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.