Probing the Beginnings of Planetary Birth in the Age of ALMA
The Atacama Large Millimeter Array (ALMA) has begun operations in the high desert of northern Chile offering unprecedented spatial resolution and sensitivity within the millimeter and sub-millimeter atmospheric windows. The study of planet birth is one of the key science areas enabled by ALMA due to the ability to resolve both gas and dust emission within the planet formation zones of young gas-rich circumstellar disks. This is highlighted by the fantastic high resolution image of HL Tau showing significant structure in the emission from pebbles within a young disk that is still accreting from its natal envelope. In this talk I will explore the related physics and chemistry of gas-rich disks and emphasize new breakthroughs in our understanding brought about by ALMA in concert with data from the Herschel Space Observatory. In particular I will report on the physical/chemical links in terms of snow-lines and the likely formation of pebbles and possibly planetesimals. Snow-lines represent chemical transitions (ice to vapor) in the disk and have long been posited as favorable sites for planet formation. With ALMA we have now directly and indirectly resolved the carbon monoxide snow-line in several disk systems. I will present these data and show compelling new evidence that grain growth is fostered at these locations, perhaps giving rise to the fantastic structure seen in HL Tau. Furthermore the formation of ice-coated pebbles in the increasingly dust rich midplane must deplete the upper layers, and due to radial drift, the outer disk of key ices that carry C, H, O, N. We will show that there is strong evidence for missing volatiles in the disk surfaces layers of the nearest disk system (TW Hya) with an apparent radial gradient in the carbon to oxygen content in the gas and solids. This elemental abundance gradient will likely be imprinted within the atmospheres of forming gas giants and sets constraints on the location of the volatile reservoir needed to form habitable terrestrial worlds.