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

T. Alonso-Albi, P. Riviere-Marichalar, A. Fuente, S. Pacheco-Vázquez, B. Montesinos, R. Bachiller and S. P. Treviño-Morales. 2018. Revisiting the case of R Monocerotis: Is CO removed at R < 20 au?. Astronomy and Astrophysics 617, DOI: 10.1051/0004-6361/201731658

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)-C-12 1 -> 0 and 2 -> 1 interferometric observations and the PACS CO data to investigate the disk structure. Interferometric detections of (CO)-C-13 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)-C-12 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 less than or similar to 20 au 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 greater than or similar to 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.

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