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Project Progress

Co-I Hollenbach is working with collaborator Gorti to model photoevaporation processes in protoplanetary disks around young stars. An objective for this year was to examine the dispersing effects of EUV (Lyman continuum), FUV, and X-rays from the central star. This photoevaporation effect clears the disk and helps to determine the likelihood of habitable planet formation around a star of a given mass. In the current reporting period we extended our model to address the structure of optically thick disks that are the birthplaces of planets. We find that in the central midplane regions of the disks, the gas and dust couple tightly and are at the same temperature, but near the disk surface, the gas generally becomes hotter than the dust, which inflates the atmosphere at the surface of the disk and can cause photoevaporation of the disk.

Work on the basic paper that describes this new model is still in progress. However using the earlier computer code developed for optically thin disks (the “optically thin code”) we show that in the outer regions of disks (at radii of order of hundreds of AU) a peak in the gas phase water abundance would exist near the disk surface due to photodesorption of the ice from the cold grains in these regions. The model compared favorably to a tentative detection of HDO (deuterated gas phase water) in the disk around a nearby young solar type star.

We were also able to test aspects of our optically thin code that we were improving as we worked on the optically thick code by comparing our numerical models to Spitzer Space Telescope observations of disks around nearby solar mass stars. These results are presented in Hollenbach et al (2005). Here we were able to show that the epoch of giant planet formation in this 20 million year old system had ended.

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 reported in Dullemond et al (2006) and Hollenbach & Gorti (2006). We find that the EUV has little effect on the disk until the accretion rate onto the central stars has diminished to less than about a few times 10^-10 solar masses per year, because of the EUV opacity in the protostellar wind launched from the disk near the star. However, the FUV and X rays do penetrate and photoevaporate the outer disk (beyond about 10 to 30 AU). FUV tends to dominate over X-rays. FUV evaporation, in contrast to EUV evaporation, works from the outside of the disk inward, stalling when the disk has shrunk to about 10-30 AU because the stellar gravity in the inner regions is sufficiently strong to hold the warm surface gas to the disk. EUV photoevaporation is strongest at a critical radius of a few AU, and EUV photoevaporation there eventually creates a gap, after which the inner disk rapidly accretes onto the central star, while the EUV rapidly evaporates the outer disk from inside out. As reported in these two papers, photoevaporation is being studied as a function of stellar mass, to determine which mass star has the longest-lived disks and therefore the highest probability of forming planets.

One of the major works of the year was the preparation of the invited review paper for the prestigious Protostars and Planets V conference (Dullemond et al 2006). Hollenbach organized the proposal for this chapter and played a major role in its writing. This chapter focuses on theoretical models of the thermal and density structure of disks, and how the resultant spectra from these models compare with observation.

Co-I Davis and collaborator Richard continue to investigate aspects of the thermodynamic and physical environment of protoplanetary disks. A new cumulative-planetary-mass-model predicts the surface density distribution of the primeval solar nebula and is shown to be more consistent with planetary data than that obtained from the commonly-used Hayashi (1981) minimum mass distribution (S. Davis, Ap. J. Letters, 2005). In a related investigation, the condensation/sublimation front in the early solar nebula was examined to determine the phase state (gas or ice) of important species relating to organic chemical processes. The two-dimensional water condensation/sublimation front was computed and contrasted with that predicted by one-dimensional theory (S. Davis, Ap. J., 2005). This work was expanded in the latter part of the reporting period to consider the relative abundance of water ice/vapor and CO ice/vapor. It is found that water ice is a ubiquitous part of the protoplanetary disk, as ice forms as close as 1 AU from stars that are generally similar to our Sun. Furthermore the ice/vapor boundary at any radial distance from the star possesses a complex vertical phase structure in planet-forming regions. This structure exhibits a layered vertical distribution of water vapor that covers water ice that in turn covers water vapor again. The results of this new finding are now being prepared for publication.

D. Richard continues work on chemical-physical processes as reported in Richard & Davis (2005) and Richard & Davis (2006). The chemical evolution of protoplanetary disks is a fundamental step towards understanding the formation and composition of planets and discriminating habitable zones that can shelter life. This year we focused on characteristics of the chemical network that will enable us to model reactions for specific chemical species. Species of particular interest include, among others, H₂O, CO, OCS, CH₃OH and CH₃OCH₃ as examples of simple molecules that are related to necessary building blocks for more complex organics in the protoplanetary nebula.

We simulate the chemical evolution in a protoplanetary disk model between the radii of 0.4 and 300 astronomical units. The chemical network, which including 395 species and 3864 reactions, is built upon the UMIST rate database. The network is evolved in time until a stationary state is reached. For each species of interest, the corresponding sub-network is analyzed to identify which reactions are the most active, at different times.

We identified a small sub-set of reactions (2-5) as clearly dominant locally (although this subset may change with location) and accounting for more than 90% of the production-destruction activity for a given chemical compound. There are two major benefits to identifying these reactions. The first is to reduce the number of chemical reactions required to compute realistic abundances, and the second benefit is to pick some reactions to be part of a current project to refine their rates using computational quantum chemistry techniques in order to address a major shortcoming: the lack of information or reliability concerning the temperature dependence of the reaction rates outside of the experimental window for which data was collected. In this effort we plan to generate an efficient minimal set to study the evolution and movement of organic species in the nebula and the probability of these species appearing in habitable planet regions.

Co-Is Young and Hollingsworth continued their work on modeling extreme atmospheric environments to identify habitable planet climates. The overall objective of this investigation is to model Earth and Earth-like planets with the goal of understanding planetary implications of extreme climate regimes on the “habitable zone” (HZ) of terrestrial-like planets.

Preliminary extreme orbital configuration simulations for the Earth’s climate system have been completed using a realistic 3D climate model based on a recent version of the National Center for Atmospheric Research (NCAR), Community Atmospheric Model (CAM). The current code has built-in modules and parameters to allow generalized orbital settings and configurations so that climate simulations with extreme configurations can be pursued. In the current reporting period we altered in our model the planet’s eccentricity from its present value (epsilon = 0.017) to a highly elliptical orbit (epsilon = 0.500) and performed annual-cycle simulations. We are now analyzing the implications of this orbital change.

We also performed initial 3D climate simulations of the atmosphere of Venus using a simplified model. In particular, we studied an interesting Venusian feature called atmospheric super-rotation. Our initial calculations, which imposed realistic “radiative-convective equilibrium” temperature fields with realistic Newtonian cooling rates (i.e., thermal relaxation time scales), show only a series of “stacked” Hadley circulation cells from the surface to upper levels. In addition, these stacked overturning circulation cells are symmetric about the equator. These numerical experiments along with others now underway will help better constrain future-planned climate simulations of Venus’ environment as well as expanding our database on extra terrestrial planetary system.

Co-I Laughlin and students Wolf and Meschiari completed the “Systemic” public-domain radial velocity data analysis package. The goal of this work is to facilitate a detailed analysis of 100,000 radial velocity data sets which mimic a wide distribution of underlying planetary systems. By comparing results emerging from the analysis with the known underlying distribution, it will be possible to answer important statistical questions regarding the distribution of planets that comprise the galactic planetary census. The study is available from their website http://www.oklo.org, in both downloadable form and as a web-based Java applet. We have also completed a collaborative web-based environment where a community of users can submit fits, discuss their properties, and participate in further analyses, c.f. http://www.oklo.org/php/login.php.

At the present time, the project is rapidly expanding its user-base as a result of promotional public talks by Laughlin and Wolf. Further publicity should be forthcoming with the release of a feature article by Laughlin in Sky and Telescope to appear in either the August or September 2006 issue.

Laughlin and Wolf also used the computer codes that underlie the systemic console to analyze the “Rossiter McLaughlin” effect for the recently discovered large-core transiting planet HD 149026b. Their analysis showed that the spin axis of the star is well-aligned with the angular momentum vector of the planetary orbit, supporting formation scenarios that involve quiescent accretion in a disk, as opposed to scenarios involving planet-planet collisions. This article is accepted by the Astrophysical Journal and is currently in final proofing stages.

Co-I Lissauer and postdoc Quintana continue their research on simulations of the late stages of terrestrial planet formation within disks orbiting binary stars. Binary stars are ubiquitous in our Galaxy; indeed, single stars like our Sun are in the minority. These studies will aid in the search for zones where habitable planets can form. It was concluded from the simulations that tight binary stars with maximum separations of less than 0.2 AU and small eccentricity had little effect on the accreting bodies. In most of these simulations terrestrial planets formed over essentially the entire range of the initial disk mass distribution (and even beyond 2 AU in many cases). The effects of the stellar perturbations on the inner edge of the planetesimal disk become evident in systems with larger binary separation. Terrestrial-mass planets can still form around binary stars with significant eccentricity, but the planetary systems tend to be sparser and more diverse. Binary stars with maximum separation of 0.3 AU perturb the accreting disk such that the formation of Earth-like planets near 1 AU is unlikely. Despite these constraints, at least one terrestrial planet (at least as massive as the planet Mercury) formed in each of our simulations.

Co-I O. B. Toon and colleagues at the University of Colorado published several papers with partial support from the Ames Team, (as described in this and the following four paragraphs). A hydrodynamic escape model was applied to an extrasolar planet (Tian, F., et al., ;Astrophys. J. 621 (2): 1049-1060 Part 1 Mar 10, 2005) and was also applied to Earth (Tian F., Toon O.B., Pavlov A.A., DeSterck, H., Science, 308, 1014, 2005); published online 7 April 2005 (DOI: 10.1126/science.1106983). They proposed that early Earth had an atmosphere with tens of percent H₂, and an H₂/CO₂ ratio greater than 1. Perhaps the early atmosphere was indeed a key source of organic molecules for prebiotic processes. A follow up response was also published (Tian F., Toon O.B., Pavlov A.A., Science, 311, 5757, 2006) that addressed the outstanding issues that need to be resolved in the early hydrogen atmosphere model.

The passage of the solar system through dense interstellar dust clouds was shown to likely cause particles to build up in the stratosphere and block enough sunlight to plunge Earth into a snowball state, and how this might be shown from the geologic record using various elements and isotopes. (Alexander A. Pavlov, et al., Geophys. Res. Lett., 32, L03705,doi 10.1029/2004GL021890, 2005); We also showed that significant ozone loss can occur following passage though low density clouds ( A. A. Pavlov et al., Geophys. Res. Lett., 32, doi 10.10229/2004GL021601, 2005).

We also examined the formation of Martian gullies. Martian gullies are likely to form under present conditions, and mark an excellent spot to search for life on mars (J. L. Heldmann, et al., J. Geophys. Res. 110, E05004, doi:10.1029/2004JE002261, 2005). We also used data from the Arctic to develop a numerical model for flows under sub-freezing conditions (Heldmann et al., Arct. Antarct. Alpine Res., 37, 126-134, 2005.)