Draft version July 8, 2018 Preprint typeset using LATEX style emulateapj v. 8/13/10 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, and T. Szalai11 Draft version July 8, 2018 ABSTRACT Although photometric and spectroscopic surveys with the Spitzer Space Telescope increased re- markably the number of well studied debris disks around A-type and Sun-like stars, detailed analyzes of debris disks around F-type stars remained less frequent. Using the MIPS camera and the IRS spectrograph we searched for debris dust around 82 F-type stars with Spitzer. We found 27 stars that harbor debris disks, nine of which are new discoveries. The dust distribution around two of our stars, HD50571 and HD170773, was found to be marginally extended on the 70µm MIPS images. Combin- ing the MIPS and IRS measurements with additional infrared and 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 5 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. Subject headings: circumstellar matter — infrared: stars 1. INTRODUCTION bodies in their vicinity, initializing a collisional cascade. Nearly all young stars harbor circumstellar disks, As they become more energetic, collisions result in the which initially serve as a reservoir for mass accretion, erosion of planetesimals and the production of small and later can become the birthplace of planetary systems. dust grains. An optically thin debris disk is formed, in During this latter process, the originally submicron-sized which the second generation, short-lived dust grains are dust grains start growing, and their aggregation is be- continuously replenished by collisions and/or evapora- lieved to lead to km-size planetesimals (for a review, tion of planetesimals (Backman & Paresce 1993; Wyatt see Apai & Lauretta 2010, and references therein). Non- 2008). This self-stirring mechanism is not the sole fea- destructive collisions between planetesimals result in the sible way to incite destructive collisions between minor formation of subsequently larger bodies. These events bodies. Giant planets, formed previously in the pri- happen first in the inner disk due to the shorter colli- mordial disk, or stellar companions can also dynami- sional timescales, then the process propagates outwards cally excite the motion of planetesimals via their secu- (Kenyon & Bromley 2004a). The newly formed Pluto- lar perturbation, even at a significant distance from the sized protoplanets stir up the motion of leftover smaller planetesimal disk. Thus these large bodies can also ini- tiate and govern the formation and evolution of a de- [email protected] bris disk (Mustill & Wyatt 2009), providing an alterna- 1 Konkoly Observatory of the Hungarian Academy of Sci- tive stirring mechanism. In a debris disk, mutual col- ences, PO Box 67, H-1525 Budapest, Hungary lisions grind down planetesimals to small dust grains arXiv:1012.3631v1 [astro-ph.SR] 16 Dec 2010 2 Space Telescope Science Institute, 3700 San Martin Drive, that are then ejected by radiation pressure, or in more Baltimore, MD 21218, USA 3 Leiden Observatory, Leiden University, Niels Bohrweg 2, tenuous disks removed by the Poynting-Robertson drag NL-2333 CA Leiden, The Netherlands (Dominik & Decin 2003; Wyatt 2005). This process is 4 Laboratoire AIM, CEA/DSM, IRFU/Service accompanied by the depletion of the reservoir planetesi- d’Astrophysique, 91191 Gif-sur-Yvette Cedex, France 5 Sydney Institute for Astronomy, School of Physics A28, mal belt and eventually leads to the decline of the debris University of Sydney, NSW 2006, Australia production (Wyatt et al. 2007a; L¨ohne et al. 2008). 6 NASA Goddard Space Flight Center, Code 667, Greenbelt, Due to the strong link between the debris dust and MD 20771 the unseen planetesimals, the investigations of the small- 7 Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602 est particles of debris systems can lead to a better un- 8 Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, 69117 derstanding of the formation and evolution of planetes- Heidelberg, Germany imal belts and, eventually, the formation and evolution 9 Research School of Astronomy and Astrophysics, The of planetary systems. The observational verification of Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia the different aspects of planetesimal formation and evo- 10 Gothard Astrophysical Observatory, ELTE University, lutionary model predictions requires a detailed study of 9707 Szombathely, Hungary the incidence of stars with infrared (IR) emission due to 11 Department of Optics and Quantum Electronics, University debris dust and investigating the change of debris disk of Szeged, 6720 Szeged, D´om t´er 9., Hungary 2 Mo´or et al. properties (e.g. radius of the dust ring, fractional lu- tended planetesimal disk both the stirring by Pluto-sized minosity) with age. The ideal way would be to resolve planetesimals that were born in the same belt, and the and observe many debris disks in scattered light or in dynamical excitation by secular perturbation of distinct thermal emission from optical to millimeter wavelengths giant planets is thought to be accompanied by the out- with good wavelength coverage. In reality, however, the ward propagation of the dust production site with time number of resolved disks is very limited and the spec- (Kenyon & Bromley 2008; Mustill & Wyatt 2009). The tral energy distribution (SED) of the dust emission was observational evidence for such a delayed initiation of measured for most debris systems only in a few infrared the collisional cascade as the function of radial loca- bands. The fundamental parameters of the disks have tion is not yet conclusive. Some surveys did not report to be estimated from these sparsely sampled SEDs. The any trend in the evolution of the radius with age (e.g. interpretation of SEDs is ambiguous (e.g. considering Najita & Williams 2005), while studying debris disks the radial location of the dust) but by handling a debris around B- and A-type stars Rhee et al. (2007) found disk sample as an ensemble, one can obtain a meaning- some evidence that the radius of dust belts is increas- ful picture about the basic characteristics of the parent ing with stellar age. planetesimal belt(s) and about the evolutionary trends. Thanks to the recent photometric and spectroscopic The current theoretical models dealing with the build surveys with the Spitzer Space Telescope, the num- up of planetesimals (Kenyon & Bromley 2004a, 2008) ber of debris disks with detailed spectral energy dis- and with the steady-state collisional evolution of the tribution at mid- and far-IR wavelengths has been in- planetesimal belts (Dominik & Decin 2003; Wyatt et al. creased significantly (Chen et al. 2006; Rieke et al. 2005; 2007a; L¨ohne et al. 2008) predict how the fundamental Su et al. 2006; Carpenter et al. 2008; Rebull et al. 2008; properties of debris disks evolve with time. At a spe- Trilling et al. 2008). This improvement is especially re- cific radius, the peak of the dust emission is believed markable for disks around A-type and Sun-like stars (late to coincide with the formation of 1000–2000km sized F, G, and K-type stars). The comparison of these data planetesimals. After this stage – parallel with the de- with the predictions of quasi-steady state evolutionary pletion of planetesimals – the dust emission decreases models showed that most observed trends for A-type with time. The evolution of the disk can be traced both and Sun-like stars can be reproduced adequately (Wyatt in the variation of incidence of disks with time and in 2008; Carpenter et al. 2009a). Kennedy & Wyatt (2010) the evolution of the brightness of dust emission. The confronted the Spitzer observations of A-type stars with different models predict that the dust fractional lumi- an analytic model that also take into account the effects nosity, the ratio of the energy radiated by the dust to of the self-stirring on the disk evolution. Utilizing this the stellar luminosity, varies with time as t−n, where model they were able to reproduce the observed trends n = 0.3 − 1 in disks where collisions are the dominant and they obtained rough estimates for some initial pa- removal process (Dominik & Decin 2003; Wyatt et al. rameters (e.g. average mass) of disks around A-type 2007a; L¨ohne et al. 2008; Kenyon & Bromley 2008). The stars. It was also concluded that debris disks are nar- unique sensitivity of the Spitzer Space Telescope in the row belts rather than extended disks. According to the MIPS 24µm band allowed the detection of stellar photo- models, F-type stars are expected to be an intermedi- spheres and a small amount of excess for a large number ate type between the A-type and Sun-like stars in terms of field stars (e.g. Rieke et al. 2005; Meyer et al.