By Andrea Banzatti, Postodctoral Fellow at STScI
Antoine de Saint-Exupéry, the world-famous writer of The Little Prince, served as an aviator for the French Aéropostale over the Paris-Dakar route, crossing the African desert for many years in the 1930s. In his book Wind, Sand and Stars he reports of having once perceived the forthcoming conflagration of a sand storm before taking off from an intermediate station in the Sahara, by observing the peculiar behavior of two dragonflies. “What filled me with joy was that I had understood a murmured monosyllable of this secret language, that I had been able to read the anger of the desert in the beating wings of a dragonfly.” This joy is not unknown to the astrophysicist. Here is a story.
Over the last year, I happened to be working on a survey of infrared emission spectra of carbon monoxide (CO) observed in young “protoplanetary” disks, the birthplaces of the plethora of exoplanets detected so far. The CO molecule is generally abundant in planet-forming regions, at disk radii comprised to within approximately 10 Astronomical Units (AU) from the central star. CO had been observed in disks for over thirty years , and recent instrumental developments had made possible to perform a survey of unprecedented sensitivity, spectral resolution, and sample size in the years 2007-2010 . While studying the peculiar flickering behavior of CO and water emission from the disk of a variable star, I noticed that the CO spectra looked like the superposition of two emission line components, one being distinctly broader than the other . I attempted a spectral decomposition analysis, encouraged by the exquisite quality of the data, and found that while many protoplanetary disks showed both CO components, some had only the narrow one . By measuring the temperature (from the line flux ratios) and the disk radius of CO emission (from the line widths) in each disk of the survey, I composed the diagram shown below. When I and Klaus Pontoppidan, my collaborator and mentor, looked at it, we were astonished by the appearing of a sequence.
Figure (click to enlarge): The temperature-radius (T-R) diagram of rovibrational CO emission in disks . The red and blue data points are individual disks from high quality, high spectral resolution surveys done with CRIRES at the VLT (resolving power of ~100,000) [2,5]. The sample spans a range in stellar masses of 0.5-3 solar masses (indicated by the symbol size). The location of each disk in the diagram indicates the vibrational temperature of the innermost CO gas present in the disk. At the bottom of the figure, for comparison, are shown the Solar System planets, together with the distribution of semi-major axes of observed exoplanets with Msini > 0.5 Jupiter masses .
Given its high dissociation temperature, CO traces the innermost disk radius where molecular gas can survive in any disk. Therefore, the location of each disk in the diagram indicates the temperature of the innermost molecular gas present in its planet-forming region. The red disks in the diagram are those found to have two CO components and are identified as “primordial”, where the inner radius is set by the stellar magnetospheric accretion or by dust sublimation (truncating the disk out to ~0.1 AU at most for the whole sample). Blue disks lack the broad CO component, and have something else going on preventing CO gas from extending all the way to the smallest distance allowed by the stellar properties…
As the CO emission analyzed here is rovibrational, the measured line ratios give a vibrational temperature, which is a sensitive thermometer of the local radiation field. The temperature-radius (T-R) diagram, taken as a whole, reveals a sequence composed of two regimes. In the inner 0.03-2 AU the temperature decreases as a power-law profile, as expected for the dust temperature in models of inner disks irradiated by the central star. This regime is identified as due to infrared pumping of CO by the local warm dust, and provides an empirical temperature profile for inner disks around solar-mass stars. The second regime takes over beyond ~2 AU, and shows an inversion in the temperature. This temperature inversion strongly points at another excitation mechanism that is known to effectively populate CO lines in low-density and cold environments: ultraviolet (UV) fluorescence . In order for UV radiation to be effective at such large distances from the central star (2-20 AU), the innermost region of these disks must be largely depleted in both dust grains and molecular gas. These disks must host large inner gaps in their radial structure. Overall, CO emission suggests that all blue disks are developing or have developed large inner gaps, and some of them (filled symbols in the figure) have already been identified as “transitional” by dust emission modeling or by direct imaging. The T-R diagram has the power to provide prime targets for direct imaging campaigns, pushing inward the detection of inner gaps to radii that will become accessible to future infrared imagers (e.g. by E-ELT-METIS ).
But the best is yet to come. This research provides an empirical framework to investigate gap-opening processes in disks, including planet formation and migration. Comparison of the CO temperature sequence to the distribution of giant exoplanets detected so far reveals two interesting facts. The so-called “hot”-Jupiters are found at the innermost radial location of CO gas in disks, ensuring that abundant gas is present to allow gas-supported planet migration as proposed by models . The distribution of exo-Jupiters, instead, rises at the break point between the two regimes in the CO diagram, supporting the existence of a link between exo-Jupiters formation and the opening of gaps in the natal disks , which eventually leads to their dispersion through the “debris disk” phase . The journey of an exoplanet from its birth is long and can be full of surprises, ending up in the large diversity suggested by the foremost research in planetary architectures and compositions [e.g. 12]. And for us, at the horizon, stands the possibility of finding something similar to what we know here on Earth, a journey that is breathtaking for our entire world. It may still be far ahead in time, but every word we catch of this secret story of nature is welcomed with joy by those who spend their lives aspiring to hear it in full. Sometimes, these words are found in the most unexpected data, or in a diagram composed almost by chance. Sometimes, we can understand a murmured monosyllable of this secret language simply by “following the wings of a dragonfly”.