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Project Progress

Module 3: Nature of Planetary Systems

Progress over the past year has concentrated on increasing the achievable direct imaging contrast at 4.8 μm wavelength using a novel diffraction device called an Apodizing Phase Plate (APP), and surveying a sample of ~20 stars at previously unreachable sensitivity levels.

Separate direct imaging surveys at both H band and L band have been completed. The observations failed to discover any giant planets at large separations. These surveys indicate that giant planets at large separations must be rare, informing our understanding of planet formation and migration.

Modeling work has focused on interpretation of results from Spitzer transit observations in particular. This work is laying the foundation for understanding what other stellar systems
may be hospitable to life and how we identify such systems.

Building on observations with the APP, we have developed a technique to suppress scattered light, called Phase Sorting Interferometry (PSI). The technique promises to further
improve the achievable contrast and inner working angle when used with the APP. Observational tests in Spring 2008 have verified the basic technique.

Highlighted Accomplishments

• Perfected observational technique of an APP, and began a survey to detect planet mass objects a factor of two closer to their star compared to direct imaging searches.
• Analyzed results from the 1.5 um survey with MMT and VLT to constrain the existence of giant planets at wide separations.
• Completed and analyzed results from a 3.8 um survey with the MMT to search for cooler giant planets.
• Modeled expected secondary eclipse characteristics for hot Jupiters for comparison with Spitzer observations.
• Developed a completely new technique to suppress scattered light in direct imaging observations called Phase Sorting Interferometry (PSI)

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, work is underway to use the high Strehl capability of the MMT to implement a coronagraphic mode of imaging with Clio which An Astronomical Search for the Essential Ingredients for Life:
Placing our Habitable System in Context
significantly enhances 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 (see Figure 1). The tests have demonstrated improved detection performance (Kenworthy et al. 2007). A survey is currently underway using the APP to observe 25 nearby bright stars at
separations that correspond to the ice line (~ 4AU) around these objects. The survey is obtaining sensitivity to giant planets a factor of two closer than the previous direct imaging survey at 3.8 microns. 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.

High-contrast adaptive optics (AO) observations near stars have to contend with the telescope’s diffraction halo, a rapidly-changing cloud of residual atmospheric speckles, and a host of faint but persistent quasi-static speckles caused by various imperfections and aberrations.
It is these quasi-static speckles that typically limit the detection sensitivity near stars as they are
easily confused with faint stellar companions. Since they are coherent with the starlight, it is possible to suppress the quasi-static speckles and other residual diffraction halo over a search region by applying small offsets to the AO system’s deformable mirror (DM). Computing the required offsets requires knowledge of the location, brightness, and phase of the speckle relative to the star’s PSF core. Codona and Kenworthy (2008) have developed a new wavefront sensing technique, called Phase Sorting Interferometry (PSI) for measuring the static halo that uses the
randomly-changing residual AO speckles as interferometric probes. Doing this requires simultaneous short-exposure frames from a mid-IR science camera and measurements of the residual closed-loop wavefront using the AO system’s wavefront sensor (WFS). These data streams are combined to construct a map of the quasi-static halo’s complex amplitude near the bright core of a star’s PSF, permitting adaptive halo suppression. Implementing this new WFS and halo-suppression servo requires no new hardware, just new processing applied to the existing AO system. By suppressing the quasi-static speckles, we are left with only the fast
speckle noise, which should average to a smooth background.

Near-Infrared Survey

Laird Close, along with graduate students Beth Biller and Eric Nielsen have completed and analyzed an adaptive optics survey at H (1.65 μm) 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 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. (2008) showing that planets above approximately 4 MJ 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). Deep observations were acquired at L’band for 50 stars using the MMT.

The thermal infrared survey was completed in April 2007. Like the Near-Infrared (NIR) sample, no new planets were detected in this survey. Deep observations and limits are reported for Vega and epsilon Eri in Heinze et al. (2008). The statistical analysis of the data is underway.
The lack of detection places constraints on planets at separations outside of 10 AU, constraining
the formation or migration of giant planets at regions corresponding to the outer part of our own planetary system. Constraints at both near and mid infrared wavelengths is important since the SEDs of giant planets at younger ages is relatively uncertain (Fortney et al. 2008).

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. 2008a). Selfconsistent
albedo models have been developed for comparison with data such as the MOST observations. Models that best fit observational data indicate the existence of a temperature inversion layer in many transiting planets caused by an, as yet, unidentified species. The
inversion layer correlates with stellar insolation (Burrows et al. 2008b) . The inversion can create emission features of molecular species such as water in the infrared. The range of observational
data now available is allowing detailed atmospheric models to be developed and improved for giant planets in close proximity to their star. These models, in turn, provide a starting point for
models of cooler and less massive planets that will be observable in the near future.

At lower masses, Dr. Alex Pavlov, of LPL, a recent faculty hire for Astrobiology, is studying the early atmosphere of Earth. Work is concentrating on feedback effects and
conditions that can change the temperature of Earth, such as the rise of cyanobacteria (Schwartzmann et al. 2008). This work improves our understanding of the width of the habitable zone around other stars, and the range of conditions on Earth-like planets that can support life. It
has particular application to future exoplanet missions that could search for Earth-like planets.

PECO space mission study
PECO is a mission concept for a space coronagraph with potential for observation of planets extremely close to their star, close to an angular spacing on lambda/D. The mission was
originally proposed as a Small Explorer, and was selected for more detailed study.

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