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Project Progress

This module uses observations of the gas and dust in (a) planet-forming accretion disks surrounding young stars and (b) debris disks surrounding more mature stars to understand key steps and timescales in the formation of planetary systems and their evolution, and to constrain outcome planetary system architectures. Another objective of the module is to provide candidate targets for direct detection and characterization of planets.

This module addresses:

• Goal 1 of the Astrobiology Roadmap: Understand the nature and distribution of habitable environments in the Universe, Objective 1.1 Models of Formation and Evolution of Habitual Planets

Highlighted Accomplishments

• Determination of the rate of loss of gas from debris disks shows that little is left after 30My. As a result, giant planet formation must be very rapid, and terrestrial planets must circularize their orbits rapidly too.
• Despite this, older dusty disks have been found both in the Pleiades and HD 12039.
• Kuiper Belt analogs have been found in 10-20% of stars.
• The study of CO in ground studies of circumstellar disk have found that corrections are needed for the underlying stellar spectrum.
• Completion of the transit telescope to study photometric variability of stars in clusters.

Dust Evolution in Protoplanetary Disks
Dániel Apai and Ilaria Pascucci are continuing the study of dust evolution in disks around young stars aiming to understand how the primordial 0.1 m-sized amorphous dust grains are processed into planet-forming material. Their recent Spitzer spectroscopic study of the dust composition in disks around young brown dwarfs discovered highly processed, crystalline dust in the their inner disk (Apai et al. 2005 Science, Apai & Pascucci 2005, Luhman et al. 2006). Figure 2.1 These large dust grains and crystals resemble the dust observed in Solar System comets, preserved from the epoch of early planetesimal formation. The growth of the dust grains and their observed settling toward the disk mid-plane are the first steps toward planet formation, suggesting that even very low-mass disks might form rocky planets. Figure 2.2 The highly crystalline dust around the relatively cool brown dwarfs also contradicts previous predictions suggesting a more complex picture of dust processing and radial mixing than previously anticipated (Pascucci et al. 2006, Apai et al. 2006). In a follow-up theoretical study Daniel Apai and collaborators showed that the intial conditions of the collapsing protostar influence the crystallinity of the emerging protoplanetary disk, potentially explaining the observed high crystallinity values (Dullemond et al. 2006).

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Evolution in the Gas Content of Disks

Understanding the evolution of the gas in disks can provide insight into the processes of giant and terrestrial planet formation. In the context of giant planet formation, both total disk masses (which are dominated by the gaseous component) and the lifetime of gas in the giant planet region of the disk can be used to constrain the dominant mode(s) of giant planet formation. The lifetime of gas in the terrestrial planet region of the disk is also of interest. This is because residual gas in this region of the disk can affect the outcome of terrestrial planet formation, i.e., the masses and eccentricities of planets and their consequent habitability. Only a narrow range of gas column densities is expected to produce terrestrial planets with the Earth-like masses and low eccentricities that we associate with habitability on Earth.

These considerations motivate the characterization of the gaseous component of disks over a range of radii, in both the giant and terrestrial planet regions of the disk (1 – 20 AU) and over a range of masses from the large gas masses characteristic of giant planet formation (1 MJ) down to the residual gas masses of interest for the outcome of terrestrial planet formation. The availability of both sensitive near- and mid-infrared high resolution spectrographs on ground-based 8-10 meter class telescopes and lower resolution spectrographs on board the Spitzer Space Telescope provides a powerful set of tools for probing the gas content of disks and the radial distribution of gas within disks.

Several advances have been made on this topic in the past year. Pascucci, Najita, Meyer, Malhotra, Lunine, and coworkers have used mid-infrared spectroscopy carried out with the Spitzer Space Telescope and observations of millimeter transitions of CO to place constraints on the lifetime of gas in the giant and terrestrial planet regions of disks. The observations suggest that less than 10 MEarth of gas remains in the disk even in systems younger than 30 Myr. This suggests that giant planets must accrete their gaseous envelopes on relatively short timescales (< 10-20 Myr). Similarly, the upper limits on the column density of gas in the terrestrial planet region of the disk indicate that if terrestrial planets form frequently and their orbits are circularized by gas, the circularization occurs early.

In a complementary study, Najita and coworkers have been using ground-based spectroscopy of the CO fundamental lines to probe the gas content in the terrestrial planet region of the disk at earlier epochs (< 10 Myr). They have found that it is critical to correct for the structure in the underlying stellar photosphere in order to detect weak emission lines that indicate the presence of residual gas. Modeling of the underlying stellar photospheres is in progress and results are expected later this year.

Identifying and Characterizing Forming Planetary Systems

Najita, Strom, and Muzerolle have explored the nature of “transition objects”, i.e., young stars surrounded by optically thin inner disks and optically thick outer disks. These unusual properties suggest that significant disk evolution has occurred, possibly as a result of planet formation. They found that the demographics of transition objects (their stellar accretion rates and disk masses compared to those of accreting T Tauri stars of comparable age) suggest that they have diverse origins. One group of transition objects (those with high disk masses) is found to have systematically low accretion rates for their disk masses, consistent with the theoretical expectation for disks that have formed a Jovian mass planet. Another group (those with low disk masses) is likely to be in an advanced state of disk photoevaporation and not forming any planets at all. These inferences can be verified by measuring the radial filling factor of the gas in the disk (r < 20AU). These observations are underway using both ground-based Near-Infrared Echelle Spectrograph (NIRSPEC) /Keck and space-based Spitzer/Infrared Spectrograph (IRS) facilities.

Production and Evolution of Collisional Debris

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We have also continued to study the properties of debris disks and the constraints that they place on our understanding of the formation and evolution of planetary systems. Our Spitzer program has produced a first round of publications and the second set are in preparation.

Scientific highlights include:

• Measuring the lifetime of inner circumstellar disks (< 3 AU) at 1-10 Myr (Silverstone et al. 2006).
• An upper-limit to the lifetime of gas-rich disks < 10 Myr (Hollenbach et al. 2005; Pascucci et al. 2006).
• The discovery of “asteroid like” debris disk surrounding HD 12039 (Hines et al. 2006).
• New debris disks discovered around sun-like stars in the Pleiades open cluster at an age of 100 Myr (Stauffer et al. 2005).
• A measurement of the frequency of cold outer Kuiper Belt analogues at 10-20%
• (Kim et al. 2005; Hillenbrand et al., submitted).

Ongoing work includes the study of:

• Evolution in dust disk mineralogy (Bouwman et al., submitted)
• The connection between debris disk evolution and the presence/absence of gas giant planets (Moro-Martin et al., submitted)
• Evolution in the mid-infrared excess emission between 3-300 Myr correspond to the epoch of terrestrial planet formation in our solar system (Meyer et al. submitted)
• A connection between host star metallicity and the presence/absence of dust debris (Najita et al. in preparation)
• A global analysis of observed spectral energy distributions in light of current models (Carpenter et al. in preparation).

Much of this work on disk evolution is summarized in a review article to appear in Protostars and Planets V (Meyer et al.).

Debris Disks, Stellar Metallicity, and Planets

In one of the above ongoing studies, we investigate whether there is a relation between stellar metallicity and the presence of collisional debris. One might expect that such a correlation might exist, since it is now well-established that the presence of extrasolar giant planets is correlated with stellar metallicity. This relation likely indicates that higher metallicity molecular cloud material enhances the likelihood of forming giant planets. By extension, then, since the production of collisional debris requires both planetesimals (the building blocks of giant planets) as well a source of dynamical stirring for the planetesimals (a giant or lower mass planet), a correlation might be expected between stellar metallicity and the presence of collisional debris. Thus far, we find a weaker correlation between debris and stellar metallicity than that between giant planets and stellar metallicity, i.e., collisional debris can arise over a wide range of stellar metallicity rather than solely under the high metallicity conditions required to produce a giant planet. This suggests that a broad range of planetary architectures (including those without giant planets) is capable of generating planetesimals and producing collisional debris.

In a related study, Chen et al. have used the Spitzer IRS to constrain the structure and composition of the dust in debris disks. They find that the IRS spectra of the majority of debris disks are well-modeled assuming that the dust is located in a narrow ring around the star, consistent with the idea that collisions are the primary destruction mechanism for the dust. The origin of the central clearings in these disks is not well-understood although they may be the result of planets sweeping dust from the inner portions of the disk. Five debris disk systems are found to possess a population of warm dust grains, suggesting that these systems possess multiple debris belts analogous to the asteroid belt and the Kuiper Belt in our Solar System.

Adaptive Optics, Spectral Difference Imaging, and other Follow up Observations

More generally, we are in the process of conducting follow-up observations on the above list of results based on these results with a variety of ground- and space-based telescopes. We are utilizing the Multiple Mirror Telescope Adaptive Optics system (MMTAO) to search for gas giant planets in systems we know to have debris disks with large inner holes. We are pursuing near-IR observations for the youngest stars (using the Simultaneous Differential Imaging (SDI) technique to search for methane-bearing companions with the ARIES camera) and mid-IR observations for the older nearby stars (searching for cool companions with the Clio 3-5 micron camera in collaboration with graduate student Ari Heinze and Phil Hinz). We also have a program underway at the European Southern Observatory Very Large Telescope (ESOVLT) to utilize NACO/SDI to search for planets around FEPS targets (led by Daniel Apai with help from graduate student Beth Biller). In a related program, we have recently identified a candidate planetary-mass companion to a young brown dwarf in the NGC 1333 star cluster. If confirmed as an association member, this companion would be 3-5 Mjup surrounding a very low mass primary with a projected separation of 300 AU (in a paper recently submitted by graduate student Julia Greissl).

We are also conducting follow-up with MMTAO using the MIRAC-BLINC 5-25 micron camera attempting to resolve debris disks with warm (terrestrial temperature) material. In particular, our high resolution observations of the unique Spitzer target HD 69830 will provide constraints on disk models (second year project of graduate student Wayne Schlingman with support from post-doctoral researcher Daniel Apai). In addition to IR excess emission thought to emanate from dust comparable to that found in solar system comets (Beichman et al. 2005), this system was recently discovered to harbor three extra-solar planets within 1 AU of the central star!

We are also using Hubble Space Telescope (HST) for follow-up observations of specific Formation and Evolution of Planetary Systems (FEPS) targets to search for extended emission produced by dust seen in scattered light. Observations of HD 107146, a now-famous Spitzer target from our FEPS program which has been resolved with the James Clerk Maxwell Telescope (JCMT) and Owens Valley Radio Observatory (OVRO) in the millimeter, in the visible with HST, and we have new observations in the near-IR with Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Through Life and Planets Astrobiology Center (LAPLACE), we are leading a broad effort to model the spectral energy distributions, and resolved observations of disk emission in one or more wavelengths in a self-consistent way (involving graduate student Stephanie Cortes). In support of our work on the gas context in circumstellar disks from which giant planets are formed, post-doc Ilaria Pascucci has led 230 and 340 GHz observations of CO in targets observed with Spitzer for remnant molecular gas disks where upper limits provide important constraints for chemical models of the disk. Using these observations in combination with Spitzer data, Pascucci et al. were able to also constrain the timescale for the formation of Uranus and Neptune as well as dynamical effects of remnant gas on forming terrestrial planets.

Renu Malhotra continues to work on understanding the dynamics of planetary systems around other stars as well as our own solar system. In work with former student Amaya Moro-

Martin, Malhotra is attempting to understand the physical processes that lead to structure in circumstellar dust distributions such the dynamics of embedded planets and dust outflow (Moro-Martin and Malhotra 2005a; Moro-Martin et al 2005a,b). She continues work aimed at understanding the migration of Neptune and interactions with the planetesimals in the Kuiper Belt (Hahn and Malhotra, 2005), and the sculpting of the Asteroid Belt to the migration of Jupiter and Saturn (Strom et al. 2005).

In Strom et al 2006, we reported our results of a comparison of the long-known terrestrial planets\’ impact crater record with the recently-characterized size distributions of the main belt and near Earth asteroids. We showed that the impact crater record of the terrestrial planets is owed to two populations of impactors that are distinct in their size distributions. the ‘old’ crater population (older than 3.8 gigayears, as determine dradiometric ages of Apollo lunar samples) is virtually identical in size distribution to the present main belt asteroids; the younger crater population matches closely the size distribution of the near earth asteroids. These results confirm that an inner solar system impact cataclysm — the so-called Late Heavy Bombardment — occurred 3.9 Gy ago, and also provide compelling new evidence that identifies the main asteroid belt as the source of the LHB impactors. A plausible dynamical explanation is that many asteroids were ejected from the main belt on a dynamical timescale by sweeping gravitational resonances during an epoch of orbital migration of the giant planets in early solar system history.
These results have important implications for planetary geology, in the detailed interpretations of the geological history of the planets which are based on the crater record. They have implications for the interpretation of astronomical debris disk observations, as the LHB phenomenon would be a major effect on circumstellar dust evolution. These results are also very significant in understanding the early astronomical environment on Earth when life arose.

We have two new successful Cycle #3 Spitzer programs in this area in collaboration with colleagues at the Max Planck Institute for Astronomy in Heidelberg (also Large Binocular Telescope (LBT) collaborators). In a related project, support for undergraduate Praveen Kundurthy (attending graduate school at University of Washington (U.W.) in astrobiology this fall) has led to further understanding the connection between young star rotation and circumstellar disk structure. Stars that are rotating more slowly show strong disk signatures in the mid-infrared and this has implications for the evolution of angular momentum evolution of sun-like stars (cf. work of colleagues Giampapa and Strom at National Optical Astronomy Observatories (NOAO)) as well as planet formation in the terrestrial planet zone. This work comprised Praveen\‘s honors thesis at the University of Arizona which was recently submitted to the Astronomical Journal.

Characterization of the ambient radiation fields in solar systems
Following the epoch of disk dispersal, the evolution of terrestrial planets is driven in part by the evolution of their central stars as they enter the main sequence phase of hydrogen burning. Stars like the Sun vary in energy output on a range of timescales. Our investigation seeks to systematically quantify such variations on both short and long time scales for stars spanning a range of ages.

The characterization of the changing output of solar-type stars (total brightness, UV, X-ray, visible)—from the time of planet-formation to the current epoch—is needed to understand: (i) the formation and early evolution of planetary atmospheres (t < 500 Myr); and (ii) climatic variations among more mature planets (t > 500 Myr). Understanding the nature of the joint variability of irradiance and magnetic activity on Sun-like stars spanning a range of ages will provide crucial insight on how frequently earth-like atmospheres are likely to form and survive, and how frequently exo-Earths encounter benign climatic variations. We are, therefore, pursuing an observational program to delineate the joint evolution of activity—such as flares, spots, and magnetic cycles—and irradiance variability in solar-type stars that may be the hosts of planetary systems.

Our present program is focusing on the implementation of high precision photometric monitoring of two open clusters: the young Pleiades (age ~ 100-120 Myr) and the solar-age cluster, M67 (age ~ 4-5 Gyr). In addition, we are conducting spectroscopic surveys of these and other clusters to investigate the range of magnetic field-related activity as manifested in key spectral line diagnostics. A paper summarizing our spectroscopic study of magnetic activity in M67 has just been accepted for publication (Giampapa et al. 2006; Astrophysical Journal, in press). We recently received an allocation of time on the 3.5-m telescope WIYN on Kitt Peak to carry out a similar study for the solar-type stars in the Pleiades. In addition, we are pursing an opportunity to extend our M67 survey and to expand this investigation to the intermediate-age open cluster, NGC 752 (age ~ 2 Gyr), using the MMT telescope on Mt. Hopkins.

In the parallel photometric study, we completed work on the construction of the small transit telescopes for the M67 program and obtained a large set of unfiltered test data. This instrumentation is now ready for the upcoming observing season. We completed writing the photometric processing software and we were able to obtain our first season of data for the Pleiades. We are in the process of reducing the 2.5 months accumulation of data for this cluster. Finally, we are investigating opportunities for expanding the photometric investigation to other clusters.

Acronyms

ARIES Near Infrared Imager and Spectrograph
ESOVLT European Southern Observatory Very Large Telescope
FEPS Formation and Evolution of Planetary Systems
JCMT James Clerk Maxwell Telescope
HST Hubble Space Telescope
IRS Infrared Spectrograph
LAPLACE Life and Planets Astrobiology Center
LBT Large Binocular Telescope
MMT Multiple Mirror Telescope
MMTAO Multiple Mirror Telescope Adaptive Optics system
NACO/SDI Near Infrared Adaptive Optics Camera/Spectral Difference Imager
NASA/JPL National Aeronautics and Space Administration Jet Propulsion Lab
NOAO National Optical Astronomy Observatories
NICMOS Near-Infrared Camera and Multi-Object Spectrometer
NIRSPEC Near-Infrared Echelle Spectrograph
OVRO Owens Valley Radio Observatory
SDI Simultaneous Differential Imaging
U.W. University of Washington
WIYN Consortium, which consists of the University of Wisconsin, Indiana University, Yale University, and the
National Optical Astronomy Observatories (NOAO).