New Debris Disks Around Young, Low Mass Stars Discovered With The Spitzer Space Telescope

Nearly twenty-five years ago, the Infrared Astronomical Satellite (IRAS) launched the study of infrared excesses around stars that we attribute to mature extra-solar planetary systems (e.g., Rhee et al. 2007;Zuckerman 2001;Fajardo-Acosta et al. 2000;Mannings & Barlow 1998;Backman & Paresce 1993;Walker & Wolstencroft 1988;Aumann et al. 1984). Parent bodies in a planetary system -∼1 m and larger aggregates of rock and ice analogous to asteroids and Kuiper-belt objects in our own Solar System -collide and produce dusty debris that is heated by incident stellar radiation. This optically thin "debris disk" reemits the absorbed radiation at infrared wavelengths; it is detected in excess of the expected stellar radiation around ∼15% of main sequence stars (e.g., Lagrange et al. 2000). Ground-based infrared and sub-millimeter efforts (e.g., Lestrade et al. 2006;Liu et al. 2004;Weinberger et al. 2004;Song et al. 2002), the Infrared Space Observatory (ISO; de Muizon 2005; Laureijs et al. 2002;Habing et al. 2001;Spangler et al. 2001) and the Spitzer Space Telescope (Werner et al. 2004) have discovered >100 nearby stars with infrared excess. These discoveries include several dozen debris disks around young stars less massive than the Sun (e.g., Rebull et al. 2008;Meyer et al. 2007;Smith et al. 2006;Low et al. 2005;Chen et al. 2005a). Large samples of these systems observed with the Spitzer Space Telescope are useful in determining overall trends such as disk frequency, disk infrared luminosity, and dust dynamics as a function of age or effective stellar temperature both in the field (e.g., Carpenter et al. 2009;Hillenbrand et al. 2008;Trilling et al. 2008;Wyatt 2008;Gautier et al. 2008;Wyatt 2007;Beichman et al. 2006a,b;Bryden et al. 2006;Su et al. 2006;Rieke et al. 2005;Chen et al. 2005a) and in clusters and associations (e.g., Currie et al. 2009;Dahm & Carpenter 2009;Cieza et al. 2008;Currie et al. 2008b;Hernandez et al. 2008;Cieza & Baliber 2007;Currie et al. 2007;Dahm & Hillenbrand 2007;Gorlova et al. 2007;Hernandez et al. 2007a,b;Carpenter et al. 2006;Chen et al. 2005b;Low et al. 2005;Stauffer et al. 2005;Gorlova et al. 2004). High-contrast, high-resolution direct and coronagraphic imaging from a number of telescopes has spatially resolved ∼15 debris disks around solar-type stars in scattered and/or thermal emission that show disk structures and clearings indicative of possible unseen planets (e.g., Fitzgerald et al. 2007;Telesco et al. 2005;Krist et al. 2005a,b;Marsh et al. 2005;Kalas et al. 2004;Stapelfeldt et al. 2004;Holland et al. 2003;Wahhaj et al. 2003;Heap et al. 2000;Schneider et al. 1999;Jayawardhana et al. 1998;Koerner et al. 1998). The recent reported discoveries of Fomalhaut b (Kalas et al. 2008;Chiang et al. 2009) and a possible planet orbiting Beta Pic (Lagrange et al. 2009) support the hypothesis that planets can directly influence debris disk structure. Spatially resolved systems provide further information about the dust dynamics and evolution, but discerning overall trends is limited by the small number of resolved systems.

In parallel with infrared observations and high-contrast imaging of young stars, over 300 extrasolar planets have been discovered, primarily with the radial velocity technique, in the past decade, but also through direct imaging, transits, microlensing, and pulsar timing (Marois et al. 2008;Kalas et al. 2008;Butler et al. 2006;Marcy et al. 2005;Bond et al. 2004;Beaulieu et al. 2006;Wolszczan & Frail 1992, and references therein). The synergy of these planet-finding methods with the study of debris disks can yield new insights into the overall circumstellar architecture of exo-planetary systems. For example, radial velocity discovered extrasolar planets have revealed trends such as the Jovian planet frequency -host star metallicity correlation (Gonzalez 1997). This correlation is not observed for debris disks (Beichman et al. 2006a).

Fundamental questions remain about the dust dynamics, properties, and evolution of debris disks around young stars. For example, the evolutionary time-scales for both primordial and debris disks appear to be dependent on spectral type. Optically thick primordial disks are common (∼50%) at ages of ∼1 Myr around stars with spectral types of A through M (Haisch et al. 2001;Meyer et al. 1997). These primordial disks provide the material to form planets and parent bodies that in turn can generate secondary debris disks. In the 3-5 Myr Upper Sco association, Carpenter et al. (2006) observes that primordial disks continue to persist in an optically thick state around K and M dwarfs, but have already transitioned to optically thin disks around earlier type stars. It might follow that debris disks should similarly persist for longer around K and M dwarfs due to lower stellar luminosities and masses ( §A.2). Debris disks can persist for stars older than ∼1 Gyr around A,F, and G-type stars (Trilling et al. 2008;Meyer et al. 2007;Su et al. 2006;Bryden et al

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