Dust without Na I D Trace: The Case of Highly Attenuated Type Ib SN 2024vjc

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 Ib) SN 2024vjc, followed for ∼130 days postexplosion. SN 2024vjc exhibits only weak Na I D absorption, indicating only a modest host attenuation with E ( B − V ) 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 Type Ib supernovae. 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 H ii region at the explosion site; this scenario is directly supported by the detection of narrow H α , [N ii ], and H β emission lines and a blue continuum excess in late-time spectroscopy. SN 2024vjc represents a clear counterexample to the common assumption that weak or absent Na I D absorption implies negligible host-galaxy attenuation, and highlights the importance of employing multiple, independent attenuation diagnostics.