2026/07/26 by Javier Silva-Farfán, Keiichi Maeda, Andrea Pastorello +19
#astro-ph.HE
Understanding dust attenuation toward extragalactic transients is critical for recovering their intrinsic properties and probing the local environments of distant galaxies. A popular diagnostic is the Na I D absorption equivalent width widely applied to extragalactic transients. In this paper, we present early-time optical and near-infrared observations of the Type Ib supernova (SN) SN 2024vjc, followed for ∼130 days post-explosion. SN~2024vjc exhibits only weak Na~I~D absorption, indicating only a modest host attenuation with E (B-V)\rm host ∼ 0.18 mag; if this argument were applied, SN~2024vjc would be a peculiar, faint, and red SN Ib while showing the light-curve shape and spectral evolution broadly consistent with those of canonical SNe Ib. We show that this is not the case; SN-based diagnostics (intrinsic color templates, color-curve evolution, and spectral dereddening) together with the Balmer decrement indicate substantial attenuation, E(B-V) ∼ 0.45 - 0.8 mag. The weak Na I D absorption may result from photoionization of neutral sodium by the intense radiation field of a young H2 region at the explosion site; this scenario is directly supported by the detection of narrow Hα, [N2], and Hβ emission lines and a blue continuum excess in late-time spectroscopy. SN~2024vjc represents a clear counterexample to the commonly assumption that weak or absent Na I D absorption implies negligible host-galaxy attenuation, and highlights the importance of employing multiple, independent attenuation diagnostics.