Revisiting the case of R Monocerotis: Is CO removed at R < 20 au?.

Context: To our knowledge, R Mon is the only B0 star in which a gaseous Keplerian disk has been detected. However, there is some controversy about the spectral type of R Mon. Some authors propose that it could be a later B8e star, where disks are more common.

Aims: Our goal is to re-evaluate the R Mon spectral type and characterize its protoplanetary disk.

Methods: The spectral type of R Mon has been re-evaluated using the available continuum data and UVES emission lines. We used a power-law disk model to fit previous CO 1→0 and 2→1 interferometric observations and the PACS CO data to investigate the disk structure. Interferometric detections of CO J=1→0, HCO 1→0, and CN 1→0 lines using the IRAM Plateau de Bure Interferometer (PdBI) are presented. The HCN 1→0 line was not detected.

Results: Our analysis confirms that R Mon is a B0 star. The disk model compatible with the CO 1→0 and 2→1 interferometric observations falls short of predicting the observed fluxes of the 1431 CO lines suggest the existence of a region empty of CO at R≲20 au in the proto-planetary disk. The intense emission of the HCO and CN lines shows the strong influence of UV photons on gas chemistry.

Conclusions: The observations gathered in this paper are consistent with the presence of a transition disk with a cavity of R ≳20 au around R Mon. This size is similar to the photoevaporation radius that supports the interpretation that UV photoevaporation is main disk dispersal mechanism in massive stars.