2018A&A...617A..31A

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 ¹²CO 1-0 and 2-1 interferometric observations and the PACS CO data to investigate the disk structure. Interferometric detections of ¹³CO 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 ¹²CO 1-0 and 2-1 interferometric observations falls short of predicting the observed fluxes of the 14<J_u<31 PACS lines; this is consistent with the scenario in which some contribution to these lines is coming from a warm envelope and/or UV-illuminated outflow walls. More interestingly, the upper limits to the fluxes of the J_u>31 CO lines suggest the existence of a region empty of CO at R≥20au in the protoplanetary 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_in≥20au 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