The IACOB project XV. Updated calibrations of fundamental parameters of Galactic O-type stars

Modern spectroscopic surveys combined with Gaia distances are enabling reliable estimates of fundamental parameters for hundreds of Galactic O-type stars and the full range of spectral types and luminosity classes. Here we provide updated, statistically robust empirical calibrations of the fundamental parameters of Galactic O-type stars, as well as of their absolute visual magnitudes (Mv) and bolometric corrections (BC), based on high-quality observational data. We perform a homogeneous analysis of a sample of 358 Galactic O-type stars, combining high-resolution spectroscopy and Gaia distances. A subset of 234 stars meeting strict quality criteria involving parallax, extinction, and multi-band photometry was used to derive empirical calibrations of fundamental parameters. For those same stars, calibrated parameters were estimated from their measured Mv using the derived relations, allowing us to assess the internal consistency and predictive power of the calibrations. We present updated spectral-type-based calibrations of fundamental parameters for luminosity classes V, III, and I. Compared to previous works, we find systematic shifts, particularly in effective temperature for dwarfs and in Mv across all classes, which propagate into derived quantities. Applying the Mv calibrations to the full sample yields consistent estimates of radius and luminosity, while spectroscopic mass (Msp) shows significant scatter. We also evaluate the FW3414 parameter (from the Hbeta line) as a calibrator for Mv, useful in large surveys lacking reliable spectral classification. Excluding SB1 systems has a noticeable impact only on the Msp calibration for LC V. These updated empirical calibrations offer a robust reference for Galactic O-type stars and will support studies of massive star populations in both Galactic and extragalactic contexts, particularly in the era of large spectroscopic surveys.