A 91bg-like supernova fainter than any of its class raises new questions about thermonuclear explosions

SN 2023vjh was detected in the direction of the elliptical galaxy MCG+04-10-013, located at approximately 271 million light-years from Earth. What makes the environment noteworthy is not only the passive, gas-poor nature of the host — typical for 91bg-like events — but also the extreme projected distance between the explosion and the galactic center: 6.8 kiloparsecs, or roughly 22,000 light-years. Such an offset is significant because it substantially reduces the likelihood that interstellar dust in the host galaxy could be blocking or reddening the light we receive from the supernova.

What makes the 91bg-like subclass so peculiar

Since the discovery of SN 1991bg more than three decades ago, astronomers have used that event as the prototype for a distinct subclass of Type Ia supernovae characterized by unusually low peak luminosities, rapid photometric decline, cool photospheres and deep absorption features from intermediate-mass elements such as titanium and chromium. Unlike the more famous, standardizable Type Ia explosions that serve as cosmological distance indicators, 91bg-like events sit at the low-luminosity, fast-declining end of the thermonuclear supernova family. Their progenitor scenario remains a matter of active debate, with double-detonation of sub-Chandrasekhar-mass white dwarfs and mergers of double white dwarfs among the leading candidates.

The team behind the new work obtained optical and near-infrared spectroscopy of SN 2023vjh and confirmed that the event clearly belongs to this subclass. The light-curve shape parameter Δm₁₅(B) — the amount by which the supernova dims in the B band during the first fifteen days after peak — was measured at 1.89 ± 0.01 magnitudes, and the color-stretch parameter s_BV came out at 0.45 ± 0.03. Both values place SN 2023vjh firmly in the fast-declining, subluminous corner of the parameter space, alongside canonical 91bg-like objects. Near-peak spectra show prominent absorption from Si II, Ca II and Ti II, indicating a cool photosphere, while late-phase near-infrared spectra display the Ca II NIR triplet together with Fe II and Co II absorption features, but no obvious H-band break.

The excess reddening problem

Where SN 2023vjh diverges from the well-behaved members of its subclass is in the amount of reddening its light appears to have suffered on its journey to Earth. Standard photometric and color analyses point to a host-galaxy color excess E(B−V) of between 0.2 and 0.35 magnitudes — a range typically associated with dusty, star-forming environments rather than the essentially dust-free interstellar medium of an elderly elliptical galaxy.

Yet the near-peak spectra of SN 2023vjh show no detectable Na I D absorption, the classical spectral tracer of interstellar sodium along the line of sight. In practice, this means that whatever is dimming and reddening the light of this supernova is not standard interstellar dust in its host galaxy. Combined with the enormous projected offset of 22,000 light-years from the galactic center, the case for a benign line of sight is strong. And yet the object still comes across as unusually faint, both against explosion-model predictions and against the observed luminosities of other 91bg-like events studied in detail.

Circumstellar material as a plausible answer

The most interesting hypothesis explored by the authors is that circumstellar material — gas and dust in the immediate vicinity of the exploding white dwarf itself — is contributing to the observed reddening. When alternative extinction prescriptions consistent with a CSM origin are included in the analysis, the agreement between the observed blue-band light curves and the theoretical predictions improves noticeably, although some residual discrepancies remain.

If confirmed by further modeling or follow-up observations, the presence of CSM around a 91bg-like supernova would carry meaningful implications for how astronomers understand the progenitors of these events. Circumstellar material is typically associated with mass loss from a companion star or from the progenitor system itself in the years preceding the explosion — an ingredient not usually invoked for 91bg-like scenarios, which are generally modeled as clean detonations of sub-Chandrasekhar-mass white dwarfs.

Why this matters

Type Ia supernovae have been at the center of some of the most important astrophysical discoveries of the past three decades, including the measurement of the accelerating expansion of the universe. Their apparent uniformity — after empirical light-curve corrections — is what makes them useful as standardizable candles. Peculiar events at the edges of this uniformity, especially the subluminous 91bg-like subclass, are essential probes of the physics that governs thermonuclear detonation and the diversity of white dwarf progenitor systems.

SN 2023vjh sits at the boundary of that diversity. Fainter than models predict, redder than a dust-free line of sight can explain, and possibly wrapped in unseen circumstellar material, it forces theorists to sharpen their explosion scenarios and observers to look more carefully at how the environments closest to a white dwarf shape the light we finally receive across hundreds of millions of light-years.

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Reproduction in whole or in part without express permission is prohibited. Original source: arXiv preprint (2026). DOI: 10.48550/arxiv.2607.08821.