🔭 astrophysics

The type Ia supernova 2023vjh: a peculiar 1991bg-like SN with unusually faint light curves

This paper presents observations of the peculiar, unusually faint 1991bg-like Type Ia supernova 2023vjh, which exhibits significant deviations from standard models and other events in its class, potentially due to circumstellar material extinction despite minimal interstellar reddening.

Original authors: M. Kopsacheili, L. Galbany, G. Folatelli, M. M. Phillips, C. R. Burns, M. D. Stritzinger, H. -Y. Miao, M. González-Bañuelos, R. García-Benito, T. E. Müller-Bravo, E. Y. Hsiao, K. Auchettl, J. P. Ander

Published 2026-07-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: M. Kopsacheili, L. Galbany, G. Folatelli, M. M. Phillips, C. R. Burns, M. D. Stritzinger, H. -Y. Miao, M. González-Bañuelos, R. García-Benito, T. E. Müller-Bravo, E. Y. Hsiao, K. Auchettl, J. P. Anderson, C. Ashall, T. -W. Chen, D. D. Desai, M. E. Huber, T. de Jaeger, J. M. DerKacy, M. Gromadzki, J. T. Hinkle, W. Hoogendam, C. Jimenez-Palau, K. Matilaine, P. A. Mazzali, K. Medler, T. Pessi, C. Pfeffer, K. Phan, G. Pignata, R. Sanfeliu, B. J. Shappee, M. A. Tucker, H. Xiao, D. R. Young

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, glittering stage where stars occasionally go out with a spectacular bang. These explosions, called supernovae, are the universe's way of recycling heavy elements and lighting up the cosmos. Among them, a specific group called "Type Ia" supernovae are famous for being the universe's "standard candles"—reliable lighthouses that astronomers use to measure how far away things are and how fast the universe is expanding.

But not all Type Ia supernovae are created equal. Some are the bright, steady giants, while others are the shy, dim cousins. Enter SN 2023vjh, a peculiar new star that exploded on October 17, 2023. It belongs to the "1991bg-like" family, a group known for being fast-fading and dimmer than their famous relatives. Think of them as the "sprinters" of the supernova world: they rise quickly, peak early, and fade away faster than the marathon runners.

The Mystery of the Extra-Dim Star
When astronomers first looked at SN 2023vjh, they found something strange. While it fit the general profile of a 1991bg-like supernova (it had the right speed and shape for its light curve), it was systematically fainter than anyone expected. It was dimmer than the standard models predicted and even dimmer than other well-studied members of its own family.

Usually, when a star looks dimmer than it should, astronomers suspect it's being covered by a dusty veil. They calculate how much dust is blocking the light, a measurement called "reddening." For SN 2023vjh, the math suggested a surprisingly thick veil of dust, with a reddening value of $E(B-V)_{host} \sim 0.2 – 0.35$ mag. That's a lot of dust for this type of star!

The Detective Work: Ruling Out the Dust
Here is where the story gets tricky. The team played detective to see if this dust was real. They looked at the star's location: it exploded 6.8 kpc away from the center of its host galaxy, MCG+04-10-013. This host is an elliptical galaxy, which is like a quiet, old neighborhood with no new construction (no new stars forming) and very little dust.

If there were a thick cloud of dust blocking the light, the team expected to see a specific chemical fingerprint in the star's light: a dark line caused by sodium (Na I D). But when they scanned the spectra, no sodium absorption was found. It was as if they were looking for a shadow in a room that was supposed to be full of dust, but the room was empty.

This creates a puzzle: The math says there's a lot of dust, but the chemical evidence says there isn't.

The "What If" Scenarios
So, what's going on? The authors suggest a few possibilities, but they are careful not to claim they have solved the mystery yet.

The "Intrinsic" Dimness: Maybe the star just is naturally dimmer than the models predict. The explosion might have produced less energy or less of the radioactive fuel (nickel-56) that powers the light.
The "Circumstellar" Veil: The team suggests the dust might not be in the galaxy's general space (interstellar medium) but could be a specific cloud of material right around the star itself (circumstellar material, or CSM). This would explain why the light is dim and red, but why the usual sodium fingerprint is missing (because the dust is in a different configuration).
The Model Glitch: When they compared the star's light to computer models of explosions (specifically the DDC25 and SCH2p0 models), the models didn't quite match the data. The models worked okay for blue light, but the star remained too faint in the red $i$ -band. The authors suggest this might mean the computer models aren't perfect at predicting how light behaves in the red part of the spectrum, rather than the star being a total mystery.

The Takeaway
SN 2023vjh is a "peculiar" case. It fits the category of fast-declining, dim supernovae, but it pushes the boundaries. It is fainter than predicted by current explosion models and shows signs of unusual reddening that doesn't quite match the standard rules of dust in its host galaxy.

Why Should We Care?
Even though this star is weird, it's still useful. The team used SN 2023vjh to help measure the Hubble constant (the rate at which the universe is expanding). By applying special math tricks designed for these fast-fading stars, they calculated a value of $H_0 = 71.56 \pm 3.09$ km s $^{-1}$ Mpc $^{-1}$ using one method and $H_0 = 74.60 \pm 2.22$ km s $^{-1}$ Mpc $^{-1}$ using another. These numbers are consistent with what other astronomers have found, suggesting that even the "oddballs" of the supernova world can be trusted to help us map the universe, as long as we know how to read their unique signals.

In short, SN 2023vjh is a dim, fast-fading star that refuses to follow the rulebook perfectly. It might be hiding behind a veil of its own making, or it might just be showing us that our computer models need a little tuning. Either way, it's a fascinating new piece of the cosmic puzzle.

Technical Summary of "The type Ia supernova 2023vjh: a peculiar 1991bg-like SN with unusually faint light curves"

Problem Statement
Type Ia supernovae (SNe Ia) serve as standardizable candles for cosmological distance measurements, yet a specific subclass known as 91bg-like SNe presents challenges. These events are subluminous, fast-declining, and often exhibit intrinsic colors and light-curve shapes that deviate from normal SNe Ia, leading to their frequent exclusion from cosmological samples. While 91bg-like SNe are generally found in old stellar populations (elliptical galaxies) and show low host-galaxy extinction, SN 2023vjh presents a paradox: it is located in a passive elliptical galaxy at a large projected distance (6.8 kpc) from the center, yet photometric analyses suggest unusually high host-galaxy reddening ( $E(B-V)_{host} \sim 0.2–0.35$ mag). Furthermore, the event appears systematically fainter than predicted by standard explosion models and other well-studied 91bg-like SNe. The paper aims to characterize SN 2023vjh's photometric and spectroscopic properties to determine if its faintness is due to significant extinction (potentially circumstellar), intrinsic properties, or limitations in current explosion models.

Methodology
The authors present a comprehensive dataset for SN 2023vjh, discovered on October 17, 2023, in the elliptical galaxy MCG+04-10-013 ( $z=0.019$ ).

Photometry: Optical light curves were obtained in $B, V, g, r, i, u$ bands using the POISE program (Swope 1m telescope), supplemented by ZTF and ATLAS data. Near-infrared (NIR) light curves were not explicitly detailed as a primary dataset, but NIR spectra were obtained.
Spectroscopy: Optical spectra were collected from $-8.7$ to $+47.3$ days relative to $B$ -band maximum using multiple facilities (UH88, GTC, Keck II, NOT, LT, NTT, Magellan, CAHA). Three late-phase NIR spectra were obtained at $+16.4, +20.8,$ and $+23.4$ days using FIRE (Baade telescope) and EMIR (GTC).
Analysis Techniques:
- Light Curve Fitting: The SNooPy fitter was used with 91bg-like templates to derive peak magnitudes, decline rates ( $\Delta m_{15}(B)$ ), and color-stretch parameters ( $s_{BV}$ ). Three models were tested: max_model, EBV_model2, and color_model.
- Extinction Assessment: Host extinction was evaluated via SNooPy fitting, the updated Lira relation, and the absence of Na I D absorption lines in high-S/N spectra. The authors also tested the impact of circumstellar material (CSM) using the Goobar (2008) extinction law.
- Model Comparison: Observed light curves were compared against the Chandrasekhar-mass delayed-detonation model (DDC25) and the sub-Chandrasekhar central detonation model (SCH2p0) from Blondin et al. (2018).
- Spectroscopic Analysis: Expansion velocities and pseudo-equivalent widths (pEW) were measured for features like Si II, Ca II, O I, and Ti II. NIR spectra were analyzed for Fe II, Co II, and the Ca II NIR triplet.
- Cosmological Application: The Hubble constant ( $H_0$ ) was estimated using the Tripp relation and a simplified "Color method" tailored for fast-declining SNe Ia (Phillips et al. 2026).

Key Results

Classification: SN 2023vjh is confirmed as a 91bg-like SN Ia with $\Delta m_{15}(B) = 1.89 \pm 0.01$ mag and $s_{BV} = 0.45 \pm 0.03$ . It falls within the "cool" and "extreme cool" regions of classification diagrams.
Photometry: The event is exceptionally faint. While the light curve shape is consistent with other 91bg-like SNe, the absolute magnitude is lower than predicted by explosion models.
Extinction Paradox:
- Photometric fitting suggests significant host extinction ( $E(B-V)_{host} \approx 0.35$ mag).
- However, spectroscopic evidence contradicts this: no Na I D absorption is detected (3 $\sigma$ upper limit EW < 0.53 Å), and the SN is located 6.8 kpc from the center of a passive elliptical galaxy, where interstellar extinction is expected to be minimal.
- The authors propose that the observed reddening and faintness may be influenced by circumstellar material (CSM) or intrinsic effects, rather than standard interstellar dust.
Spectroscopy:
- Optical spectra show strong Si II, Ca II, and Ti II features, consistent with a cool photosphere.
- NIR spectra display Ca II NIR triplet, Fe II, and Co II absorptions. Notably, the characteristic $H$ -band break (emission feature) seen in other 91bg-like SNe is absent, though the authors note this could be due to signal-to-noise limitations.
Model Comparison: The sub-Chandrasekhar SCH2p0 model combined with MW-type CSM extinction provides the best match to the observed light curves, particularly in the blue bands. Discrepancies remain in the $i$ -band, which the authors attribute to limitations in the model's spectral energy distribution in the red/NIR regime (e.g., line blanketing effects).
Hubble Constant: Using SN 2023vjh as a distance indicator yields $H_0 = 71.56 \pm 3.09$ km s $^{-1}$ Mpc $^{-1}$ (Tripp method) and $H_0 = 74.60 \pm 2.22$ km s $^{-1}$ Mpc $^{-1}$ (Color method). These values are consistent with recent determinations from normal SNe Ia and other fast-declining samples.

Significance and Claims
The paper claims that SN 2023vjh is a distinctive case within the 91bg-like subclass. Its primary significance lies in the tension between its photometrically inferred high reddening and the lack of spectroscopic evidence for host-galaxy extinction. The authors conclude that SN 2023vjh is systematically fainter than typical 91bg-like SNe and explosion models. They suggest that while circumstellar material (CSM) could explain the extra reddening and faintness, the residual discrepancies in the $i$ -band highlight limitations in current explosion models regarding the red/NIR spectral energy distribution.

The study reinforces the utility of fast-declining SNe Ia as distance indicators when appropriate standardization methods (like the Color method) are applied, even for objects with peculiar properties. However, the authors remain modest regarding the definitive nature of the extinction source, stating that the contribution of CSM "cannot be excluded" but requires further investigation, and that the faintness may also reflect intrinsic properties or model inadequacies.

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