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

We present observations of the 1991bg-like type Ia supernova (SN Ia) 2023vjh associated with the elliptical galaxy MCG+04-10-013. Its light-curve shape parameters (Δ m_ 15 (B)=1.89 ± 0.01 mag and s_ BV = 0.45 ± 0.03) together with its spectroscopic evolution place it firmly within the class of fast-declining subluminous SNe Ia. The near-peak spectra show prominent features of , , and consistent with a cool photosphere and place SN 2023vjh within the `cool' and extremely cool regions on the classification diagrams. In addition, the three late-phase near-infrared (NIR) spectra display the NIR triplet, , and absorptions but no obvious H-band break. Although SN 2023vjh falls in the same regions of the classification diagrams as other well-studied 91bg-like events, it shows some deviation within this class. In particular, it is systematically fainter than predicted by explosion models and fainter than other well-studied 91bg-like SNe. Light-curve fitting and colour-based analyses indicate a relatively large reddening (E(B-V)_ Si II Ca II Ti II Ca II Fe II Co II host ∼ 0.2 – 0.35 mag), which is unusual for 91bg-like SNe. Yet the lack of detectable Na I D absorption in its spectra, along with its large projected distance from the centre of its passive host galaxy (6.8 kpc), suggests that interstellar extinction along the line of sight is minimal. SN 2023vjh appears to be fainter than typical 91bg-like SNe, and it could be affected by circumstellar material (CSM). Comparisons with explosion models and alternative extinction prescriptions show that including CSM-like extinction improves agreement in the blue bands, but residual discrepancies in the i-band may reflect limitations in the model.