The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks

Abstract

A snow-line is the region of a protoplanetary disk at which a major volatile, such as water or carbon monoxide, reaches its condensation temperature. Snow-lines play a crucial role in disk evolution by promoting the rapid growth of ice-covered grains 1−6. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged with in the disks surrounding the pre-main-sequence stars TW Hydra 7−9 and HD163296 [3,10] , at distances of about 30 astronomical units (au) from the star. But the water snow-line of a protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto been seen, as it generally lies very close to the star (less than 5 au away for solar-type stars 11). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation 5 , and the formation of comets, ice giants and the cores of gas giants 12. Here we report ALMA images at 0.03-arcsec resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an outburst in luminosity arising from a temporary increase in the accretion rate 13. We find an intensity break corresponding to an abrupt change in the optical depth at about 42 au, where the elevated disk temperature approaches the condensation point of water, from which we conclude that-2-the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions 14 : dust fragmentation and the inhibition of grain growth at higher temperatures results in soaring grain number densities and optical depths. As most planetary systems are expected to experience outbursts caused by accretion during their formation [15,16] our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation. V883 Ori is an FU Ori object identified as such by [17] from followup spectroscopy of deeply embedded sources from the Infrared Astronomical Satellite (IRAS). It is located in the Orion Nebula Cluster, which has a distance of 414±7 pc [18]. It has a disk mass of 0.3 M and a bolometric luminosity of 400 L [19]. We have obtained 230 GHz/1.3 mm (band-6) observations of V883 Ori using the Atacama Large Millimeter/submillimeter Array (ALMA) in four different array configurations with baselines ranging from 14 m to 12.6 km, which were taken in ALMA Cycle-2 and Cycle-3. These new ALMA observations include continuum and the 12 CO, 13 CO, and C 18 O J = 2-1 spectral lines. We use the C 18 O gas line to investigate the dynamics of the system at 0.2 (90 au) resolution and the continuum data to constrain the physical properties of the dust in the V883 Ori disk at 0.03 (12 au) resolution. In Figure 1 (top panel) we show our Cycle-3 continuum image at 0.03 resolution, the highest resolution ever obtained for a FU Ori object at millimeter wavelengths. We find that the V883 Ori disk has a two-region morphology, with a very bright inner disk (r ∼ 0.1 , 42 au) and a much more tenuous outer disk...

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