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

¹ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
e-mail: t.alonso@oan.es, a.fuente@oan.es
² Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), 28049 Cantoblanco, Madrid, Spain
e-mail: pablo.riviere@csic.es
³ Departamento de Astrofísica, Centro de Astrobiología (CAB, CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, Villanueva de la Cañada, 28692 Madrid, Spain
⁴ Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

Received: 27 July 2017
Accepted: 20 May 2018

Abstract

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 ≲ 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 ≳ 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