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Project Progress

Co-I Hollenbach working with collaborator Gorti is modeling photoevaporation processes in protoplanetary disks around young stars. This year we continued the work examining the dispersing effects of EUV (Lyman continuum), FUV, and X-rays from the central star. This photoevaporation effect clears the disk and helps determine the likelihood of habitable planet formation around a star of a given mass. In the current reporting period we finished the extensive testing of our model, particularly with respect to the X-ray heating and ionization of the surface layers of gas, and the effects of coagulation and settling of the dust. We have used our model to analyze the observations of the nearest solar type star in the process of planet formation, TW Hya. We find that photoevaporation has already dispersed the gas from about 100 AU to 200 AU from the star, leaving behind large dust particles that could not be carried in the flow. Soon the gas will be dispersed inside of 100 AU, and the formation of gas giant planets will be truncated. The lack of gas will have major implications as well on the formation of terrestrial planets, and the characteristics of their orbits.
We tested aspects of our optically thin code that we were improving as we worked on the optically thick code by comparing our numerical models to observations by Spitzer Space Telescope of disks around nearby solar mass stars. We published these results. We demonstrated that no more that a small fraction of a Jupiter mass of gas remains in this sample of stars whose ages span 5 to 50 Myr. Thus the epoch of giant planet formation is over in these systems and there is too little gas to circularize the orbits of any Earth-like planets.

Gorti and Hollenbach are using the optically thick code to model the photoevaporation of young disks by the EUV, FUV, and X ray radiation from the central star. This work was described in Dullemond et al (2007) and Richling, Hollenbach, and Yorke (2006). As discussed in these papers, photoevaporation is being studied as a function of stellar mass in order to determine which mass star has the longest-lived disks and therefore the highest probability of forming planets. An additional outcome of these models is the prediction of emission lines from ionized species on the surface of the disk created by the EUV and FUV photons from the central star. One of the strongest emission lines originates from single ionized neon in the mid infrared, at a wavelength of 12.8 microns, and the Spitzer Space Telescope has now detected this line. Pascucci, Hollenbach et al (2007) discuss these observations and analyze the data with the models of Hollenbach and Gorti. One key conclusion is that this neon emission line is currently the most sensitive way to detect gas in the planet-forming region of a young star. More work is planned to exploit this sensitive probe of planet forming regions.

In recognition of the significance of this work, Hollenbach was invited to present review lectures at two major international conferences on planet formation in the reporting year. He lectured on “The Dispersal of Protoplanetary Disks and its Effect on Planet Formation” and Cambridge University on July 15, 2006 (at the Conference “The Planet-Disc Connection”), and on “Photoevaporation of Disks around Young Stars” at the University of Florida on April 11, 2007.

S. Davis and D. Richard investigated the thermodynamic, physical and chemical environment of protoplanetary disks. Gas and ice are very unevenly distributed in a protoplanetary disk and they are a key determinant in the search for habitable regions in extra solar planetary systems. The presence of gas and ice is widely variable due to the diverse thermodynamic environment in protoplanetary disks. A new condensation model is developed to account for the relative proportion of gas and ice at each point in the nebula and is reported in ApJ, 660, 1580 (2007). The method is based on computing the chemical evolution of selected abundant species until partial pressures are sufficiently high to de sublimate the gaseous species into ice. The point at which this occurs relative to its steady state values determines final gas/ice ratios. It is found that for the solar nebula the largest gradients occur near the spatial sublimation boundaries (“the snow line”). Depending on the volatility of the species, significant differences can occur in the inner high-density planet forming regions and the lower density photospheric regions. We found that, although ice dominates the mid and far nebula, water vapor is predominant in the centerplane region of the near nebula and above the disk photosphere. An interesting near nebula effect is the appearance of a cloud of water ice at the temperature inversion elevation surrounded by vapor above and below. Results from this new finding will aid in the interpretation of data from early extra-solar protoplanetary disks and determining which are most likely to support life.

D. Richard continues work on chemical-physical processes (Richard and Davis, 2006). Understanding the chemical evolution of protoplanetary disks is also a fundamental step towards understanding the formation and composition of planets and discriminating habitable zones that can shelter life. The chemical network has been studied to highlight the most important reactions. We determined that for most species only a small subset of reactions is necessary to describe the evolution of abundances. This work will help in future computational optimizations of the code. Transport by radial and vertical turbulent diffusion, as well as convection by global meridianal circulation have been implemented. Computations are ongoing to quantify the effect of each of these transport mechanisms on abundances.

J. Lissauer worked with collaborator Quintana to model the late stages of terrestrial planet formation within binary star systems. This year we completed and published a study of planetary growth around individual stars within binary systems in which the stellar periapse distances were 5 – 10 AU (Quintana et al. 2007). We calculated the late stages of terrestrial planet accumulation around a solar-type star that has a binary companion with semimajor axis significantly larger than the terrestrial planet region. We performed more than 100 simulations to survey binary parameter space and to account for sensitive dependence on initial conditions in these dynamical systems. As expected, sufficiently wide binaries leave the planet formation process largely unaffected. As a rough approximation, binary stars with periastron qB > 10 AU have a minimal effect on terrestrial planet formation within ~ 2 AU of the primary, whereas binary stars with qB <~ 5 AU restrict terrestrial planet formation to within ~ 1 AU of the primary star. Given the observed distribution of binary orbital elements for solar-type primaries, we estimated that about 40% to 50% of the binary population is wide enough to allow terrestrial planet formation to take place unimpeded. The large number of simulations allowed us to determine the distribution of results-the distribution of plausible terrestrial planet systems-for effectively equivalent starting conditions. We presented (rough) distributions for the number of planets, their masses, and their orbital elements in ApJ, 660, 807-822 (2007)
Co-I Laughlin has developed a hydrodynamical simulation code that can track the surface flow patterns on extrasolar planets under conditions of non-synchronous planetary rotation. The code is now well tested, and has been used to model a number of well-observed hot-Jupiter type planets (Langton & Laughlin 2007, ApJ, submitted). Significantly, the code can be used to model the atmospheric dynamics of newly discovered worlds, such as Gl 581 c, which lie at the boundary of potential habitability. This aspect of the research will be an important focus in the coming year.

Laughlin’s second goal has been to broaden public participation in research related to extrasolar planets. There is considerable public interest in worlds orbiting other stars, and we are leveraging this interest to our advantage. Using funds from our ongoing consortium, we have also produced a state-of-the-art software package – the Systemic Console — for analyzing the dynamics of planetary systems. The package has been developed from scratch by the PI, Aaron Wolf, Eugenio Rivera and Stefano Meschiari, and represents the primary current product arising from the grant. The package currently contains over 30,000 lines of code and incorporates a sophisticated graphical user interface linked to state-of-the-art few-body integrators. It is fully capable of providing interactive self-consistent fits to radial velocity datasets for an arbitrary number of interacting planets. In the past year, we have added considerable additional functionality to this software, including (1) long-term stability analysis, (2) F-test analyses of competing fits, (3) orbital waveform sonification (for aural evaluation of stability properties), (4) interactive system plotting, (4) Levenberg-Marquardt fit polishing, and (5) bootstrap-method calculation of uncertainties in the orbital elements of fits. The console is written in standard Java, and as such, it can be freely downloaded for immediate use on Windows, Mac OSX and Linux-based architectures. It has been successfully installed by thousands of users worldwide. Despite its ease of use, it is highly sophisticated and indeed is easily the world’s most advanced tool for extracting planetary systems from radial velocity data.

Public response to the systemic project (see www.oklo.org) has been tremendous. As of March 1, 2007, we have attracted visits by 83,990 unique IP addresses for a total of 212,366 visits to the site. The average visit duration is currently 258 seconds, which is an extraordinary statistic. We have served 2,411,876 page views, and the site has recorded 7,582,753 hits. Over a thousand users have registered for ongoing participation at the site. The project has been the focus of strong media attention, including an AP wire story that ran in newspapers and media outlets worldwide. A technical paper describing the software and the design of our large-scale dynamical experiments is planned for submission very shortly, and we believe that we are poised to make important scientific advances over the coming year.

K. Zahnle and collaborators modeled the aftermath of the Moon-forming impact that left Earth with a hot CO₂-rich steam atmosphere. Water oceans condensed from the steam after 2 Myr, but for some 10-100 Myrs the surface would have stayed warm (~500 K), for how long depending on how quickly the CO₂ was removed into the mantle. Thereafter a lifeless Earth, heated only by the dim light of the young Sun, would have evolved into a bitterly cold ice world. The cooling trend was frequently interrupted by volcanic or impact induced thaws. J. Hollingsworth co-authored a paper on assessing habitability of planets about M Dwarf Stars.