AN EXCESS DUE TO SMALL GRAINS AROUND THE NEARBY K0 V STAR HD 69830:ASTEROID OR COMETARY DEBRIS?C. A. BeichmanMichelson Science Center, California Institute of Technology, M/S 100-22, Pasadena, CA 91125; [email protected]. Bryden, T. N. Gautier, K. R. Stapelfeldt, and M. W. WernerJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109andK. Misselt, G. Rieke, J. Stansberry, and D. TrillingSteward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065Receivved 2005 January 7; accepted 2005 February 21ABSTRACTSpitzer photometry and spectroscopy of the star HD 6983 0 re veal an excess of emission relative to t he stellarphotosphere between 8 and 35 m dominated by strong features attributable to crystalline silicates with an emittingsurface area more than 100 0 times that of our zodiacal cloud. The spectrum closely resembles that of the cometC/1995 O1 (Hale-Bopp). Since no excess is detected at 70 m, the emitting material must be quite warm, be con-fined within a few AU of the s tar, and originate in grains with low, long-wavelength emissivity, i.e., grains muchsmaller than 70 m/2 10 m. The strong mineralogical features are evidence for even smaller, possibly submicron-sized grains. This small grain size is in direct contrast to the 10–100 m grains that dominate the relatively featurelessspectra of our zodiacal dust c loud a nd most other ma in-sequenc e stars with e xcesses. The upper limit at 70 malsoimplies that any Kuiper Belt analog must be either very cold or less massive than 5 times our own Kuiper Belt.With collisional and Poynting-Robertson drag times of less than 1000 yr for small grains, the emitting material musteither (1) b e created throug h continual grind ing down of material in a dense asteroid belt, or (2) originate in com-etary debris arising from either a sing le ‘‘supercome t’’ or a very l arge number of individual comets arriving from adistant reserv oir. In the case of a cometary origin for the emission, the mass re quirements for continuous generationby many individual comets are unreason able, an d we favor the ca pture of a sin gle super come t into a 0.5–1 AUorbit, where it can evolv e a large number of small grains over a 2 Myr pe riod.Subject headinggs: circumstellar matter — comets: general — infrared: stars — Kuiper Belt — planetary systemsOnline material: color figures1. INTRODUCTIONThe debris disk phenomenon, first discovered by the InfraredAstronomy Satellite (IRAS: Aumann e t al. 19 84; Gillett 1986;Backman & Paresce 1993), is o f gre at interest because of theclues these disks offer toward understanding the formation andevolution of planetary systems. Toward this end, aspects of debrisdisks have been studied with the Infrared Space Observatory(ISO: Habing et a l. 2001; Decin et al. 2003), optical imaging(e.g., Smith & Terrile 1984; Heap et al. 2000), and at submil-limeter wavelengths (Holland et al. 1998; Greaves et al. 1998;Dent et a l. 2000).Information on the nature of the dust grains in these disks isparticularly important. Spectro scopic observations toward lu-minous Herbig Ae/Be stars such as HD 100546 (Grady et al.1997; Malfait et al. 1998; Meeus et al. 2001) and in youn gdebris stars such as Pic (Telesco & Knacke 1991) and 51 Oph(F ajardo-Acosta et al. 1993) revealed dust compos ed, at least inpart, of small (submicron) grains of crystalline silicates such asforsterite and enstatite. The similarity of these spectral featuresto those seen in come t C/1995 O1 ( Hale-Bopp: Crovisier et al.1996; G ru¨n et al. 2001; Wooden et al. 2000) suggests that thiscircumstellar material may represent cometary debris. Yet thesespectral features are not present in all or eve n most debris disks.More than a dozen classic debris disks (including Fomalhaut)examined by Spitzer (Jura e t al. 2004; Stapelfeldt et al. 2004)show little or no spectral structure, suggesting that the grainsin these systems are larger than 10 m. These grains may besimilar to those in our own zodiacal cloud, which are predom-inantly larger than 10–100 m with only a small admixtureof smaller silicate grains yielding a weak 10 m emissionfeature (Kelsall et al. 1998; Fix sen & Dwek 2002; Reach et al.2003).In this paper we report the detection of a large exce ss due tohot grains orbiting the star HD 69830. These grains are domi-nated by a population of crystalline silicates with prominent fea-tures in the Infrared Spectrograph (IRS) spectrum.2. PROPERTIES OF HD 69830HD 69830 (Gliese 302, BD1 2 2449, H R 3259) is a bright(V ¼ 5: 95 mag), nearb y (12.6 pc), m ain-seque nce dwarf of so-lar metallicity ½Fe/H ¼0:015 (Cayrel de Strobel et al. 2001).Its spectral type has been variously classified as between G8(Skiff 20 03) an d K 0 (Song et al. [2000] and other referencesin SIMBAD). We adopt a K0 V spectral type througho ut thispaper, which implies an effective temperature of 5150 K, a massof 0.8 M, and a total lu minosity of 0.45 L(Cox 2000). Usingisochrone fitting, Li absorption, Ca ii HandKlineactivity,weakX-ray emission, and space motions, Song et al. (2000) suggestedan age older than 0.6 Gyr and up to 2 Gyr, with a preference forthe older age. Examination of Ca ii H and K activity indicesA1061The Astrophysical Journal, 626:1061–1069, 2005 June 20# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.leads G. W. Marcy & D. Fischer (2005, private communication)and Wright et al. (2004) to prefer an age of 2–4. 7 Gyr, whileIUE spectra suggest an age of 3 Gyr ( Hufnagel & Smith 1994).We adopt 2 Gy r as the pro bable age for this star.No plane ts have been detected orb iting HD 69830 using theradial velocity technique with a limit on any planet with a massM sin (i) > 0:5 MJlocated within a distance of 3 AU of the star(G. W. Marcy & D. Fischer 2005, private communication). Al-though Mannings & Barlow (1998) noted the presence of a small‘‘hot’’ excess based on IRAS observations at 25 m(3.8;Table 1), IRAS did not detect the star at either 60 or 100 m. Thestar was not observed with ISO.3. SPITZER OBSERVATIONS3.1. Observations and Data ReductionSpectra of HD 69830 were obtained with Spitzer on 2004April 18 (AOR 4016640). These me asurements were part of aprogram that has observed 3 6 main-sequence stars to look fordisks (Beichma n et al. 2005; G. Bryden et al. 2005, in prepara-tion) u sing IRS (Ho
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