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

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 1431 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. 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.

High spatial resolution imaging of SO and H2CO in AB Auriga: The first SO image in a transitional disk.

CONTEXT: Transitional disks are structures of dust and gas around young stars with large inner cavities in which planet formation may occur. Lopsided dust distributions are observed in the dust continuum emission at millimeter wavelengths. These asymmetrical structures can be explained as being the result of an enhanced gas density vortex where the dust is trapped, potentially promoting the rapid growth to the planetesimal scale. AIMS: AB Aur hosts a transitional disk with a clear horseshoe morphology which strongly suggests the presence of a dust trap. Our goal is to investigate its formation and the possible effects on the gas chemistry. METHODS: We used the NOEMA (NOrthern Extended Millimeter Array) interferometer to image the 1mm continuum dust emission and the 13CO J=2 →1, C18OJ=2 →1, SO J=56 →45, and H2CO J=303 →202 rotational lines. RESULTS: Line integrated intensity ratio images are built to investigate the chemical changes within the disk. The I(H2CO J=303 →202)/I(C18O J=2→1) ratio is fairly constant along the disk with values of ~0.15±0.05. On the contrary, the I(SO J=56 →45)/I(C18O J=2 →1) and I(SO J=56 →45)/I(H2CO J=303 →202) ratios present a clear northeast-southwest gradient (a factor of 3-6) with the minimum towards the dust trap. This gradient cannot be explained by a local change in the excitation conditions but by a decrease in the SO abundance. Gas densities up to ~109 cm-3 are expected in the disk midplane and two-three times larger in the high pressure vortex. We have used a single point (n,T) chemical model to investigate the lifetime of gaseous CO, H2CO, and SO in the dust trap. Our model shows that for densities >107 cm-3, the SO molecules are depleted (directly frozen, or converted into SO2 and then frozen out) in less than 0.1 Myr. The lower SO abundance towards the dust trap could indicate that a larger fraction of the gas is in a high density environment. CONCLUSIONS: Gas dynamics, grain growth and gas chemistry are coupled in the planet formation process. We detect a chemical signature of the presence of a dust trap in a transitional disk. Because of the strong dependence of SO abundance on the gas density, the sulfur chemistry can be used as a chemical diagnostic to detect the birthsites of future planets. However, the large uncertainties inherent to chemical models and the limited knowledge of the disk's physical structure and initial conditions are important drawbacks.