innovations. For students, it underscores research careers' excitement—opportunities abound in stellar evolution.