Re-visiting the case of R Mon: Is CO removed at R<20 au?
  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. We have re-evaluated the spectral type of R Mon using the
available continuum data and UVES emission lines. We used a power-law disk
model to fit previous 12 CO 1-0 and 2-1 interferometric observations and the
PACS CO data to investigate the disk structure. 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<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.
The observations gathered in this paper are consistent with the presence around
R Mon of a transition disk with a cavity about 20 au. This size is similar to
the photoevaporation radius that supports the interpretation that UV
photoevaporation is main disk dispersal mechanism in massive stars
