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2007 Annual Science Report

University of Arizona Reporting  |  JUL 2006 – JUN 2007

Module 2: Formation and Evolution of Habitable Worlds

Project Summary

Current ideas concerning the origin of life require both complex organic chemistry capable of extracting energy from the environment through metabolic processes, and environments such as planets conducive to reproductive strategies that promote evolution.

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Field Sites

Project Progress

Current ideas concerning the origin of life require both complex organic chemistry capable of extracting energy from the environment through metabolic processes, and environments such as planets conducive to reproductive strategies that promote evolution. Thus the chemical evolution of circumstellar disks as well as the formation and evolution of planetary systems are central to understanding the origin of life and its prevalence in the Universe. Yet the processes that transform the ingredients found in the interstellar medium into dense hot gas-rich disks and, ultimately, planets capable of sustaining life are poorly understood. To give one example, current the ories for the formation of terrestrial planets are considered to be relatively mature, yet none of the m can explain the carbon deficiency thought to obtain in the inner solar system! Similarly, the origin of Earth’s water is poorly understood with the ories ranging from accretion of water-rich planetesimals followed by outgassing from the differentiated planet, dynamical exogenous delivery from ice-rich planetesimals just beyond the ice-line, and late bombardment by comets. The efforts of LAPLACE Module #2 are centered on understanding the formation of planets and constraints on the ir habitability. Most of our research is focused on observational studies of gas and dust in circumstellar disks in order to test the ories of planet formation. Additional work provides a dynamical context to understand the evolution of planetary systems. We continue efforts to combine what we know from our own solar system’s history with constraints on the formation and evolution of typical planetary systems in our galaxy to more fully understand the prospects for life in the Universe. Our research module addresses primarily

Module 2 Research Highlights:

  • Mid-infrared excess emission toward sun-like stars traces processes consistent with models of planet formation. Statistics suggest that many, if not most sun-like stars could be forming terrestrial planets.
  • No strong connection is found between the presence of radial-velocity planets and evidence for debris dust from far-infrared observations. Results suggest that remnant planetesimals may be more common than gas giant planets.
  • High resolution imaging survey of Spitzer targets indicates that cold debris disk systems lack massive giant planets at large radii.
  • Dynamical study suggests that mass loss from the sun early in the history of the solar system cannot explain sustained temperatures that permitted liquid water through orbital evolution of the Earth.
  • First photometric results from survey telescope constrain variability of young sun-like stars.

Module 2 Progress:

Primordial Gas-rich Disks:

Primordial gas-rich disks provide the initial conditions from which habitable planetary systems are thought to form. Working with Phil Hinz, graduate student Wilson Liu published his results from a high resolution imaging survey of disks around intermediate mass Herbig Ae/Be stars using the novel technique of nulling interferometry. Fewer systems were resolved than expected suggesting some revisions of standard models are needed. These results comprised a significant portion of Wilson ‘s PhD the sis. Undergraduate student Praveen Kundurthy (now Astrobiology graduate student at U.W.) published his senior the sis demonstrating a correlation between stellar rotation period and the presence of an inner accretion disk. Ilaria Pascucci et al. report detection of [NeII] emission in circumstellar disks around young stars from the Formation and Evolution of Planetary Systems (FEPS) Spitzer Legacy Science Program (Meyer et al. 2006). Such observations help constrain models for the photoevaporation of gas-rich disks. She and J. Najita participated in a follow-up study resolving the line profile using a ground-based echelle spectrometer suggesting that the emission comes from relatively close to the star. Members of Module #2 participated in a Spitzer study that investigated dust processing and grain growth ( the first step toward planet formation) in young circumstellar disks (Bouwman et al. 2007). The results demonstrate that grain growth is also accompanied by settling of the grains to the midplane and decreased flaring. C. Chen (along with J. Najita) participated in a number of studies concerning the processing of dust in primordial and debris disks with Spitzer. Despite the large samples collected to date, it is still a mystery what controls the processing of pristine interstellar dust grains into heavily altered crystalline forms as the re is no strict correlation of dust properties with stellar age, luminosity, or observable disk property. Pascucci and Module #2 colleagues Meyer and Apai completed a study of grain processing in young T Tauri binaries indicating that the presence of a binary companion also does not alter the processing history of dust. Our results concerning the evolution of gas-rich disks (reported last year to the NAI) suggest that the timescale to form giant planets ranges from 3-10 Myr, were published this year (Pascucci et al. 2006). Finally, Meyer participated in a study of primordial disk lifetime as a function of stellar mass. Evidence is mounting that circumstellar disks last longer around stars of lower mass (Carpenter et al. 2006).

Debris Disks and Planets:

Dust generated through collisions of planetesimals creates observable excess emission in the infrared and the star systems exhibiting this phenomenon are commonly referred to as “debris disk systems”. These planetesimal belts are thought to be dynamically stirred by the presence of (dwarf) planets as small as Pluto, though any larger planets present in the systems will also wreak havoc. Meyer et al. (2007) report observations of mid-infrared excess emission toward sun-like stars from the FEPS project (Figure 2-1). These data place constraints on models of terrestrial planet formation and suggest that such planets could be quite common around typical stars in the galaxy. Working with several members of Module #2, Amaya Moro-Martin (former graduate student of LAPLACE member Renu Malhotra) searched for a connection between evidence for the presence of cold outer debris disks from Spitzer and radial velocity detections of inner gas giant planets. No connection was found and one interpretation is that debris disks such as the Kuiper Belt in our own solar system are more common that close-in gas giant planets. The star HD 38529 was found to possess both multiple gas giants and outer debris. In a follow-up paper, Moro-Martin et al. provide a consistent dynamical model for the system that takes both observations into account. Christine Chen was a co-author on a debris disk survey targeting mature binary star systems finding that companions do not inhibit the formation of planetesimal belts (Trilling et al. 2007). She also completed a study of the remnant gas and dust in the Beta Pictoris system in which she introduced a new mechanism to produce debris gas (Chen et al. 2007). Finally, Apai et al. (2007) searched for gas giant planets around Spitzer targets sporting cold outer debris disks using adaptive optics on the European Sou the rn Obsrevatories Very Large Telescope in Chile . They find no evidence for extra-solar giant planets surrounding the se debris disk systems with larger inner holes (Figure 2.1).

Evolution of Planetary Systems:

Graduate student Jade Bond, working with planetary scientist Dante Lauretta, and astronomical colleagues, published a study concerning the distribution of heavy elements in stars known to host extra-solar planets. Curtis Cooper, who defended his PhD in 2006 and became a NAI post-doctoral fellow, completed a number of research projects related to the structure and dynamics of the atmospheres of “hot Jupiters”, gas giant planets that reside very close to the ir host stars. Ca the rine Neish, working with LAPLACE colleague Jonathan Lunine, completed a study of the pre-biotic chemistry on Titan in comparison to Earth based on results from the Huygens probe. Graduate student David Minton completed a study of the dynamical history of the solar system driven by mass loss from the Sun, under the supervision of Renu Malhotra. They find that invoking orbital evolution of the Earth due to solar mass loss is an unlikely solution to the “faint young Sun” paradox. Finally, members of the National Solar Observatory led by Mark Giampapa have succeeded in obtaining first results from a new transit telescope project whose main goal is to study the variability of sun-like stars as a function of age.

Figure 2.1: Sensitivity of new observations from Apai et al. (2007) to giant planets as a function of orbital radius for stars with cold outer debris disks identified with the Spitzer Space Telescope (Meyer et al. 2006). The vertical dashed line indicates the minimum inner radius of the detected dust disk. The dotted-lines indicate the minimum planet mass required to clear the circumstellar disk of planetesimals. Light shaded regions indicate the sensitivity appropriate for optimistic (young) estimates of the stellar age, while darker regions correspond to pessimistic assumptions (older ages). No planetary mass companions were detected placing strong constraints on the ories of dynamical interactions.
Figure 2.2: Frequency of mid-infrared excess emission observed towards sun-like stars as a function of age (Meyer et al. 2007). Such emission is consistent with dust generated through the collisional evolution of warm planetesimal belts such as those required to form the terrestrial planets. The observations indicate that this behavior persists around 10-20 % of sun-like stars with ages between 3-300 Myr. If the duration of this evolutionary phase is short compared to the width of the age-bins, the n the se data support the notion that > 60 % of sun-like stars could form terrestrial planets.
Figure 2.2: Frequency of mid-infrared excess emission observed towards sun-like stars as a function of age (Meyer et al. 2007). Such emission is consistent with dust generated through the collisional evolution of warm planetesimal belts such as those required to form the terrestrial planets. The observations indicate that this behavior persists around 10-20 % of sun-like stars with ages between 3-300 Myr. If the duration of this evolutionary phase is short compared to the width of the age-bins, the n the se data support the notion that > 60 % of sun-like stars could form terrestrial planets.

Variability of Sun-like Stars Through Time Introduction:

The next stage of evolution of terrestrial planets after disk dispersal is evolution in the presence of stars that have entered the Zero Age Main Sequence stage of evolution but are still characterized by variability in the ir luminous output and possibly the generation of energetic particles. Solar-like stars vary in energy output on a range of timescales. The effort discussed here seeks to quantify the range of variability on short and long time scales in young stars (ages ~ 100 Myr) and solar-age stars (age ~ 5 Gyr)

The changing output of solar-type stars (total, UV, and X-ray)—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 Gyr). Delineating irradiance and associated activity variability on sun-like stars spanning a range of ages thus provides crucial insight on how frequently earth-like atmospheres are likely to form and survive, and how frequently exo-earths encounter benign climatic conditions. The portion of the module discussed herein seeks to describe the nature and evolution of the ambient radiative and inferred energetic particle environment in which the exoplanetary atmospheres evolve.

Accumulate nightly V-band photometric observations of a field in the Pleiades open cluster (age ~ 80—100 Myr) that includes a number (~ 20) of young solar analogs.

Commence nightly observations of solar analogs in the solar-age open cluster M67 (age ~ 5 Gyr). Refine data processing pipeline for batch reduction of photometric data combined with the analysis of variability. Fur the r developed hardware for future data acquisition, including the adaptation of a pointed, automated telescope for pointed “stare-mode” observations of a cluster field during the night analyze the data acquired during the Pleiades season

During the reporting term of 1 July 2006—30 June 2007, we obtained observations of the Pleiades and M67 open clusters, respectively, during each clear night, utilizing small-aperture, automated transit telescope systems. We have focused on the Pleiades in our reduction and analysis since we expect to see the most variability and since young planetary atmospheres are still evolving at this young age.

Our instrumentation consisted of a transit telescope based on a conventional Newtonian design with a 20-cm aperture, f/5.44 primary of low-expansion Pyrex glass and a 3.8-cm minor axis secondary, also of Pyrex. A temperature-compensating optical support structure using the differential expansion of steel and aluminum rods eliminates the need for focus adjustments during the course of a night.

A home-built camera based upon an existing design (Tucker 1995, {\it A Public Domain CCD Camera Design}, BAAS, 185, {\#} 63.04) was implemented utilizing athinned, back-illuminated 512 \times 512 CCD from Scientific Imaging Technologies ( Beaverton , Oregon ). The TK512 device has 27 micron pixels and, in combination with the telescope system, delivers an image scale of 5.04 arcsec per pixel. The resulting field of view is 43.0 arcmin.

The observations are obtained in ‘drift-scan" mode. In this technique, the image of the sky is allowed to drift across the CCD at the focal plane of the telescope while clocking the parallel registers of the CCD at a rate that transports the accumulating charge at the same rate that the field is moving across the CCD. In our case, we utilize the stationary telescope described above and drift motion due to the earth’s rotation. The results reported here were obtained in this mode of operation using a V-band filter on 23 nights during 5 December 2005—18 January 2006.

Our pipeline uses DAOFIND to identify all of the stars on each image. It the n measures aperture photometry using an aperture with a radius of 4 pixels. We calculate differential magnitudes for each target star using between 4 and 20 comparison stars that are within 20 arc minutes of the target star. The zero point of our magnitudes has been roughly matched to the known V magnitudes of Pleiades members.

We encountered a number of data reduction challenges related to factors involving elongated images (because of the relatively high declination of the cluster for fixed transit telescopes) and variable extinction across the cluster combined with few usable comparison stars in the field. We the refore had to spend significant effort in revising our data processing pipeline. While this work is still in progress, we were able to obtain some preliminary results, which we summarize in the following.

Module 2 Collaborative Efforts:

Module 2 members are heavily involved with several large programs involving NASA telescopes Spitzer Space Telescope including: a) The Spitzer Legacy Science Program “Formation and Evolution of Planetary Systems” (M. Meyer, J. Najita, S. Strom, R. Malhotra, J. Lunine, I. Pascucci, S. Cortes); b) the Spitzer IRS GTO Disks Team (C. Chen, J. Najita); and c) the Orion Nebula Cluster Hubble Space Telescope Treasury Program (M. Meyer, J. Najita). In addition, Module #2 members are PIs and Co-I’s on a number of programs on Spitzer, Hubble, Chandra, XMM-Newton, FUSE, and (soon) Herschel that relate to the research goals of LAPLACE . Affiliates of Module #2 are also members of the science and analysis teams for the Stardust (Lauretta), Cassini (Lunine), and recently launched Phoenix missions. Module #2 researchers are also participating in new proposals for solar system mission s such as OSIRIS (Lauretta), as well as concept studies for new astrophysics missions such as TOPS (Angel, Meyer, and Woolf). TOPS utilizes the new technique of Phase Induced Amplitude Apodization (PIAA) developed by PI Olivier Guyon in a 1-2 meter class telescope to search for earth-like planets in reflected light. The TOPS program would be based in Arizona with PI Guyon and Deputy-PI Meyer with support from NASA-Ames and JPL, and NAI colleagues from U.W. and Penn State among the science team.

Within the NAI, members of Module 2 are leading a program selected for support from the Director’s Discretionary Fund (DDF) “Chemical Constraints on the Formation of Habitable Worlds: A Combined Astronomical/Laboratory/Theoretical Approach” (PI: M. Meyer). Module 2 members are also participating in a DDF program led by Steinn Sigurdsson (PSU) concerning “Extreme Habitability. M. Meyer was a co-I on the proposal and D. Apai participated in a workshop supported through this program in June, 2007.

Finally, Module #2 researchers make significant use of the University of Arizona Observatories which include major ground-based collaborative telescope facilities. The MMT 6.5 meter optical/infrared telescope on Mt. Hopkins in sou the rn Arizona is a partnership of the University of Arizona and the Harvard-Smithsonian Center for Astrophysics. The Magellan Project manages two 6.5 meter telescopes at Las Campanas Observatory in Chile through a consortium comprised of the Observatories of the Carnegie Institution of Washington, Harvard University , MIT, the University of Michigan , and the University of Arizona . The Large Binocular Telescope Project operates two 8.4 meter telescopes on Mt. Graham in sou the rn Arizona in partnership with Germany and Italian research consortia, the Ohio State University, the Research Corporation (coordinating the Universities of Virginia, Minnesota, and Notre Dame), and the University of Arizona. The UofA is also a partner with the Vatican Observatory in operating a 1.8 meter telescope on Mt. Graham . The Arizona Radio Observatories operates a 12 meter millimeter wave telescope on Kitt Peak as well as the 10 meter Sub-millimeter Telescope on Mt. Graham . In addition, colleagues at NOAO are responsible for administering time on telescopes at Kitt Peak , Cerro Tololo Inter-American Observatory in Chile , the twin 8 meter Gemini telescopes in Hawaii and Chile , as well as time available on privately run telescopes on behalf of the national community. The University of Arizona is also a founding partner in the Giant Magellan Telescope consortium which is planning to build a 24.5 meter telescope in Chile . Researchers within Module #2 are central to this effort (Angel, Hinz, Meyer). Module #2 researchers at NOAO (Strom, Najita) have also participated heavily in plans for access to the next generation of extremely large telescopes on behalf of the national community.

Module 2 Other:

  • Gas giants jump into planet formation. Based on work of I. Pascucci and collaborators. bin/WebObjects/UANews.woa/wa/MainStoryDetails?ArticleID=13445
  • Hosted international exchange student from the University of Heidelberg to conduct a practicum in Astrobiology.
  • Guest Lecturers at the Vatican Observatory Summer School on Exoplanets and Brown Dwarfs (M. Meyer, J. Liebert, and J. Lunine, June, 2007).
  • M. Meyer received the Blitzer Award for Excellence in Teaching of Physical Sciences from the University of Arizona .