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Confirming Nunki as the closest core collapse progenitor candidate to the Sun

Space

Key takeaway

Astronomers have identified a nearby supernova candidate star Nunki, located just 160 light-years from Earth. This discovery provides scientists with a rare opportunity to closely study the final stages of a star's life before it explodes.

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Quick Explainer

Nunki is a close binary star system that may be the nearest candidate to eventually undergo a core collapse supernova. By combining high-precision interferometric observations, the researchers were able to determine Nunki's orbital parameters, including a surprising spin-orbit misalignment. This suggests the binary will undergo an eccentric Roche lobe overflow process, potentially leading to a merger into a more massive star. Given Nunki's relative proximity to Earth, it represents the closest known core collapse progenitor candidate, providing a rare opportunity to study this critical pre-supernova phase in detail.

Deep Dive

Technical Deep Dive: Confirming Nunki as the Closest Core Collapse Progenitor Candidate

Overview

  • Nunki (σ Sagittarii) is a close binary system that may be the closest progenitor candidate for a future core collapse supernova to the Sun.
  • The system was previously suggested to be a 6.5 M⊙ + 6.3 M⊙ binary at a projected separation of 0.60 au.
  • This work aims to confirm the orbital parameters of Nunki in order to better understand its evolution and fate.

Methodology

  • Combine multiple epochs of VLTI/PIONIER and VLTI/GRAVITY interferometric observations to measure the orbital parameters of the Nunki binary.
  • Analyze archival spectroscopic data to investigate the detectability of the binary.

Data & Experimental Setup

  • VLTI/PIONIER observations: 3 epochs in 2017 using 3-4 telescopes, providing astrometric positions and flux ratios.
  • VLTI/GRAVITY observation: 1 epoch in 2024, providing high-precision astrometry.
  • 2 archival high-resolution spectra (FEROS and HERMES) to study the detectability of the binary.

Results

  • Determined the orbital parameters of Nunki:
    • Period: 134.8 ± 0.025 days
    • Eccentricity: 0.492 ± 0.003
    • Semi-major axis: 1.25 ± 0.05 au
  • The low orbital inclination (i ≈ 20°), high projected rotational velocity (v sin i ≈ 160 km/s), and lack of a decretion disk suggest significant spin-orbit misalignment.
  • Archival spectra show no evidence of binarity, demonstrating that equal-mass binaries with small radial velocity separations can remain undetected spectroscopically.

Interpretation

  • The significant eccentricity (e ≈ 0.5) means Nunki will undergo "eccentric Roche lobe overflow", a poorly understood process that may lead to a common envelope event and merger into a ≳ 10 M⊙ star.
  • Given its proximity (d ≈ 69 pc), closer than both Spica and Bellatrix, Nunki can be considered the closest core collapse progenitor candidate to the Sun, interestingly as the result of a merger event.

Limitations & Uncertainties

  • No radial velocity measurements, leading to some parameter degeneracies.
  • Future evolution of the eccentric Roche lobe overflow process is highly uncertain and difficult to predict.

What Comes Next

  • Further observations, especially radial velocities, to refine the orbital parameters.
  • Detailed modeling of the eccentric mass transfer phase and its potential outcomes (merger, envelope ejection, etc.).

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