Circumstellar Disk Evolution: Constraining Theories of Planet Formation

📝 Abstract

Observations of circumstellar disks around stars as a function of stellar properties such as mass, metallicity, multiplicity, and age, provide constraints on theories concerning the formation and evolution of planetary systems. Utilizing ground- and space-based data from the far-UV to the millimeter, astronomners can assess the amount, composition, and location of circumstellar gas and dust as a function of time. We review primarily results from the Spitzer Space Telescope, with reference to other ground- and space-based observations. Comparing these results with those from exoplanet search techniques, theoretical models, as well as the inferred history of our solar system, helps us to assess whether planetary systems like our own, and the potential for life that they represent, are common or rare in the Milky Way galaxy.

💡 Analysis

Observations of circumstellar disks around stars as a function of stellar properties such as mass, metallicity, multiplicity, and age, provide constraints on theories concerning the formation and evolution of planetary systems. Utilizing ground- and space-based data from the far-UV to the millimeter, astronomners can assess the amount, composition, and location of circumstellar gas and dust as a function of time. We review primarily results from the Spitzer Space Telescope, with reference to other ground- and space-based observations. Comparing these results with those from exoplanet search techniques, theoretical models, as well as the inferred history of our solar system, helps us to assess whether planetary systems like our own, and the potential for life that they represent, are common or rare in the Milky Way galaxy.

📄 Content

arXiv:0906.4507v1 [astro-ph.EP] 24 Jun 2009 The Ages of Stars Proceedings IAU Symposium No. 258, 2009 E.E. Mamajek, D.R. Soderblom & R.F.G. Wyse, eds. c⃝2009 International Astronomical Union DOI: 00.0000/X000000000000000X Circumstellar Disk Evolution: Constraining Theories of Planet Formation Michael R. Meyer Steward Observatory, The University of Arizona 933 N. Cherry Avenue, Tucson, AZ 85721 (USA) Abstract. Observations of circumstellar disks around stars as a function of stellar properties such as mass, metallicity, multiplicity, and age, provide constraints on theories concerning the formation and evolution of planetary systems. Utilizing ground- and space-based data from the far–UV to the millimeter, astronomers can assess the amount, composition, and location of circumstellar gas and dust as a function of time. We review primarily results from the Spitzer Space Telescope, with reference to other ground- and space-based observations. Comparing these results with those from exoplanet search techniques, theoretical models, as well as the inferred history of our solar system, helps us to assess whether planetary systems like our own, and the potential for life that they represent, are common or rare in the Milky Way galaxy. Keywords. solar system: formation, stars: circumstellar matter, pre–main-seqeunce, planetary systems: protoplanetary disks, planetary systems: formation

1. Introduction Are there multitudes of planetary systems that are capable of harboring life, like our own Solar System? Answering this question motivates the research activities of a great number of astronomers, as well as scientists of many disciplines. Yet the answer depends on which aspect of our solar system to which one is comparing the physical properties of other systems. Extrapolation of radial velocity results to 20 AU suggests that planets with mass at least a third that of Jupiter’s surround 15-20 % of sun-like stars (Cumming et al. 2008). Yet lower mass planets might turn out to be even more common (e.g. Mayor et al. 2009). Enormous progress has been made in the past several years on many aspects of circumstellar disk evolution (e.g. Meyer et al. 2007), especially those that can be addressed with observations from the Spitzer Space Telescope (Werner et al. 2006). In this review we explore answers to three key questions: 1) What is the time available to form gas giant planets? 2) What is the history of planetesimal collisions versus radius? 3) How do answers to the above vary with stellar properties? Because the answers to these questions are subtle, one needs large stellar samples with reliable stellar ages from the youngest pre–main sequence stars to the oldest stars known in the galactic disk. In our attempt to study important evolutionary processes in the formation and evolution of planetary systems, we assemble groups of stars with like properties (such as a narrow range in stellar mass) as a function of age, the main topic of this symposium. We hope that by studying the mean (as well as the dispersion) in those properties of the circumstellar environment as a function of time, we can create a ”movie” (or range of plausible trajectories) that helps tell the story of how our solar system might have formed. Once completed for one range in stellar mass, we can attempt to repeat the study for other stellar masses. Examining the differences in circumstellar disk evolution as a function of stellar mass may be our best tool in delineating the most important physical processes in planet formation. It is a lofty goal, and often strong assumptions 109 110 Michael R. Meyer are required to make progress. We can only hope that most of these assumptions represent hypotheses we can test in the near future. Observations of circumstellar gas and dust, both its amount and geometrical distri- bution, can be compared to theoretical timescales for its expected evolution. Keplerian orbits can range from days to millennia. The viscous timescale in the context of an α disk model depends on the orbital radius and can be < 1 Myr within 10 AU for reasonable parameters (Hartmann 1998). Preliminary results suggest that disk chemistry proceeds more slowly than relevant dynamical times indicating that mixing could be important (Bergin et al. 2007). The inward migration of solids in the disk results in the loss of planet–building material and remains a serious problem on many scales: a) gas drag on meter–sized bodies can reach 1 AU/century (Weidenschilling, 1977); b) Type I migration of lunar–mass planetary embryos on timescales of 105 yrs; and c) Type II migration of forming gas giant planets on timescales proportional to the viscous time (e.g. Ida & Lin, 2008 and references therein). The timescale for orderly growth of bodies through colli- sions (e.g. Goldreich et al. 2004) is proportional to the product of the radius and volume density of typical particles divided by the product of the mass surface density of solids and the orbital frequency. Th

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