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Project Progress

Direct detection for extrasolar planets is fast becoming a reality, and LAPLACE is well-poised to be the forefront of this new area of research. Using multiple techniques we are now in the process of carrying out surveys for young planets as well as developing new techniques which will push our sensitivity to older planets around more nearby stars. Module 3: Nature of Planetary Systems, work has developed new instrumentation to detect these planets, begun surveys using existing adaptive optics techniques, and furthered modeling work to enable the interpretation of detections from a range of observatories, including, HST, Spitzer, and the James Webb Space Telescope (JWST). This work lay the foundation for understanding what other stellar systems may be hospitable to life and how we identify such systems.

This module 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).

Highlighted Accomplishments

• Discovered a need to modify the mass-luminosity relationship for very low mass stars and brown dwarfs, as a result of observation of AB Dor C.
• Developed capability for studying faint close companions (planets) at 3-5 microns. First there was a capability developed to search by subtracting the star diffraction pattern
• A new capability has developed as a result of making pupil plane phase masks, with the first one funded by LAPLACE. This removes the diffraction pattern over half the image and allows the system to reach a limit set by the residual speckles uncorrected as a result of wind patterns above the telescope.
• A comprehensive invited review “A theoretical look at the direct detection of giant planets outside the Solar System” published in Nature.

Ongoing Near-Infrared Survey

Laird Close is leading an adaptive optics survey at H (1.65 micrometers) 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. Graduate student Eric Nielsen is carrying out the survey. Their 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.

One of the significant early results from this survey is the discovery of a close companion to AB Dor (Close et al. 2005) Figure 3.1 Dynamical determination of the mass, combined with the brightness measurement at H band indicates that current theoretical models overestimate the brightness of substellar objects at young ages. Calibration of these models is important for understanding the sensitivity to giant planets of the ongoing surveys.

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An additional discovery is the detection of a brown dwarf companion to SCR 1845-6357, which at 3.9 parsecs (pc) is a close stellar neighbor to the Sun (Biller et al. 2006). The companion is likely 9-65 Jupiter masses and provides and excellent opportunity for follow-up study of a nearby substellar object.

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.

Work this past year has focused on developing techniques of improving the correction of the adaptive optics system. Currently we only correct for aberrations which manifest themselves on spatial scales larger than ~1 meter. By improving calibration of our system we expect to be able to correct for aberrations down to size scales of approximately 0.5m. Progress in this area is summarized in Brusa et al. (2004) and Kenworthy et al.(2004).

3-5 micron camera Development
The unique sensitivity of the MMTAO system has been demonstrated this past year using a newly developed 3-5 micron imager (see Freed et al. 2004), called Clio. Phil Hinz has led this effort. An early result suggests that we can detect giant planets down to nearly 6 Jupiter masses around the nearby star Vega (Hinz et al. 2006, in press).Figure 3.2 shows an image of Vega with this system.

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Figure 3.3 shows the expected contrast ratio achievable with the MMTAO system and Clio. Comparison to AO systems working at shorter wavelengths on Keck and Palomar telescopes suggests that the system is sensitive to planets of significantly lower mass.

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We have begun a survey of nearby stars with Clio and have observed approximately 20 stars as of late June 2006. 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).

We have also begun 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). Figure 3.4 shows a first test of the phase plate on the star Vega. 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. in prep) and use of the technique for observations of bright stars is planned for fall 2006.

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Modeling Work

Led by Adam Burrows, work on planet modeling this past year has focused on how planets detected by transits compare to existing models (cf. Burrows, Hubeny, and Sudarsky, 2005 and Burrows et al. 2004), as well as creating self consistent models have for the light curves and phase functions of giant planets at a broad range of orbital separations (Sudarsky et al. 2005).

Adam Burrows contributed a comprehensive invited review to Nature, entitled “A theoretical look at the direct detection of giant planets outside the Solar System”. The paper presents the most attractive spectral regions and techniques for detection based on the theoretical modeling. This work is currently our best guide to aid ongoing searches. See figure 3.5

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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 the Terrestrial Planet Finder mission.

In addition to this work, members of LAPLACE have been involved in a Discovery Mission proposal, TOPS (Telescope to Observe Planetary Systems) in collaboration with NASA Ames Research Center. Roger Angel, Adam Burrows, Michael Meyer, Tim Slater and Neville Woolf are all Co-Investigators. Olivier Guyon is the P.I. along with 3 other members from UA as Co-I’s; 7 more are UA faculty and staff (2 collaborators and 5 key personnel). In addition, members of the Penn. State, NASA Ames and VPL teams of NAI are also co-investigators.

Acronym List

AO: Adaptive Optics
HST: Hubble Space Telescope
JWST: James Webb Space Telescope
LAPLACE: Life And Planets Astrobiology Center
MMT: Multiple Mirror Telescope
MMTAO: MMT Adaptive Optics
NAI: NASA Astrobiology Institute
NASA: National Aeronautics and Space Administration
SDI: Simultaneous Differential Imaging
TOPS: Telescope to Observe Planetary Systems
VLT: Very Large Telescope
VPL: Virtual Planetary Laboratory