O stars in transition .1. Optical spectroscopy of Ofpe/WN9 and related stars

Ofpe/WN9 stars have become one of the keys for understanding the physics of very strong stellar winds and their consequences for the evolution of massive stars. This paper is the first part of a comprehensive effort to understand the role of Ofpe/WN9 stars in the uppermost part of the Herztprung-Russell diagram. First, we report the results of a complete literature search on aspects most relevant to the goal of our study. This provides a database which will be used for the interpretation of our observational and theoretical results. Second, we present a complete dataset of new ground-based data. We present here high S/N, high spectral resolution echelle observations of seven Ofpe/WN9 stars and six closely related stars, with the objective to establish similarities and differences between individual objects and stellar groups, in an attempt to shed light on the true nature of Ofpe/WN9 stars. Our findings show that the Ofpe/WN9 stars form a homogeneous group, in terms of spectral distribution and wind properties, with characteristics somewhere in between the Of stars on one hand (with the weakest winds) and the B [e] stars on the other. We also find that, at least for the Of-Ofpe/WN9 stars, this progression corresponds to an evolutionary sequence. In fact, in two of the observed Of stars in our sample (HDE 313846 and HD 152408) the appearance of P Cygni profiles and strengthened emission components in both H and He I lines seems to suggest they are in a transition phase between Of and Ofpe/WN9. We also find that five of the Ofpe/WN9 star spectra show the presence of nebular emission lines, indicating a surrounding nebulosity. We examine all the cases individually and conclude that only for S119 and BE 381 is there a clear case for an expanding shell associated with the star. This would strengthen the connection between Ofpe/WN9 stars and luminous blue variables, which also are often surrounded by circumstellar nebulae generated during one or more ''violent outbursts'' in their recent evolution history.