2025ApJ...994..213C

Weak-line quasars (WLQs) represent a subset of type 1 quasars distinguished by remarkably weak high-ionization broad emission lines, yet exhibiting normal optical/UV continua. This study establishes a physically motivated definition of WLQs using 371,091 quasars from the Sloan Digital Sky Survey Data Release 16 catalog. By analyzing outliers in three key relations, the L_1350 Å_-C IV blueshift relation, the Baldwin effect, and the $\mathrm{log}{L}_{2500\,\,\unicode{x0212B}\,}-{\alpha }_{\mathrm{ox}}$ relation, we identify two critical thresholds in C IV equivalent width (EW): 8.9 ± 0.2 Å and 19.3 ± 0.3 Å. Quasars with EW(C IV) < 8.9 ± 0.2 Å are defined as WLQs, exhibiting enhanced C IV blueshifts, significant deviations from the Baldwin effect, and a high fraction of X-ray weak objects (nearly half of this population). Quasars with EW(C IV) > 19.3 ± 0.3 Å show normal quasar properties, while objects with intermediate C IV EW (8.9-19.3 Å) are defined as “bridge quasars,” showing transitional behaviors. Systematic analysis of emission-line attenuation of WLQs reveals a clear positive correlation between the attenuation factor and ionization energy, with high-ionization lines (HILs; e.g., He II, C IV) suppressed by factors of ∼3σ-4σ compared to low-ionization lines (LILs; e.g., Mg II, O I). This ionization-stratified attenuation supports the shielding gas model, where geometrically thick inner accretion disk obscures high-energy photons, suppressing HIL emission, while the LILs remain less affected. Based on this accretion-disk geometry, we argue that WLQs and normal quasars defined in our framework generally correspond to the slim disk and standard thin disk regimes, respectively, while the bridge quasars represent a transitional phase between the two states. This work establishes a unified observational criterion for WLQs and discusses their physical implications, highlighting the role of accretion-driven shielding gas in shaping their unique spectral features.