On the origin of field O-type stars

Aims. We aim to identify the origins of field O-stars in the nearest 2 to 3 kpc around the Sun using the best presently available kinematic data on O-stars and on young open clusters. We investigate the question of whether the present-day data are consistent with the assumption that O-stars have formed in groups (clusters, associations), or in isolation. Methods. We apply the epicycle theory to back-trace the orbits of O-type stars and of candidate parent open clusters. Results. From the 370 O-stars in the Galactic O star catalog v 2.0 (GOSV2) we have investigated 93 stars classified as field, and found the origin for 73 of them in 48 open clusters younger than 30 Myr. Only for 32 stars or about 9% of all O-stars from this catalogue is the question of their origin in groups not solved; some of them may have originated in isolation or may have disintegrated the group in which they formed. Fifty percent of the young open clusters (age < 30 Myr) in the Catalogue of Open Cluster Data (COCD) have O-stars as members, or have ejected at least one O-star in the first 10 Myr of their life, or both. During this period the average mass loss from open clusters by ejecting O-stars is found to be 3 to 5 M-circle dot per Myr. We prove that. Pup had its origin in the open cluster Trumpler 10 which it left about 2.5 Myr ago, and that its present-day distance is 300 pc (compared to 440 pc before). The revised distance implies a significant revision of the stellar parameters (a radius of 14 R-circle dot, a mass of 22.5 M-circle dot, and a luminosity of log L/L-circle dot of 5.74) i.e., zeta Pup is closer, less massive, and less luminous than previously thought. Our findings provide independent estimates of the present-day distances and absolute magnitudes of field O-stars.