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 grains1,2,3,4,5,6. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged in the disks surrounding the pre-main-sequence stars TW Hydra7,8,9 and HD163296 (refs 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 stars11). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation5, and the formation of comets, ice giants and the cores of gas giants12. Here we report 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 rate13. 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 the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions14: 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 formation15,16, our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation.
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We thank the referees for their valuable comments. We also thank A. Banzatti and P. Pinilla for providing their model predictions in tabular form (Fig. 2d, e). ALMA is a partnership of the European Southern Observatory (ESO; representing its member states), the National Science Foundation (NSF; USA) and the National Institutes of Natural Sciences (Japan), together with the National Research Council (Canada) and the National Science Council and the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities Inc./National Radio Astronomy Observatory (NRAO), and the National Astronomical Observatory of Japan. The NRAO is a facility of the NSF, operated under cooperative agreement by Associated Universities. Support for this work was provided by the Millennium Science Initiative (Chilean Ministry of Economy), through grants RC130007 and IC120009. L.A.C., D.A.P., J.L.P. and C.C. acknowledge support from CONICYT FONDECYT grants 1140109, 3150550, 1151445 and 3140592, respectively. H.C. acknowledges support from the Spanish Ministerio de Economía y Competitividad under grant AYA2014-55840P. Our work made use of ALMA data available at https://almascience.eso.org/alma-data with the following accession numbers: 2013.1.00710.S and 2015.1.00350.S.
The authors declare no competing financial interests.
Nature thanks E. Bergin and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, During quiescence, the water snow-line around stars of Solar masses is located 5 au or less from the star, where the temperature of the disk reaches the sublimation point of water. b, During protostellar accretion outbursts, this line moves out to more than 40 au, where it can be detected. Outward of the snow-line, grain growth is promoted by the high coagulation efficiency of ice-covered grains (brown and blue concentric circles). Inward of this line, dust production is promoted by the high fragmentation efficiency of bared silicates (brown circles). This results in the observed break in the disk intensity profile, a steep reduction in the 1.3-mm dust opacity, and a sharp increase in the spectral index across the snow-line.
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Cieza, L., Casassus, S., Tobin, J. et al. Imaging the water snow-line during a protostellar outburst. Nature 535, 258–261 (2016). https://doi.org/10.1038/nature18612
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