The IACOB project

Context. O-type stars are key drivers of galactic evolution. For the first time, modern spectroscopic surveys combined with Gaia distances are enabling reliable estimates of the fundamental parameters of several hundred Galactic O-type stars, spanning the full range of spectral types and luminosity classes. Aims. We aim to 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, based on high-quality observational data. Methods. We performed 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 multiband photometry was used to derive empirical calibrations of fundamental parameters with respect to spectral type (SpT) and luminosity class (LC). We also evaluated the utility of the FW3414 parameter (based on the shape of the Hβ line profile) as a calibrator for MV, useful in large surveys lacking reliable spectral classification. As a consistency test, we compared radii, luminosities, and spectroscopic masses derived from two independent MV calibrations, one based on spectral classification and the other on FW3414, with their directly determined counterparts. Results. We present updated SpT-based calibrations of fundamental parameters for LCs V, III, and I. Compared to previous works, we find systematic shifts, particularly in effective temperature for dwarfs and in MV across all classes. Significant scatter in MV persists even with Gaia DR3 distances, which propagate into derived quantities. Applying the SpT-MV and FW3414-MV calibrations to the full sample yields consistent estimates of radius and luminosity, while the spectroscopic mass (Msp) shows significant scatter. Conclusions. 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.