Gaia Asteroid Phase Curve Catalog (GAPC) — HG1G2 Parameters for 128,885 Main-Belt Asteroids from Gaia DR3

The Gaia Asteroid Phase Curve Catalog (GAPC) provides HG1G2 phase curve parameters — absolute magnitude H, slope parameters G1 and G2 — for 128,885 main-belt asteroids derived from Gaia DR3 sparse photometry (sso_observation table, Galluccio et al. 2022). Method Gaia G-band single-epoch photometry was reduced to standard geometry (V_r = G − 5 log₁₀(r_h · r_geo)), corrected to Johnson V using taxonomy-dependent color terms, and fitted per object using the HG1G2 photometric system (Muinonen et al. 2010). Objects with fewer than 5 observations or a phase angle coverage below 5° were excluded. Fits were performed using sbpy 0.6 (HG1G2.from_obs with SLSQPLSQFitter) with a scipy.optimize fallback. The catalog was cross-matched with ten external datasets: MPC orbital elements, NEOWISE albedos (Masiero et al. 2011, Mainzer et al. 2019), SDSS MOC4, Bus-DeMeo PDS spectral taxonomy, LCDB rotation periods (Warner et al.), Pravec binary flags, DAMIT shape model flags, Goffin (2014) masses/densities, Milani proper elements, and the GASP spectral catalog (Scheibenpflug 2026). Taxonomy was refined using a four-tier cascade: PDS spectral classification → SDSS a* threshold → NEOWISE E/M/P albedo split → Random Forest fallback, reaching 96.8% coverage. Catalog contents 128,885 asteroids with successful HG1G2 fits 162 columns including H, G1, G2, fit uncertainties, n_obs, phase range, chi²_reduced, diameter estimates, albedos, taxonomy, rotation periods, family membership, and all cross-matched external parameters 9,748 objects additionally cross-matched with Gaia DR3 reflectance spectra (GASP) Key results The slope parameter G shows a universal size dependence (Spearman ρ(G, log D) = −0.294, p ≈ 0) that is composition-independent: the partial correlation r(G, log D | log pV) ≈ −0.28 is statistically indistinguishable across S-, M-, E-, P-, and C-type asteroids, and confirmed within individual asteroid families (Flora, Koronis, Eunomia). Albedo has no residual predictive power for G after controlling for size. Gaia DR3 reflectance spectra (16-band) cannot predict G beyond taxonomy and size (RF R² ≈ 0), suggesting G encodes sub-visual surface properties (grain size, regolith depth) not captured by optical spectroscopy. Files File Description gapc_catalog_v8.parquet Main catalog (Parquet, snappy compression) gapc_catalog_v8.csv.gz Same catalog as gzip-compressed CSV README.md Column descriptions and usage notes pipeline/ Full reproducible pipeline (Python, MIT license) Validation 75/75 independent verification checks passed (see verify_all.py). H magnitudes validated against MPC and NEOWISE with Pearson r = 0.810. Code Pipeline source code: https://github.com/esoxconsult/gapc (MIT license) Predecessor catalog GASP (Gaia Asteroid Spectral Pipeline): https://doi.org/10.5281/zenodo.19366681