Sep 162015
 

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 [1], and recent instrumental developments had made possible to perform a survey of unprecedented sensitivity, spectral resolution, and sample size in the years 2007-2010 [2]. 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 [3]. 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 [4]. 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.

TR_diagram

Figure (click to enlarge): The temperature-radius (T-R) diagram of rovibrational CO emission in disks [4]. 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 [6].

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 [7]. 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 [8]).

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 [9]. 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 [10], which eventually leads to their dispersion through the “debris disk” phase [11]. 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”.

 

References:

  1. Najita et al., 2000
  2. Pontoppidan et al., 2011
  3. Banzatti et al. 2015
  4. Banzatti & Pontoppidan, 2015
  5. van der Plas et al., 2015
  6. exoplanet.org
  7. Brittain et al. 2007
  8. Bradl et al. 2010
  9. Kley & Nelson, 2012
  10. Zhaohuan et al. 2011
  11. Wyatt et al. 2015
  12. Howard, 2013
Dec 022014
 

By Steve Lubow, Observatory Scientist at STScI

The orbits of a particle about a central point mass are well known and have analytic solutions that were determined by Newton in 1687. In the case that a second point mass is present, forming a binary system, the particle orbits are much more complicated and simple analytic descriptions are possible for only special cases, such as the famous Lagrange points. This so-called three-body problem has been the subject of many studies for several centuries by Euler, Lagrange, Poincare, and others. The three-body problem has many astronomical applications.

In the early 1960s,  Kozai and Lidov independently discovered a fairly general class of orbits in the three-body problem that has some peculiar properties. Kozai was motivated by his studies of asteroids that orbit under the influence of the Sun and Jupiter. Lidov was motivated by the analysis of orbits of artificial satellites at the beginning of the Russian space program. Consider a particle that orbits about a single central object whose motion is disturbed by a distant companion. One might think the effects of the companion do not matter much, since it is far away. However, they found that over time its effects can build up to produce strong changes in the particle’s orbit. They showed that a particle orbit that is initially circular and sufficiently inclined would undergo tilt and eccentricity oscillations. During the oscillations, the particle orbit eccentricities can become quite large for sufficiently inclined initial orbits. For example, for an initial orbit tilt of 60 degrees, an initially circular orbit reaches an eccentricity of about 0.75.

The key to understanding the Kozai-Lidov (KL) effect is that the vertical component (perpendicular to the binary orbital plane) of the particle’s angular momentum is nearly conserved, while the particle’s orbit plane undergoes tilt oscillations. As the orbit tilt evolves to a lower inclination angle over part of the oscillation cycle, its eccentricity must grow to maintain the same vertical angular momentum.

There have been many applications of this KL effect apart from these original motivations. These include triple star systems (Eggleton & Kiseleva-Eggleton 2001; Fabrycky & Tremaine 2007), extrasolar planets with inclined stellar companions (Wu & Murray 2003; Takeda & Rasio 2005), inclined planetary companions (Nagasawa et al. 2008), merging supermassive black holes (Blaes et al. 2002), stellar compact objects (Thompson 2011), and blue straggler stars (Perets & Fabrycky 2009).

Early in 2014, I was working with Rebecca Martin, a Sagan Fellow at the University of Colorado, and other collaborators on explaining some fluid (SPH) simulation results that she had obtained for Be star disks. The disks were taken to orbit about Be stars that have neutron star companions on an eccentric orbits, as suggested by observations. In addition, the simulations considered the disk to be initially circular and inclined with respect to the orbit plane of the binary. The simulations showed that the disk became substantially eccentric. We had initially thought that the disk eccentricity could be explained by some tidal effects that had been found in previous simulations and had been explained analytically. These effects were analyzed for a disk that is coplanar with the binary. For example, an eccentric binary has an eccentric component of its tidal potential that can induce eccentricity in a disk. We thought the latter was the most plausible explanation for the eccentricity, as we discussed in our paper (Martin et al 2014a).

However, I was suspicious of our explanation because I thought this tidal effect would become smaller with higher orbit inclination, while we found that eccentricity was larger at higher inclinations. So I suggested that Rebecca try numerically to determine the orbits of particles, rather than performing the computer-intensive SPH simulations. The result was that the particles underwent KL oscillations — obvious in hindsight. To confirm this result, Rebecca performed simulations with a circular orbit companion, for which the KL effect can operate, while the eccentric tidal effect would not. She found that the eccentricity growth was still present. We found that the disk undergoes coherent tilt oscillations much like a rigid body. The oscillation period agrees well with the expectations of KL theory. This is the first time that the KL effect has been found to operate on a fluid (Martin et al 2014b).

During tilt oscillations, the disk eccentricity gets fairly high, but it is reduced by disk dissipation.  After a few KL oscillations, the disk evolves to an eccentric state and at the critical minimum tilt angle for KL oscillations of about  40 degrees with respect to the binary orbit plane (see Figure 1). With Rice University postdoctoral fellow Wen Fu, we are exploring the range of parameters for which the KL effect operates on disks. Misaligned disks are likely common in young wide binary systems and so the KL effect may have an important influence on the evolution of some protoplanetary disks.

Fig 1Figure 1: Evolution of disk eccentricity and inclination in degrees as a function of time in binary orbital periods for a disk that orbits a member of an equal mass binary binary system that has an initial tilt of 60 degrees from the binary orbit plane and is initially circular.

The video (found at this link) shows the evolution of the disk in Figure 1 that undergoes KL oscillations viewed in different planes. The binary lies in the X-Y plane, while the disk is initially tilted by 60 degrees with respect to this plane. The disk precesses about the Z-axis. The disk eccentricity is apparent from the displacement of the central star from the disk center. Note that some disk mass is transferred to the companion as a consequence of the reduction of tidal forces at high inclination (Lubow et al 2014) and because the disk becomes eccentric.

References

  • Blaes, O., Lee, M. H., & Socrates, A. 2002, ApJ, 578, 775
  • Eggleton, P. P. & Kiseleva-Eggleton, L. 2001, ApJ, 562, 1012
  • Fabrycky, D. & Tremaine, S. 2007, ApJ, 669, 1298
  • Lubow, S. H., Martin, R. G., & Nixon,C. 2014, ApJ, submitted
  • Martin, R. G., Nixon,C., Armitage, P. J., Lubow, S. H., & Price, D. J. 2014, ApJ, 790, LL34
  • Martin, R. G., Nixon,C., Lubow, S. H., et al. 2014, ApJ, 792, LL33
  • Nagasawa, M., Ida, S., & Bessho, T. 2008, ApJ, 678, 498
  • Takeda, G. & Rasio, F. A. 2005, ApJ, 627, 1001
  • Thompson, T. A. 2011, ApJ, 741, 82
  • Wu, Y. & Murray, N. 2003, ApJ, 589, 605
  • Perets, H. B. & Fabrycky, D. C. 2009, ApJ, 697, 1048

 

Mar 032014
 

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.

RNO90_w

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.

EXLupi_changes

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.

Water_RD

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.

References:

  1. 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)
  2. Pontoppidan et al. 2009 (ApJ, 704, 1482)
  3. Banzatti et al. 2012 (ApJ, 745, 90), 2014 (ApJ, 780, 26)
  4. Ciesla & Cuzzi 2006 (Icarus, 181, 178)
  5. Banzatti 2013 (PhD thesis)