HCO+, CN, and 13CO maps of R Mon

Interferometric detections of ^13^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. Our analysis confirms that R Mon is a B0 star.

The disk model compatible with the ^12^CO 1-0 and 2-1 interferometric observations falls short of predicting the observed fluxes of the 14<Ju<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 Ju>31 CO lines suggest the existence of a region empty of CO at R<=20au in the proto-planetary disk.

The intense emission of the HCO^+^ and CN lines shows the strong influence of UV photons on gas chemistry. The observations gathered in this paper are consistent with the presence of a transition disk with a cavity of Rin>=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