PLANETS AND INFRARED EXCESSES: PRELIMINARY RESULTS FROM A SPITZER MIPSSURVEY OF SOLAR-TYPE STARSC. A. Beichman,1G. Bryden,1G. H. Rieke,2J. A. Stansberry,2D. E. Trilling,2K. R. Stapelfeldt,1M. W. Werner,1C. W. Engelbracht,2M. Blaylock,2K. D. Gordon,2C. H. Chen,1K. Y. L. Su,2and D. C. Hines2Receivved 2004 October 18; accepted 2004 December 10ABSTRACTAs part of a large Spitzer MIPS Guaranteed Time Observation program, we have searched for infrared excessesdue to debris disks toward 26 FGK field stars known from radial velocity (RV) studies to h ave one or more planets.While none of these stars show excesses at 24 m, we h ave detected 70 m excesses around six s tars at the 3 confidence level. The excesses are produced by co ol material (<100 K) located beyond 10 AU, well outside the‘‘habitable zones’’ of these systems and consistent with the presence of Kuiper Belt analogs with 100 times moreemitting surface area than in our own planetary system. These planet-bearing stars are, by selection for RV studies,typically older than 1 Gyr, and the stars identified here with excesses have a median age of 4 Gyr. We find apreliminary correlation of both the frequ ency and the magnitude of dust emission with the presence o f knownplanets. These are the first stars outside the solar system identified as having both well-confirmed planetary systemsand well-confirmed I R exce sses.Subject headinggs: infrared: stars — Kuiper Belt — planetary systems: formation —planetary systems: protoplanetary disks1. INTRODUCTIONFirst discovered by the Infrared Astronomical Satellite (IRAS),the ‘‘Vega phenomenon’’ is characterized by a significant de-viation from the expected Rayleigh-Jeans emission of quies-cent main-sequence stars (Aumann & Good 1990; Gillett 1986).The excess IR emission from other stars has been attributed todust orbiting at distances anywhere from less than 1 to more than100 AU, analogous to the zodiacal cloud and the Kuiper Belt inour own solar system. Because of effects such as radiation pres-sure and Poynting-Robertson drag, the lifetime of the dust is gen-erally much shorter than the age of the system; any dust observedmust have been recently produced. In the solar system, dust is gen-erated by collisions between larger bodies in the asteroid andKuiper Belts, as well as from outgassing comets. Extrasolar sys-tems with IR excess presumably have their own supply of large,solid planetesimals, and, perhaps, large planets like those in oursystem.IRAS found that approximately 15% of main-sequence stars(Plets & Vynckier 1999), predominantly A an d F stars, showexcesses attributable to dust in orbit around the stars at dis-tances between a few and a few hundred AU and he ated by a b-sorbed starlight to temp eratures o f 3 0– 300 K (Backman &Paresce 1993). Fractional luminosities, Ldust/L*, are observed aslarge as 103for stars with bright excesses such as Pic and assmall as 105for stars at the detection thr eshold. These valuescan be compared to our solar system: 108to 107for the hotmaterial in the zodiacal clo ud (1–3 AU; Dermott et al. 2002)and 107to 106predicted for the cooler Kuipe r Belt regionbeyond 10 AU (Stern 1996), which is constrained to be <105by IRAS (Aumann & Good 1990; Teplitz et al. 1999). Obser-vations by the Infrared Sp ace Observatory (ISO;Habingetal.2001; Spangler et al. 2001) suggest a decline in the fraction ofstars with excess IR emission with time, but with the possibilityof finding modest (Ldust=L > 105) excess es among older stars(Decin et a l. 2000, 20 03). Spitzer and IRAS results toward morethan 200 A stars confirm a general decline in the average amountof emissio n with little 24 m emission seen for sources olderthan 100 Myr but with large variations within each age groupstudied (Rieke et al. 2005). The variations are probably due inpart to sporadic replenishment of dust clouds by individual col-lisions between large, solid bodies, but also are likely to reflect arange in mass and exte nt for the initial planetesimal disk.Images in the optical and near-IR (Heap et al. 2000; Schneideret al. 1999), submillimeter (Holland et al. 1998, 2003; Greaveset al. 1998) and from Sp itzer itself (Stapelfeldt et al. 2004) haverevealed that in some cases the dust exists in flattened disks withwarps, gaps, and bloblike structures that have been attributedto the gravitational influen ce of planets (Wyatt & Dent 2002;Dermott et al. 2002). Still, while the frequency and magnitudeof IR excess are strongly correlated with stellar youth, no suchcorrelation has yet been found between IR excess and th e pres-ence of planets. Even b efore the first confirmed discovery of anextrasolar p lanet orbiting a Sun-like star (51 Peg b; Mayor &Queloz 1995; Marcy & Butler 1995), attempts had been madeto find links between p lanets and d usty debris around other stars(e.g., Eri; Backman & Paresce 1993). While ea rly radial ve loc-ity (RV) searches fo r pla nets found a planet-like trend for Eri(Campbell et al. 1988 ), the can didate planet remains controver-sial even tod ay owing to the extremely high level of chromo-spheric activity on this young star. The 7 yr periodicity fou ndin the star’s RV (Campbell et al. 1988; Cumming et al. 1999;Hatzes et al. 2000) may be induced by a planet orbiting at sev-eral AU (Hatzes et al. 2 000) or may in stead be a reflection of thestar ’s poorly unde rstood magnetic cycle. Conversely, 55 Cnchas a well-known planetary system, but claims of a dust diskbased on visible imaging (Trilling et al. 2000) and ISO p ho-tometry (Dominik et al. 1998) have been ruled out by HubbleSpace Telescope coronography (Schneider et al. 2001), submil-limeter mapping (Jayawardhana et al. 2002), and Spitzer Space1Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109;and Michelson Scienc e Center, California Institute of Technology, 770 SouthWilson Avenue, Pasadena, CA 91125.2Steward Observatory, University of Arizona, 933 North Cherry Avenue,Tucson, AZ 85721.1160The Astrophysical Journal, 622:1160–1170, 2005 April 1# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.Telescope Multiband Imaging Photometer (MIPS) measure-ments presented here. In spite of the many studies specificallydesigned t o find a connection between dust and planets, no sys-tems with both well-confirmed planets and well-confirmed IRexcess have yet been observed. In fact, the data to da te appea rto
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