Issue 12 - October 2013
By Wes Traub The “snow line” is an almost poetical term that tells us where it gets cold enough in a protoplanetary disk for gas molecules to freeze out as “snow.” In a recent Atacama Large Millimeter/submillimeter Array (ALMA) observation, the snow line for CO was found by the indirect trick of imaging N2H+, and noticing that in the frozen phase of CO the N2H+ remains as a gas, whereas in the gas phase of CO there is very little N2H+ because the species react, depleting the latter. Thus, the observed ring of N2H+ is a marker for frozen CO. See http://www.sciencemag.org/content/341/6146/630.full.pdf for the paper by Qi et al. A crescent of millimeter-size grains was found around IRS 48 at a radius of about 55 AU, even though the gas and micron-size dust appear in full ring-type structures. The authors explain this peculiar clumping of large particles by positing a 10-Jupiter-mass planet at about 20 AU. The paper, by van der Marel et al., with these ALMA observations and a model of the system, is at http://www.sciencemag.org/content/340/6137/1199.full.pdf. On the other hand, Lyra and Kuchner http://www.nature.com/nature/journal/v499/n7457/full/nature12281.html show that it is possible to form “sharp eccentric rings in debris disks with gas but without planets.” This mechanism provides a counterexample to the common model of a planet shepherding disk material into a ring-like structure, and makes it all the more challenging for observers to interpret their data. We continue to like the science fiction articles in the back of Nature every week. Here is another, by Gregory Benford, that the planet-characterization types among us might enjoy: http://www.nature.com/nature/journal/v499/n7458/full/499374a.html. Finally, to show that exoplanets are still high on the list of topics that the greater public cares about, we are encouraged by an article in The Economist that briefly looks at the radial velocity and transit techniques, and even mentions direct imaging as being one of the “clever tricks” we can use -- all very encouraging for future users of Wide-Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets (WFIRST-AFTA). See http://www.economist.com/blogs/economist-explains/2013/06/economist-explains-8 for the article.