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Project Progress

PART I: Overview

Module 3: the Nature of Planetary Systems focuses on the direct detection and characterization of extrasolar planetary systems. As a complement to the successful radial velocity surveys, direct detection can further our understanding of planets in wide orbits with long periods. The module activity is divided into three areas: development of high contrast techniques, survey observations, and modeling of giant planet spectral energy distributions.

Module 3 addresses:

• Goal 1 of the Astrobiology Roadmap, Understand the nature and distribution of habitable environments in the Universe, objectives 1.1 (models of formation and evolution of habitable worlds) and 1.2 (indirect and direct astronomical observations of extrasolar habitable planets).

PART II: Research Overview

• LAPLACE is developing adaptive optics, differential imaging, and diffraction suppression techniques to optimize sensitivity levels of the direct imaging surveys.
• A near infrared survey at H (1.65 µm ) band using the VLT and MMT is being carried out to obtain direct images of young planets around nearby stars.
• A the rmal infrared survey at L’ (3.8 µm ) and M (4.8 µm ) band survey using the MMT is being carried out to obtain direct images of older, cooler planets around the nearest stars.
• LAPLACE is developing detailed radiative transfer models of planetary atmospheres to predict flux levels for direct detection of planets using ground-based telescopes, HST, Spitzer, and JWST.

PART III: Module Progress

Technical development of high contrast imaging has concentrated on suppression of diffraction at 3.8 and 4.8 µm wavelength. The first long exposure images were taken using an Apodizing Phase Plate to obtain useful sensitivity as close as 3 λ/D away from a star.

We have completed two distinct surveys for giant planets over this past year. The first program surveyed 45 stars using Simultaneous Differential Imaging (SDI) at 1.65 µm wavelength and the second surveyed 50 solar-like stars at 3.8 µm wavelength for cooler planets. As well, we have an ongoing survey of M-type stars for comparison with their higher mass equivalent.

Finally, modeling work continues to enable the interpretation of detections from a range of observatories, including, HST, Spitzer, and the James Webb Space Telescope (JWST). This work is laying the foundation for understanding what other stellar systems may be hospitable to life and how we identify such systems.

Highlighted Accomplishments

• Carried out deep observations with an Apodizing Phase Plate to allow detection of planet-mass objects only 3 λ/D from a star.
• Completed 1.5 µm survey with MMT and VLT to search for young giant planets.
• Completed a 3.8 µm survey with the MMT to search for cooler giant planets.
• Started a survey of lower mass stars at 3.8 µm to compare with solar-type stars.
• Modeled expected secondary eclipse characteristics for hot Jupiters for comparison with Spitzer observations.
• Modeled nonequilibrium effects in giant planet atmospheres.

High Contrast Imaging and Adaptive Optics Development

The MMT Adaptive Optics (MMTAO) system is the only such system integrated into the telescope itself. This allows a uniquely smooth point spread function and much lower background in the infrared. Both of these increase the system’s sensitivity for extrasolar planets and allow development of high contrast techniques at high Strehl ratio.

Lead by Matt Kenworthy and John Codona, we have begun to use the high Strehl capability of the MMT to implement a coronagraphic mode of imaging with Clio which will significantly enhance our ability to detect planets in the 0.4-1 arcsecond range of separations (4-10 AU for stars at 10 pc). Using the Apodizing Phase Plate (APP) approach, the diffracted light is suppressed on one side of the star by insertion of a custom-machined phase plate in the pupil plane. The tests have demonstrated improved detection performance (Kenworthy et al. 2007). Deep observations of several trial stars were carried out in early 2007 (see Figure 1, Kenworthy et al. in prep) and a survey of the brightest 25 stars is being planned to take advantage of this new capability.

Work this past year has also focused on developing techniques of improving the correction of the adaptive optics system. Phil Hinz, along with software engineer Vidhya Vaitheeswaran has lead an effort to implement a completely PC-based control system that allows for flexible implementation of control laws for the adaptive optics system. This has allowed us to begin improving the image quality of the MMT AO system due to degradations from, for example, telescope shake due to wind excitation

Near-Infrared Survey

Laird Close, along with graduate students Beth Biller and Eric Nielsen are carrying out an adaptive optics survey at H (1.65 μ) band to look for young companions using both the 8.2m VLT in the southern hemisphere and the 6.5m MMT in the northern hemisphere. The approach uses the technique of Simultaneous Differential Imaging (SDI) in and out of methane to enhance sensitivity to giant planets in the presence of scattered light of the parent star.

The survey is now complete and results are reported in Biller et al. 2007). Although no planets were detected, a new brown dwarf was discovered as part of the survey around the very nearby M star SCR 1845-6537 (Biller et al. 2006). A Monte Carlo analysis of the results has been completed by Nielsen at al. (2007) showing that planets above approximately 4 M J and outside 20 AU are rarer than 1 in 5.

Thermal Infrared Survey

The unique sensitivity of the MMTAO system has been demonstrated using a newly developed 3-5 micron imager (see Freed et al. 2004), called Clio. Phil Hinz has led this effort. The technique is sufficient to typically detect planets down to 5 Jupiter masses into 10-15 AU around the stars being observed (Heinze et al. 2006). Over the past eighteen months a survey has been carried out of 50 stars, concentrating on the very nearest stars to the sun, and a few slightly younger, and further away stars.

The thermal infrared survey was completed in April 2007. Like the Near-Infrared (NIR) sample, no new planets were detected in this survey. The analysis of these results are still being completed. Taken together, these observations indicate that the outer regions of other planetary systems do not harbor massive planets. These results can can help constrain our understanding of models for planet formation.

Giant Planet Modeling

Lead by Adam Burrows, work on planet modeling this past year has continued to focus on how planets detected by transits compare to existing models (Burrows et al. 2007), as well as improved spectral energy distribution calculations for giant planets which take into account departure from chemical equilibrium (Hubeny et al. 2007). The models for “hot” Jupiters can reproduce the range of radii seen for the 14 known transiting planets, if both enhanced opacity is taken into account for the large planets, and larger core mass are invoked for the smaller planets. The latter conclusion correlates with the metallicity of the parent star, suggesting an interesting link with the core accretion scenario for planet formation.

Our first astrobiology tenure track faculty member is working with module 3. Dr. Alex Pavlov of LPL is studying the early atmosphere of Earth, and effects on the temperature of Earth. This has particular application to future exoplanet missions that could search for Earth-like planets.

Acronym List

AO: Adaptive Optics

APP: Apodizing Phase Plate

HST: Hubble Space Telescope

JWST: James Webb Space Telescope

LAPLACE: Life And Planets Astrobiology Center

LPL: UA Department of Planetary Sciences Lunar and Planetary Laboratory

MMT: Multiple Mirror Telescope

MMTAO: MMT Adaptive Optics

NAI: NASA Astrobiology Institute

NASA: National Aeronautics and Space Administration

NIR: Near-Infrared

SDI: Simultaneous Differential Imaging

TOPS: Telescope to Observe Planetary Systems

VLT: Very Large Telescope

VPL: Virtual Planetary Laboratory