By Andrea Banzatti, Postdoctoral Fellow at STScI
As the number of confirmed (exo)planets grows rapidly, infrared spectroscopy is providing an exceptional opportunity to study the molecular environments where planets are being formed. The Spitzer Space Telescope recently revealed a dense forest of emission lines from water, OH, and some organic molecules (Figure 1) tracing warm/hot gas (200 < T < 1000 K) in young protoplanetary disks inward of the water snow line (the condensation/evaporation boundary between gas and ice in the disk)1. This “steam” emission offers a unique observational link to a variety of processes ongoing inside planet factories.
Figure 1 : Infrared emission from water and other gas molecules in the planet formation region of a young protoplanetary disk (around RNO90, a G5 star), as observed with the Spitzer Space Telescope. All the prominent emission features, apart from those labelled differently, are due to water vapor (a model2 of water emission is superimposed in blue).
Analyzing the rich infrared molecular emission, we recently had the opportunity to investigate its connection to disk evolution and planet formation processes. By comparing infrared spectra of young solar-mass stars, taken at different phases of their accretion of circumstellar material, I and my collaborators found that strong accretion outbursts are able to dramatically affect the molecular content at planet-forming radii in the disk3. An increased heating causes a recession of the snow line to larger disk radii, probably triggering evaporation of water ice, while a harsher UV radiation photodissociates water vapor producing OH (as seen in the infrared spectrum of EX Lupi during a strong recent outburst, see Figure 2). The fate of organic molecules, which disappear during outburst, remains unclear. If accretion outbursts are ubiquitous in star formation, the evolution of material in the planet formation region may be strongly linked to the accretion histories, probably affecting also the chemical/physical properties of forming solid bodies.
Figure 2 : Changes in molecular emission from the disk of EX Lupi during a strong accretion outburst (from Banzatti et al. 2012). Water and OH emission increases, in connection to a recession of the snow line and a stronger UV radiation. The emission from organic molecules instead disappears.
Infrared spectroscopy of planet formation regions also allows us to study the migration of icy solids in the midplane of disks. As icy dust grains stick together in the outer disk, they form larger particles that at some point decouple from the gas and are dragged toward the star. When they reach and cross the snow line, the ice is evaporated providing large abundances of water vapor in the inner disk, unless forming planets accrete them at outer radii4. Evidence for ongoing ice migration is provided by inner disks where the water vapor abundance exceeds the oxygen budget available to form it in situ, while “drier” disks may be advanced in depleting the outer disk from migrators, or have already formed large accreting planetesimals outward of the snow line. In a recent work5, we have shown how a rotation diagram analysis of infrared water vapor emission offers a useful tool to distinguish between these two scenarios, from the spread of the rotational scatter (Figure 3). Such studies of water vapor emission are lifting the veil on processes taking place in disk midplanes that have been until now elusive to our observations.
Figure 3 : Rotation diagram analysis of infrared water vapor emission in protoplanetary disks (from Banzatti et al. 2013). Line opacities are color-coded in blue, while dot sizes are proportional to line intensities. The larger the amount of water vapor in the inner disk, the larger the spread of the rotational scatter in the diagram. The plot to the right shows the case of an inner disk water abundance larger than the oxygen budget available in situ, providing evidence for enrichment from inward icy migrators that evaporate at the snow line.
The number of protoplanetary disks and of molecular emission lines from planet formation regions observed with Spitzer is by far the largest provided by any other telescope to date. This unique dataset has already given the opportunity for pioneering studies of the properties and evolution of the molecular environments during planet formation. Yet, it is likely that we are just scratching the surface. These studies offer us a fertile ground for planning observations with the upcoming James Webb Space Telescope, which promises to narrow the gaps in our understanding of how planets form and to bring us closer to deciphering planet diversity, so to understand better even our own Earth.
- Carr & Najita 2008 (Science, 319, 1504); Salyk et al. 2008 (ApJ, 676, L49); Pascucci et al. 2009 (ApJ, 696, 143); Pontoppidan et al. 2010 (ApJ, 720, 887), 2014 (PPVI chapter)
- Pontoppidan et al. 2009 (ApJ, 704, 1482)
- Banzatti et al. 2012 (ApJ, 745, 90), 2014 (ApJ, 780, 26)
- Ciesla & Cuzzi 2006 (Icarus, 181, 178)
- Banzatti 2013 (PhD thesis)