2008 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2007 – JUN 2008
1. From Molecular Clouds to Habitable Planetary Systems
1. Formation of Habitable Planetary Systems
Co-investigator John Chambers has continued work on the oligarchic growth stage of planet formation, which determines many of the final characteristics of a planetary system. Chambers’ semi-analytic model for oligarchic growth has been extended to include planetesimal fragmentation and planetary migration— two processes that can substantially alter the growth and survival of planets. The new work shows that fragmentation greatly increases the rate of growth of planets and improves the chances of forming giant-planet cores within the lifetime of a protoplanetary disk. Migration, whether smooth or stochastic, typically has a negative impact on planet formation. However, giant-planet cores and the precursors of terrestrial planets are able to form and survive migration under a limited range of circumstances, in particular when the protoplanetary disk accretion rate is high.
2. Protoplanetary and Debris Disks
Co-Investigator Alycia Weinberger continued her studies of circumstellar disks to measure their compositions in as many direct and indirect ways as possible. With Postdoctoral Fellow John Debes, she determined the color of reflected light from circumstellar disks. The disk around HR 4796 was successfully modeled as composed of organic-rich materials, such as the “tholins” created to explain the atmosphere of Titan and reported in the spectra of Kuiper Belt objects. This is the first detection of organics in a debris disk; their presence implies that the basic building blocks of life are common in the later stages of planet formation. Weinberger and Debes showed that the disk around HD 15115 has complex color structure, ranging from red to blue, as well as complex physical structure with a warp and large-scale asymmetry.
Weinberger studied the stellar host of the disk around BD+20 307 using Carnegie’s Magellan telescopes and found that it was actually a close binary and a quite old one (>600 My). Her work in progress on the disk with the Spitzer Space Telescope shows that a recent massive collision happened near 0.5 AU in the disk. Such collisions at late ages are very rare and are perhaps akin to the Late Heavy Bombardment in the Solar System.
Weinberger also worked with American University undergraduate Johanna Teske to study “shell-stars” that are surrounded by debris disks. In these systems, our line of sight to the star passes through the disk, and the gas composition can be studied from its absorption spectrum. Weinberger and Teske found that one such star, HD 158326, had circumstellar calcium and sodium that may be variable.
Postdoctoral Fellow Fred Ciesla examined the transport of dust in the early solar nebula, focusing on the variations in the rates and preferred direction of transport above the nebular midplane. He applied his models to investigate the dynamical evolution of calcium-aluminum-rich inclusions (CAIs), the distribution of water in the nebula, and the delivery of high-temperature materials to the comet formation region. This latter application resulted in an explanation for the findings of the Stardust mission. He also participated in collaborations in which he applied his models of shock processing of materials in the solar nebula to investigate the chemical properties of chondrules.
3. Searching for Extrasolar Habitable Planetary Systems
Co-Investigator Alan Boss’s contribution to Carnegie’s NAI effort in the last year centered on his leadership of a new ground-based astrometric planet detection effort being conducted with the 2.5-m du Pont telescope at Carnegie’s Las Campanas Observatory in Chile. Boss and his team were awarded funds by the NSF in 2004 to build the Carnegie Astrometric Planet Search (CAPS) camera, a specialized camera that should yield astrometric accuracies better than 1 millarcsec per epoch. The $235K Teledyne Hawaii-2RG focal plane arrays (engineering and science grade) were delivered in mid-2005, along with the Barr Associates lambda/30 combination filter/window. The CAPS camera was completed in early 2007 and was mounted on the du Pont telescope during the March 2007 observing run. Engineering tests were run, and plans were made for developing a second-generation electronics board, as well as the installation of a Uniblitz electronic shutter. Boss observed for six nights on the du Pont in 2007-08 for the CAPSCam project. The electronic shutter was installed successfully during the August 2007 run. A second CAPSCam camera was built (Figure 1) and will be used for testing and development in the optical shop at the Carnegie Observatories. This camera may also be used to launch a northern hemisphere astrometric planet search, provided that a suitable telescope can be located, such as the 2.5-m telescope on Mt. Wilson.
Co-Investigator Paul Butler continued his survey of the nearest 2,000 sun-like stars within 50 pc. These surveys have uncovered about half of the 300 known exoplanets. Ongoing improvements in precision have taken this program from Jovian mass planets to Neptune mass and are now beginning to probe the terrestrial planet mass regime. These surveys use the Keck 10-m (in conjunction with Steve Vogt and Greg Laughlin at UCSC), the 3.9-m Anglo-Australian Telescope (with Chris Tinney and Hugh Jones), and the 6.5-m Magellan telescope (with Dante Minniti and Steve Shectman). Over the next year this program will incorporate three major hardware upgrades. The MIKE spectrometer on Magellan will be replaced with a custom Planet Hunting Spectrometer that is being built by Jeff Crane and Steve Shectman at the Carnegie Observatories optical shop in Pasadena. In conjunction with Greg Henry at Tennessee State University, two 0.8-m robotic telescopes will be installed at Las Campanas Observatory to photometrically survey the southern hemisphere planet-search stars. Steve Vogt is leading the effort to install a 2.4-m dedicated precision velocity telescope at Lick Observatory in California.
4. Habitable Environments in the Solar System
Co-Investigator Scott Sheppard recently received a special near-infrared filter set that is designed to allow spectrophotometry of faint distant Solar System objects in order to identify water ice and methane ice on their surfaces. The first observations with these new filters are set for Autumn 2008. Sheppard is also following up several Kuiper Belt objects that he discovered in 2007 and 2008 in order to determine their orbits and understand what they may tell us about the evolution of the Solar System, with a focus on the evolution of icy bodies.
Postdoctoral Fellow Isamu Matsuyama studied the rotational stability of planetary bodies, including the icy satellites in the outer solar system. In collaboration with the NAI’s University of California, Berkeley, team, Matsuyama found that variations in elevation of postulated shorelines on Mars can be explained by surface deformation resulting from reorientation of the rotation axis, providing evidence for the presence of an ancient martian ocean. In collaboration with Francis Nimmo of University of California, Santa Cruz, Matsuyama showed that the tectonic patterns on Saturn’s moon Enceladus and Jupiter’s moon Europa can be generated by large-scale reorientations. The reorientations required to explain the tectonic patterns suggest that the outer shells of these moons are decoupled from their cores by a liquid layer.
PROJECT INVESTIGATORS:Sean Solomon
PROJECT MEMBERS:Sean Solomon
R. Paul Butler
RELATED OBJECTIVES:Objective 1.1
Models of formation and evolution of habitable planets
Indirect and direct astronomical observations of extrasolar habitable planets
Outer Solar System exploration
Sources of prebiotic materials and catalysts