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CALTECH GE 133 - EXTRASOLAR ANALOGUES TO THE KUIPER BELT

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EXTRASOLAR ANALOGUES TO THE KUIPER BELTMARK C. WYATT, WAYNE S. HOLLAND, JANE S. GREAVES and WILLIAM R. F.DENTUK Astronomy Technology CentreAbstract. Debris disks are found around some 15% of main sequence stars and their dust is thoughtto be continually replenished in collisions between planetesimals in extrasolar Kuiper belts. Whilethey were discovered in 1984 by IRAS, it is only with more recent imaging that their true nature hasbeen revealed. This paper discusses recent debris disk images and their impact on our understandingof extrasolar systems. Importantly these images confirm the extrasolar Kuiper belt hypothesis formost (but not all) debris disk candidates and show that the planetesimals within these disks must havegrown to at least a few km. Asymmetries in imaged disk structures also provide information aboutthe planetary systems orbiting inside these planetesimal belts. The impact of debris disk studies onour understanding of the evolution of our own Kuiper belt, as well as their potential to solve puzzlessuch as the origin of the missing mass and the outer edge of the Kuiper belt, is also discussed.1. IntroductionWhile most of the mass of the solar system is tied up in its planets, these are not al-ways the most readily observable components of an extrasolar system. A relativelysmall mass of dust (< 1M⊕) can be easily detected around nearby stars because ofits lar g e surface area. The infrared satellite IRAS found that some 15% of nearbymain sequence stars exhibit far-IR emission in excess of that expected from the staritself (Backman and Paresce, 1993; Lagrange et al., 2001). The first such discov erywas of excess emission toward Vega (Aumann et al., 1984). The spectral energydistrib ution (SED) of Veg a’s excess could be fitted well with a 95 K black body(Walker and Wolstencroft, 1988), implying that the star is surrounded by relativelycold dust grains and so, in regions ∼ 80 AU from the star, are analogous to theKuiper belt. Since the dust contrib uting to the excess has a lifetime due to P-R dragand collisions that is much shorter than the age of the star, it could not be a remnantprimordial disk, rather it has to be continually replenished. This replenishment waspostulated to come from the collisional grinding down of a population of muchlarger bodies which have longer lifetimes; similar arguments also apply to the otherexcess stars. Thus these excesses are thought to be indicative of extrasolar Kuiperbelts (hereafter XKB) around the stars. Further evidence that these excesses arecaused by XKBs comes from the lack of hot dust close to the stars. This innercavity is thought to be caused by clearing of a planetary system, since without sucha mechanism the dust created in the XKB would repopulate the inner region due toEarth, Moon and Planets 92: 423–434, 2003.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.424 MARK C. WYATT ET AL.P-R drag, although other mechanisms have been proposed that could cause such ahole such as the interaction of dust grains with a gaseous disk (e.g., Takeuchi andArtymowicz, 2001).2. Debris Disk ImagesAnalysis of the IRAS database has now uncovered ∼ 300 debris disk candidates.However, the poor resolution of IRAS (∼ 1 arcmin) means that only limitedinformation about the disks is av ailable from their SEDs: their temperature andluminosity. The ring-like geometry of the disks, the radius of the XKBs, and anydefining features in their structure can only be determined by imaging. Luckily,some of the debris disk candidates are close enough and bright enough to be im-aged. However, despite significant effort, images have only been made of six diskcandidates. These images have been made using a variety of techniques, rangingfrom optical and near -IR coronographic imaging of the starlight scattered by thedisk (e.g., Smith and Terrile, 1984; Schneider et al., 1999) to mid-IR, sub-mm andmm imaging of the disks’ thermal emission (e.g., Telesco et al., 2000; Holland etal., 1998; Wilner et al., 2002). In the following subsections we discuss images ofthe thermal emission of four of these disks,in age order.2.1. HR4796The HR4796 disk is the only one that has been well resolved in the mid-IR (apartfrom β Pictoris). The challenge with mid-IR imaging is that the disks are cold,so their emission is falling off quickly on the Wien side of the black body curve;extrapolation from the IRAS far-IR fluxes is thus very uncertain. In addition it isoften difficult to untangle the disk emission from that of the stellar photospherewhich can be brighter than the disk at these wavelengths. In its favour, though,the high resolution of the mid-IR means that XKB disks have the potential to beresolved out to at least > 100 pc.HR4796A is a 10 Myr-old A0V star at 67 pc. The 18 µmdiskimageshownin Figure 1a was taken using the mid-IR camera OSCIR on Keck (Telesco et al.,2000), and images of this disk have also been made in the near-IR (Schneider etal., 1999); unresolved sub-mm observations of the disk determined its mass to be∼ 0.25M⊕(Greav es, Mannings and Holland, 2000). The central peak in the mid-IRimage is emission from the stellar photosphere, but once this has been subtractedthe remaining double-lobed structure is characteristic of an edge-on disk, in thiscase with ∼ 70 AU radius. Importantly the images confirm the dust ring with aThe other two well resolved disks are those around β Pictoris and HD141569. These are notdiscussed partly because their emission distribution is found to be radially extended from tens toseveral hundred AU, thus complicating any discussion of their XKBs, but also to limit the length ofthe review.EXTRASOLAR ANALOGUES 425(a) (b)(c) (d)F igure 1. Debris disk images: (a) HR4796 at 18 µm (Telesco et al. 2000); (b) Fomalhautat450µm(Holland et al. 2003); (c) Vega at 850 µm (Holland et al. 1998); (d) Eridani at 850 µm(Greavesetal. 1998).central cavity interpretation of the SED, though there remains debate about theexistence of an additional zodiacal cloud-like hot dust component close to the star(Augereau et al., 1999; Li and Lunine, 2003). One of the most exciting discov eriesfrom the imaging was of an asymmetry in the disk structure: the NE lobe is ∼ 5%brighter than the SW lobe. Wyatt et al. (1999) showed that this asymmetry wouldbe expected if there is a planet in the hole that has an eccentric orbit. The reasonis that long-term gravitational perturbations from the planet would hav e imposedan


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