ABSTRACT
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 â1, C18OJ=2 â1, SO J=56 â45, and H2CO J=303 â202 rotational lines. RESULTS: Line integrated intensity ratio images are built to investigate the chemical changes within the disk. The I(H2CO J=303 â202)/I(C18O J=2â1) ratio is fairly constant along the disk with values of ~0.15±0.05. On the contrary, the I(SO J=56 â45)/I(C18O J=2 â1) and I(SO J=56 â45)/I(H2CO J=303 â202) 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.
ABSTRACT
Hot Jupiters are giant Jupiter-like exoplanets that orbit their host stars 100 times more closely than Jupiter orbits the Sun. These planets presumably form in the outer part of the primordial disk from which both the central star and surrounding planets are born, then migrate inwards and yet avoid falling into their host star. It is, however, unclear whether this occurs early in the lives of hot Jupiters, when they are still embedded within protoplanetary disks, or later, once multiple planets are formed and interact. Although numerous hot Jupiters have been detected around mature Sun-like stars, their existence has not yet been firmly demonstrated for young stars, whose magnetic activity is so intense that it overshadows the radial velocity signal that close-in giant planets can induce. Here we report that the radial velocities of the young star V830 Tau exhibit a sine wave of period 4.93 days and semi-amplitude 75 metres per second, detected with a false-alarm probability of less than 0.03 per cent, after filtering out the magnetic activity plaguing the spectra. We find that this signal is unrelated to the 2.741-day rotation period of V830 Tau and we attribute it to the presence of a planet of mass 0.77 times that of Jupiter, orbiting at a distance of 0.057 astronomical units from the host star. Our result demonstrates that hot Jupiters can migrate inwards in less than two million years, probably as a result of planetdisk interactions.
