Structure and Evolution of Debris Disks Around F-Type Stars
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The Astrophysical Journal Supplement Series,193:4(25pp),2011March doi:10.1088/0067-0049/193/1/4 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ! STRUCTURE AND EVOLUTION OF DEBRIS DISKS AROUND F-TYPE STARS. I. OBSERVATIONS, DATABASE, AND BASIC EVOLUTIONARY ASPECTS A. Moor´ 1,I.Pascucci2, A.´ Kosp´ al´ 3,P.Abrah´ am´ 1,T.Csengeri4,L.L.Kiss1,5,D.Apai2,C.Grady6,7,Th.Henning8, Cs. Kiss1, D. Bayliss9,A.Juhasz´ 8,J.Kovacs´ 10,andT.Szalai11 1 Konkoly Observatory of the Hungarian Academy of Sciences, P.O. Box 67, H-1525 Budapest, Hungary; [email protected] 2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 3 Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, The Netherlands 4 Laboratoire AIM, CEA/DSM, IRFU/Service d’Astrophysique, 91191 Gif-sur-Yvette Cedex, France 5 Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia 6 NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA 7 Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602, USA 8 Max-Planck-Institut fur¨ Astronomie, Konigstuhl¨ 17, 69117 Heidelberg, Germany 9 Research School of Astronomy and Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia 10 Gothard Astrophysical Observatory, ELTE University, 9707 Szombathely, Hungary 11 Department of Optics and Quantum Electronics, University of Szeged, 6720 Szeged, Dom´ ter´ 9, Hungary Received 2010 May 30; accepted 2010 December 10; published 2011 January 20 ABSTRACT Although photometric and spectroscopic surveys with the Spitzer Space Telescope remarkably increased the number of well-studied debris disks around A-type and Sun-like stars, detailed analyses of debris disks around F-type stars remained less frequent. Using the MIPS camera and the Infrared Spectrograph (IRS) spectrograph, we searched for debris dust around 82 F-type stars with Spitzer.Wefound27starsthatharbordebrisdisks,nineofwhicharenew discoveries. The dust distribution around two of our stars, HD 50571 and HD 170773, was found to be marginally extended on the 70 µmMIPSimages.CombiningtheMIPSandIRSmeasurementswithadditionalinfraredand submillimeter data, we achieved excellent spectral coverage for most of our debris systems. We have modeled the excess emission of 22 debris disks using a single temperature dust ring model and of five debris systems with two-temperature models. The latter systems may contain two dust rings around the star. In accordance with the expected trends, the fractional luminosity of the disks declines with time, exhibiting a decay rate consistent with the range of model predictions. We found the distribution of radial dust distances as a function of age to be consistent with the predictions of both the self-stirred and the planetary-stirred disk evolution models. A more comprehensive investigation of the evolution of debris disks around F-type stars, partly based on the presented data set, will be the subject of an upcoming paper. Key words: circumstellar matter – infrared: stars Online-only material: color figure 1. INTRODUCTION perturbation, even at a significant distance from the planetesi- mal disk. Thus, these large bodies can also initiate and govern Nearly all young stars harbor circumstellar disks, which ini- the formation and evolution of a debris disk (Mustill & Wyatt tially serve as reservoirs for mass accretion and later can be- 2009), providing an alternative stirring mechanism. In a debris come the birthplaces of planetary systems. During this lat- disk, mutual collisions grind down planetesimals to small dust ter process, the originally submicron-sized dust grains start grains that are then ejected by radiation pressure, or in more growing, and their aggregation is believed to lead to km-sized tenuous disks removed by the Poynting–Robertson (PR) drag planetesimals (for a review, see Apai & Lauretta 2010,andref- (Dominik & Decin 2003;Wyatt2005). This process is accom- erences therein). Non-destructive collisions between planetesi- panied by the depletion of the reservoir planetesimal belt and mals result in the formation of subsequently larger bodies. These eventually leads to the decline of the debris production (Wyatt events happen first in the inner disk due to the shorter colli- et al. 2007a;Lohne¨ et al. 2008). sional timescales, then the process propagates outward (Kenyon Due to the strong link between the debris dust and the unseen &Bromley2004a). The newly formed Pluto-sized protoplanets planetesimals, the investigations of the smallest particles of de- stir up the motion of leftover smaller bodies in their vicinity, bris systems can lead to a better understanding of the formation initializing a collisional cascade. As they become more ener- and evolution of planetesimal belts and, eventually, the for- getic, collisions result in the erosion of planetesimals and the mation and evolution of planetary systems. The observational production of small dust grains. An optically thin debris disk is verification of the different aspects of planetesimal formation formed, in which the second-generation, short-lived dust grains and evolutionary model predictions requires a detailed study of are continuously replenished by collisions and/or evaporation the incidence of stars with infrared (IR) emission due to debris of planetesimals (Backman & Paresce 1993;Wyatt2008). This dust and investigating the change of debris disk properties (e.g., self-stirring mechanism is not the sole feasible way to incite de- radius of the dust ring and fractional luminosity) with age. The structive collisions between minor bodies. Giant planets, formed ideal way would be to resolve and observe many debris disks previously in the primordial disk, or stellar companions can also in scattered light or in thermal emission from optical to mil- dynamically excite the motion of planetesimals via their secular limeter wavelengths with good wavelength coverage. In reality, 1 The Astrophysical Journal Supplement Series,193:4(25pp),2011March Moor´ et al. however, the number of resolved disks is very limited and the A-type stars Rhee et al. (2007)foundsomeevidencethatthe spectral energy distribution (SED) of the dust emission was radius of dust belts is increasing with stellar age. measured for most debris systems only in a few infrared bands. Thanks to the recent photometric and spectroscopic surveys The fundamental parameters of the disks have to be estimated with the Spitzer Space Telescope,thenumberofdebrisdisks from these sparsely sampled SEDs. The interpretation of SEDs with detailed SED at mid- and far-IR wavelengths has been is ambiguous (e.g., considering the radial location of the dust) increased significantly (Chen et al. 2006;Riekeetal.2005; but by handling a debris disk sample as an ensemble, one can Su et al. 2006;Carpenteretal.2008;Rebulletal.2008; obtain a meaningful picture about the basic characteristics of the Trilling et al. 2008). This improvement is especially remark- parent planetesimal belt(s) and about the evolutionary trends. able for disks around A-type and Sun-like stars (late F-, G-, The current theoretical models dealing with the buildup and K-type stars). The comparison of these data with the pre- of planetesimals (Kenyon & Bromley 2004a, 2008)andwith dictions of quasi-steady-state evolutionary models showed that the steady-state collisional evolution of the planetesimal belts most observed trends for A-type and Sun-like stars can be (Dominik & Decin 2003;Wyattetal.2007a;Lohne¨ et al. 2008) reproduced adequately (Wyatt 2008;Carpenteretal.2009b). predict how the fundamental properties of debris disks evolve Kennedy & Wyatt (2010)confrontedtheSpitzer observations of with time. At a specific radius, the peak of the dust emission A-type stars with an analytic model that also take into account is believed to coincide with the formation of 1000–2000 km the effects of the self-stirring on the disk evolution. Utilizing sized planetesimals. After this stage—parallel with the depletion this model they were able to reproduce the observed trends and of planetesimals—the dust emission decreases with time. The they obtained rough estimates for some initial parameters (e.g., evolution of the disk can be traced both in the variation average mass) of disks around A-type stars. It was also con- of incidence of disks with time and in the evolution of the cluded that debris disks are narrow belts rather than extended brightness of dust emission. The different models predict that disks. According to the models, F-type stars are expected to be the dust fractional luminosity, the ratio of the energy radiated by an intermediate type between the A-type and Sun-like stars in n the dust to the stellar luminosity, varies with time as t− ,where terms of debris disk evolution as well: (1) their disks are pre- n 0.3–1 in disks where collisions are the dominant removal dicted to evolve faster than those around main-sequence stars of process= (Dominik & Decin 2003;Wyattetal.2007a;Lohne¨ later types (in disks with identical surface density distribution, et al. 2008;Kenyon&Bromley2008). The unique sensitivity of the timescale of planetesimal formation processes are thought to 1/2 the Spitzer Space Telescope in the MIPS 24 µmbandallowed be proportional to M− )and(2)F-typestarslivemuchlonger the detection of stellar photospheres and a small amount of than A-type stars (the∗ main-sequence lifetime of a 1.4 M F5- excess for a large number of field stars (e.g., Rieke et al. 2005; type star is three times longer than the main-sequence lifetime$ Meyer et al. 2008)andevenforrelativelydistantopencluster of a 2.0 M A5-type star), making it possible to follow disk members (e.g., Young et al. 2004;Gorlovaetal.2006;Siegler evolution for$ a significantly longer time. Up to now the number et al. 2007;Currieetal.2008;Balogetal.2009). The latter of detailed studies of debris disks around F-type stars is modest observations enabled the study of the evolution of warm dust compared to the A-type and Sun-like samples, preventing us around well-dated sample stars.