2009 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2008 – AUG 2009
Project 1: Looking Outward: Studies of the Physical and Chemical Evolution of Planetary Systems
We study the origin of life through a wide variety of approaches, beginning here with theoretical investigations of protoplanetary disks, the environments in which simple organic molecules first appeared and were concentrated in planetary bodies. We also study the survival of this organic matter during subsequent evolution through observations of circumstellar disks around both young and mature stars, extrasolar planetary systems, and small bodies in our Solar System, and through detailed models of planetary system formation.
Project 1: Looking Outward: Studies of the Physical and Chemical Evolution of Planetary Systems
Task 1.1 Radial Velocity Extrasolar Planet Searches
Doppler spectroscopy is uniquely able to find terrestrial-mass planets and Solar System analogs around 2,000 stars within 50 pc. Over the past twenty years Co-I Butler’s team has improved their radial velocity precision from 300 m/s to 1 m/s, and discovered half of the known extrasolar planets.Butler’s Doppler surveys are presently discovering the first terrestrial mass planets and Solar System analogs. Over the past year Butler’s group has found two super-Earths, and three Neptune-mass planets from their Keck and Anglo-Australian planet search programs, two Jupiter analogs in circular orbits, and the first long period eccentric transit planet. For the lowest mass, lowest amplitude planets, multiple telescopes improve observing cadence, and provide multiple confirmation. Over the past year Butler has received 100 nights on the 3.9-m Anglo-Australian Telescope (AAT). First light with Butler’s new Planet Finding Spectrometer on the 6.5 Magellan Telescope is scheduled for Oct 17, 2009. Butler is gearing up for over 50 nights a year on this Carnegie telescope. In early 2010 Butler’s team will begin observing 164 nights per year on the new 2.4-m Automated Planet FindingTelescope at Lick Observatory. Together with the Keck 10-m, Butler’s team will combine these telescopes to observe the best candidates sieved from the 1,500 stars that have been surveyed over the past decade. Of particular interest are M dwarfs, which will yield the first potentially habitable planets around the nearest stars. In addition Butler is collaborating with Greg Henry on the construction and operation of two 0.8-m robotic photometry telescopes at Carnegie’s Las Campanas Observatory. These new robotic telescopes will afford an unprecedented ability to monitor stars with better than 1 milli-mag precision simultaneously with Magellan Doppler velocity observations. For the first time it will be possible to track correlations between variations in stellar velocity and brightness due to stellar rotation, spot cycles and pulsation. This is critical for distinguishing the lowest amplitude planets from stellar “jitter”. These robotic telescopes will enter operation in early 2010. Butler and his team have been monitoring the nearby (8.5 pc) G5 dwarf 61 Vir with the AAT and Keck for the past several years. Both data sets reveal the two planets shown in Figure 1 (hd115617.jpg) and an additional 25 Earth-mass planet in a 123-day orbit (Vogt et al. 2009). Figure 2 (hd115617_phase.jpg) shows over 200 AAT and Keck observations, phased to the orbital periods of the 3 planets.
Task 1.2 Astrometric Search for Planetary Systems Hospitable for Life
Co-I Boss leads the Carnegie Astrometric Planet Search (CAPS) project, while Co-I Weinberger and NAI Fellow Anglada-Escude are key members of the CAPS team. The CAPS team is undertaking an astrometric search for gas giant planets and brown dwarfs orbiting nearby low mass dwarf stars with the du Pont telescope, located at Carnegie’s Las Campanas Observatory in Chile. The plan is to follow about 100 nearby (primarily within about 10 pc) low mass stars, principally late M, L, and T dwarfs, for 10 years or more, in order to detect very low mass companions with orbital periods long enough to permit the existence of habitable, Earth-like planets on shorter-period orbits. These stars are generally too faint and red to be included in ground-based Doppler planet surveys, which are often optimized for FGK dwarfs. The smaller masses of late M dwarfs also yield correspondingly larger astrometric signals for a given mass planet. The CAPS search will also help to determine whether gas giant planets form primarily by core accretion or by disk instability around late M dwarf stars. We have a paper in press showing that our preliminary analysis of the target star NLTT 48256 (see Figure 3 (caps.jpg)) yields an astrometric accuracy better than 0.4 milliarcsec over a time scale of several years with CAPSCam-S on the du Pont 2.5-m. We expect the astrometric accuracy to improve to below 0.3 milliarcsec once full color corrections are included in the data analysis, using the several years of Tek5 color photometry taken while building CAPSCam-S. Such a precision is sufficient to detect a Jupiter-mass companion orbiting 1 AU from a late M dwarf 10 pc away with a signal-to-noise ratio of about 4. Given this level of performance, we have proposed for another 30 nights of du Pont bright time in 2010 to continue our astrometric planet search program.
Task 1.3 From Disks to Planets
Co-I Boss is partially supported by the NASA Origins of Solar Systems Program to work on mixing and transport in marginally gravitationally unstable disks. This work is being continued and extended to include an analysis of the time history of a population of individual dust grains, as they traverse high and low temperature regions of the disk. This will allow a determination of the extent to which water is transported as a solid by the dust grains, through a collaborative effort with Prof. Morris Podolak of Tel Aviv University, who has developed a model for silicate dust grains with water ice mantles. Prof. Podolak visited Boss at DTM in August 2009, and plans were made for accomplishing this interaction. By following the extent to which water ice condenses or sublimates on a population of grains being transported around the disk, a better understanding of the distribution of water in the solar nebula will be achieved, with important implications for the delivery of water to the terrestrial planets. Co-I Boss has calculated a new set of 3D marginally gravitationally unstable (MGU) disk models that are similar to those studied previously but with several variations. A color field is used to represent the evolution of isotopes or volatiles residing in solids small enough (~cm-size or less) to remain tied to the disk gas. Two models are identical to a previous model except for having the number of terms in the spherical harmonic expansion for the gravitational potential solver changed from 32 to either 16 or 48. Since MGU disks evolve solely as a result of gravitational torques, these changes have the effect of varying the numerical resolution. Two other models are identical to a previous model except for starting with a more gravitationally stable disk, as quantified by initial minimum Toomre Q values of 1.8 and 1.9, respectively, compared to 1.4 for the previous model. All four of these new models evolved in much the same manner as in the previous model: the initially high degree of heterogeneity is lowered by mixing within 1000 yrs to a dispersion of ~10% inside 5 AU and ~2% from 5 AU to 10 AU. Combined with previous results for disks extending to 20 AU, gradients in heterogeneity are to be expected in MGU disks. 3D MGU disk models are consistent with rapid mixing of initially highly heterogeneous distributions of volatiles to levels of ~10% or less in both the inner (< 5 AU) and outer (> 10 AU) nebula, and with even lower levels (~2%) in intermediate regions, where gravitational torques are most effective at mixing.
Task 1.4 Late Mixing and Migration in the Solar System
The dynamical and physical properties of small bodies in our Solar System offer one of the few constraints on the formation, evolution and migration of the planets. The strong dynamical connection of the asteroids to the planets make determining their population and orbital properties valuable for gaining insight into Solar System formation and planet evolution. As an example, the recent discovery by Co-I Sheppard of a high inclination Neptune Trojan indicates that Neptune was once on a much more eccentric orbit than now. Co-I Sheppard and his colleagues have been conducting further wide-field observations in order to discover faint Neptune Trojans and objects beyond the Kuiper Belt edge at 50 AU. Discovery of these types of objects will allow us to further determine how the planets in our Solar System may have moved around during their formation and evolution. Sheppard’s survey to date shows there is a turnover at the faint end of the Cumulative Luminosity Function (CLF) for Neptune Trojans. This result is similar to recent CLF studies obtained for faint Jupiter Trojans and Kuiper Belt objects and shows that the sparse number of small objects in the various small body reservoirs is likely of primordial origin. The lower than expected number of objects in the tens of kilometer size range in the various outer Solar System reservoirs may make us have to rethink the source of the abundant kilometer sized comets.
Task 1.5 Compositions of Circumstellar Disks and Delivery of Volatiles to Comets
Co-I Alycia Weinberger examined the colors and structures of spatially resolved debris disks in order to help determine their composition and evolution. Imaging of the HR 4796 disk revealed two significant flux density asymmetries: preferential forward scattering by the disk grains and an azimuthal surface brightness anisotropy. The former should help constrain the grain composition and size, but Mie scattering models are doing a poor job of reproducing the low forward anisotropy. Weinberger found that the debris ring was offset from the location of the star by 1.4 AU, but that this shift was insufficient to explain the differing brightnesses of the northeast and southwest ansae. Sculpting of the disk by the companion or by an unseen planetary-mass perturber is necessary. Multi-color imaging of the HD 32297 debris disk (Figure 4 (hd32297.jpg)) revealed complex colors and morphology. Weinberger and her colleagues have interpreted the asymmetry in color and size of the two sides of the disk as due to interaction of disk grains with interstellar medium material as HD 32297 moves into a cloud. Evidently the environment may significantly shape the outer parts of disks.
PROJECT INVESTIGATORS:Alan Boss
Co-InvestigatorR. Paul Butler
PROJECT MEMBERS:John Chambers
RELATED OBJECTIVES:Objective 1.1
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