7 Apr 2025

Royal Holloway – University of London leads a new breakthrough in the study of supernovae, with astronomers uncovering clear signatures of late-time shock interaction in the nearby Type II supernova SN 2023ixf, marking the first identification of such interaction in this event. The research, led by Dr Amit Kumar, utilised the newly commissioned WEAVE spectrograph on the William Herschel Telescope (WHT) to obtain a spectrum nearly one year after the explosion. The resulting spectrum reveals intricate emission features that offer new insights into the complex processes shaping the final stages of massive star evolution.

SN 2023ixf, located in the nearby galaxy M101, is the closest Type II supernova discovered in the past decade. Using WEAVE's large integral field unit, LIFU, the team obtained the first-ever supernova spectrum captured by the instrument. In the late-time spectrum, they identified a strikingly complex H-alpha emission profile, revealing distinct high-velocity components moving at ±5650 kilometres per second. These features indicate that the supernova ejecta is colliding with dense shells of circumstellar material (CSM) expelled hundreds of years before the explosion, injecting additional energy into the system. This is the first time such shock interaction signatures have been observed in SN 2023ixf.

Further analysis of forbidden oxygen and magnesium emission lines suggests that the ejecta is moderately aspherical and clumpy, rather than arranged in a toroidal or disk-like geometry. Measurements of the oxygen mass and comparison with model spectra constrain the progenitor star's initial mass to be less than 12 times the mass of the Sun, consistent with other studies using pre-explosion imaging and hydrodynamic modelling. SN 2023ixf's late-time spectrum also shows red-blue asymmetries in several emission lines, likely influenced by dust formation within the ejecta.

These results highlight the power of WEAVE for detailed spectroscopic investigation of transient phenomena. By capturing the shock-driven signatures shaping SN 2023ixf's evolution, the study provides key insights into how red supergiants lose mass in the centuries leading up to their collapse, enriching our understanding of massive star life cycles.

The study was carried out by a team led by Royal Holloway researchers, including Dr. Amit Kumar and Dr Justyn R. Maund (Royal Holloway – University of London, UK), Dr Raya Dastidar (Instituto de Astrofísica – Universidad Andres Bello & Millennium Institute of Astrophysics, Chile), Adam J. Singleton (University of Sheffield, UK), and Dr Ning-Chen Sun (University of Chinese Academy of Sciences, NAOC, and Beijing Normal University, China).

WEAVE funding and construction

The ING initiated plans to build WEAVE after extensive consultation with the ING user community about what was needed for the future. There was a broad consensus that a world-class wide-field multi-object spectrograph was required to exploit from the ground the huge surveys being undertaken by powerful telescopes such as as ESA's Gaia, thereby helping to address the main astrophysical challenges foreseen for the next decade.

In 2016, the multi-lateral agreement to design and build WEAVE was signed by the countries of the ING partnership (the UK, Spain and the Netherlands), joined by France and Italy, with each country contributing major components as listed below, and with the ING providing auxiliary systems and overall project management.

The consortium is led by Gavin Dalton from the University of Oxford and RAL Space as Principal Investigator, Scott Trager from the University of Groningen as Project Scientist, Don Carlos Abrams from the ING as Project Manager, and Chris Benn from the ING as Instrument Scientist. The main components of WEAVE are:

• Fibre positioner, developed by the University of Oxford in the UK, with support from the Instituto de Astrofísica de Canarias (IAC) in Spain.
• Prime-focus system, designed by ING, IAC and SENER, provided by the IAC and manufactured by SENER. Support from Konkoly Observatory (HU). Lenses were polished by KiwiStar in New Zealand, funded by STFC, NOVA, INAF, IAC and ING, and mounted at SENER Aeroespacial (ES) by SENER and ING.
• Spectrograph, built by NOVA in the Netherlands with optical design by RAL Space in the UK, optics manufactured at INAOE (MX) and with support from INAF (IT) and the IAC.
• Field rotator, provided by IAC and manufactured by IDOM (ES). Optical fibres, provided by the Observatoire de Paris in France, manufactured in France, Canada and USA.
• LIFU, built by NOVA (NL).
• CCD detector system, provided by Liverpool John Moores University in the UK.
• Data processing, analysis and archiving led by the University of Cambridge (UK), IAC (ES) and FGG-INAF (IT), respectively.
• Observatory control system, built by the ING.

[Image]

(A) Left: the figure displays the Pan-STARRS1 image of the site of SN 2013ixf in M101 before explosion, with the footprint of the WEAVE LIFU observation indicated by the hexagon. Right: WEAVE restored image from a collapsed datacube, with blue, green and red colours derived from wavelength regions corresponding to the Sloan gri filters. The position of the supernova SN 2023ixf is marked by cross-hairs.

(B) Nebular spectrum of SN 2023ixf at 363 days post-explosion, compared with model spectra featuring varying shock powers. The inset zooms in on the H-alpha and [O I] regions, highlighting clear signatures of shock interaction.

(C) WEAVE prime-focus corrector and positioner.

(D) The William Herschel Telescope (WHT).