The apparent lack of massive O-type stars near the Zero Age Main Sequence, or ZAMS, (at ages <2Myr) has been a topic widely discussed in the last 40 years. Different explanations for the elusive detection of these young massive stars have been proposed from both the observational and theoretical side, but no firm conclusions have been reached yet. The aim of this work is to perform a reassessment of this empirical result benefiting from the high quality spectroscopic observations of (more than 400) Galactic O-type stars gathered by the IACOB and OWN surveys. We use effective temperatures and surface gravities resulting from a homogeneous, semi-automatized, IACOB-GBAT/FASTWIND spectroscopic analysis to locate our sample of stars in the Kiel and spectroscopic Hertzsprung-Russell (HR) diagrams. We evaluate the completeness of our magnitude limited sample of stars - as well as the existence of potential observational biases affecting the compiled sample - using information from the Galactic O star catalog (GOSC). We discuss limitations and possible systematics of our analysis methodology, and compare our results with other recent studies using smaller samples of Galactic O-type stars. We mainly base our discussion on the distribution of stars in the spectroscopic HR diagram in order to avoid the use of still uncertain distances to most of the stars in our sample. However, we also perform a more detailed study of the young cluster Trumpler-14 as an illustrative example of how Gaia cluster distances can help to construct the associated classical HR diagram. We find that the apparent lack of massive O-type stars near the zero-age main sequence with initial evolutionary masses in the range between ~30 and 70M_{sun} still persist despite using spectroscopic results from a large, non-biased sample of stars. We do not find any correlations between the dearth of stars close to the ZAMS and obvious observational biases, limitations of our analysis methodology, and/or the use of one example spectroscopic HR diagram instead of the classical one. Finally, by investigating the impact of the efficiency of mass accretion during the formation process of massive stars, we conclude that an adjustment of the mass accretion rate towards lower values than canonically assumed could reconcile the hotter boundary of the empirical distribution of optically detected O-type stars in the spectroscopic HR diagram and the theoretical birthline for stars with masses above 30M{sun}_. Last, we also discuss how the presence of a small sample of O2-O3.5 stars found much closer to the ZAMS than the main distribution of Galactic O-type star could be explained in the context of this scenario taking also into account the effect of non-standard star evolution (e.g. binary interaction, mergers, and/or homogeneous evolution).

Cone search capability for table J/A+A/638/A157/tablee1 (O-type stars considered in this work with spectroscopic parameters)