Debris discs in the 27 Myr old open cluster IC4665
aa r X i v : . [ a s t r o - ph . S R ] O c t Mon. Not. R. Astron. Soc. , 1–14 (2010) Printed 25 October 2018 (MN L A TEX style file v2.2)
Debris discs in the 27 Myr old open cluster IC4665
R. Smith ⋆ and R. D. Jeffries and J. M. Oliveira Astrophysics Group, Lennard-Jones Laboratories, Keele University, Keele, Staffordshire, ST5 5BG
Accepted October 2010
ABSTRACT
We present Spitzer IRAC and MIPS 24 µ m imaging of members of the 27 ± +18 − % of the solar-type (F5-K5) cluster members have excess emissionat 24 µ m indicative of these debris discs, the highest frequency of the clusters studiedwith Spitzer to date. The majority of these discs have intermediate levels of excess( F /F phot < µ m excess in this cluster sample. Onlythe early-type star TYC424-473-1 ( T eff ∼ K ) has significant near-infrared excessfrom 4.5 µ m as measured with IRAC. Two solar-type targets have low significance8 µ m excess but no significant 24 µ m excess. All other targets show no evidence fornear-infrared excess which could indicate the presence of an optically thick primordialdisc, demonstrating that the observed 24 µ m excess arises from a debris disc. Key words: circumstellar matter – infrared: stars.
A key question in astronomy today is whether the SolarSystem’s architecture is a typical outcome of planet forma-tion processes. Integral to this question is whether Earth-likeplanets exist in other systems. However, the direct detectionof terrestrial planets is difficult due to the limitations ofcurrent planet detection techniques. An alternative methodfor determining how the planet formation process proceedsaround other stars is through observation of the remnantsof these processes.Current models of planet formation propose thatdusty discs around a new star settle and km-sizedplanetesimals aggregate on a short ( < > µ m. In the ab-sence of gas the lifetime of such dust is short ( < ⋆ E-mail: [email protected] stellar wind drag and radiation pressure forces (Poynting-Robertson drag). The presence of a mid-infrared excess thusindicates a transient source or continuous replenishment ofthe dust population, a natural consequence of the ongoinggrowth and development of planetary systems.The MIPS instrument on the
Spitzer Space Telescope al-lows the detection of 24 µ m excess in nearby, young stars. Ex-cesses detected at 24 µ m imply a temperature of 100-150K.Assuming thermal equilibrium this translates to an offsetof 3-30AU around A and early F-type stars and 0.5-3AUin F5-K5-type stars (hereafter known as “solar-type stars”).These regions are precisely those in which we may expect tofind planets.Recent studies with Spitzer have explored the evolutionof 24 µ m excess in a statistical manner (see e.g. Rieke et al.2005, Su et al. 2006, Siegler et al. 2007, Rebull et al. 2008,Carpenter et al. 2009). These studies have tried to answerthe question of why two apparently similar stars can havevery different levels of excess emission. To date, the clearestdependency is on age. In A and early F-type stars there isevidence for a peak in the upper envelope of excess emissionat 10-20Myr before a decay in proportion with time (see e.g.Wyatt 2008 and references therein). For solar-type stars thenumber of observed objects is smaller and so correlationsare harder to establish. Based on current evidence the de-cay of 24 µ m excess around solar-type stars appears to followa similar pattern to the A stars but on a timescale that isan order of magnitude shorter (drop from 40% to 20% ofstars with 24 µ m excess occurs from 10-100Myr for solar-type c (cid:13) R. Smith, R. D. Jeffries and J. M. Oliviera stars, and from 100-500Myr for A-type stars, see Figure 6 ofSiegler et al. 2007). In general the levels of excess emissionare also smaller around lower mass objects ( < ∼ ∼ ± E ( B − V ) = 0 . ± . E ( V − I ) = 0 . ± .
06 and A V = 0 . ± .
16 for theintrinsic colours of a low mass PMS star, Bessell et al. 1998).Open clusters provide a homogeneous, chemically uniformcoeval population for which debris disc incidence rates canbe calculated. IC 4665 is one of only 5 clusters to have anage measured using the lithium depletion boundary giving amore accurate age determination than from H-R diagramanalysis. The determined age of 27 ± µ mphotometry of confirmed members of IC 4665. We determinethose stars with excess 24 µ m emission based on their posi-tion in a K s −
24 vs. V − K s colour-colour diagram and onSED fitting. We discuss how the rates and levels of excessemission compare with other studies of solar-type stars andhow these observations constrain models of terrestrial planetformation. Our primary aim in this study is to determine the disc popu-lation in the solar-type stars in IC 4665. We base our sampleon the study by Jeffries et al. (2009) who used fibre spec-troscopy to establish cluster membership from a sample of452 photometric candidates. Membership was assessed pri-marily from radial velocity, giving a candidate list of 56stars. This was further refined through measurements of Ca I lines at 6439 and 6463˚A to filter out contamination by K-giants. Mean proper motions for 45 of the candidate stars(taken from the NOMAD database, Zacharias et al. 2005) Figure 1. A V vs B − V colour-magnitude diagram of brightcandidate members of IC 4665 from Tycho. Here we showonly sources with proper motions consistent with cluster mem-bership. Those sources with colours consistent with member-ship are marked by large crosses. Overplotted is a 30Myrisochrone from Siess et al. (2000) is shown with conversion fol-lowing Kenyon & Hartmann (1995), adjusted for a distance of370pc, A V =0.59mag and E ( B − V )=0.18. are -0.7 ± − in RA and -6.2 ± − in Dec. Two stars were found to have proper motions in-compatible with cluster membership at the 3 σ level. A finalsample of 40 candidates were confirmed as low mass clustermembers. For these targets we used the 2MASS catalogue(Skrutskie et al. 2006) to determine the K s band magnitudeof the targets. Temperatures were taken from Jeffries et al.(2009). These sources are listed in Table 1.To complement the low mass sample we searched theTycho catalogue (Høg et al. 2000) for higher mass candidatemembers within a 1 degree radius of the center of our Spitzerobservations at RA17 h m . s
0, Dec5 ◦ ′ ′′ (see section 3for details of the observed region). A candidate list of 132stars with 6 < V <
12 were selected. This list was refinedfirstly by excluding candidates with proper motions incom-patible with the mean cluster proper motion (at the 3 σ levelas determined by Jeffries et al. 2009). This reduced the can-didate high mass population to 41 stars. This list was furtherreduced by examination of the source colours in a B − V vs V colour-magnitude diagram (Figure 1). We adopt a 30Myr oldisochrone from Siess et al. (2000). Seven stars are found tohave colours incompatible with cluster membership, and ourfinal list of additional bright targets comprises of 33 mem-bers. We further add to our list the solar-type targets P39and P155 from the study of IC 4665 by Prosser & Giampapa(1994). These targets were identified as possible membersbased on their radial velocities and were not excluded byJeffries et al. (2009) but not observed by them spectroscop-ically. Temperatures for these sources were determined fromthe B and V magnitudes of the star B − V = − .
684 log T eff + 14 . . Our final list of additional targets and their parameters isgiven in Table 2. c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Identifier RA Dec
V V − K s T eff , KJCO1 427 266.814 5.94327 13.0690 2.200 5742JCO1 530 266.885 5.87545 12.5180 1.655 6389JCO2 145 266.631 6.09112 12.2720 1.638 6450JCO2 213 266.483 6.04839 13.2180 2.037 6062JCO2 220 266.599 6.04287 16.4200 4.079 3856JCO2 373 266.530 5.93018 12.4560 2.019 6214JCO2 637 266.662 6.03693 17.2000 4.936 3456JCO3 065 266.379 6.11816 15.0450 3.377 4375JCO3 285 266.149 5.95576 13.3360 1.721 6340JCO3 357 266.112 5.90146 13.5170 2.476 5128JCO3 395 266.354 5.86076 13.7160 2.280 5426JCO3 396 266.242 5.85915 13.4260 2.086 5758JCO3 770 266.112 5.86267 17.5610 5.105 3470JCO4 053 267.021 5.81170 13.5280 2.663 5071JCO4 226 266.899 5.64882 13.7810 2.359 5730JCO4 337 266.954 5.54434 13.9130 3.052 4584JCO4 437 266.735 5.79573 16.7160 4.364 3806JCO4 459 266.833 5.78159 17.8930 4.866 3488JCO4 591 266.876 5.68092 16.2360 3.912 4007JCO5 179 266.550 5.69051 14.4070 2.840 4900JCO5 280 266.471 5.60814 14.2430 2.479 5166JCO5 282 266.412 5.60645 12.1460 1.626 6608JCO5 296 266.603 5.59392 13.9400 2.447 5243JCO5 472 266.579 5.77233 16.9100 4.260 3738JCO5 515 266.671 5.73845 17.8510 5.045 3458JCO5 521 266.444 5.73190 17.0620 4.527 3638JCO6 088 266.331 5.79448 13.7160 2.233 5587JCO6 095 266.113 5.78877 13.5110 2.244 5668JCO6 111 266.333 5.77487 15.9930 3.745 4120JCO6 240 266.171 5.69883 11.6580 1.214 7418JCO7 021 266.889 5.52944 13.1260 2.316 5635JCO7 079 266.826 5.50173 13.0880 1.929 6286JCO7 088 266.818 5.49708 15.4110 3.971 4001JCO7 670 266.935 5.36262 17.3490 5.028 3489JCO8 257 266.603 5.28927 12.9210 2.109 5459JCO8 364 266.648 5.51877 17.5300 4.847 3478JCO8 395 266.394 5.49829 17.6510 4.710 3507JCO8 550 266.694 5.37568 17.8720 5.340 3430JCO9 120 266.203 5.48869 14.4050 2.414 5132JCO9 281 266.364 5.40240 14.9560 3.096 4521 Table 1.
Target sources in this study. These sources, taken fromJeffries et al. (2009), are confirmed members of IC 4665.
Data were obtained with the Spitzer Space Telescope IRAC(Fazio et al. 2004) and MIPS (Rieke et al. 2004) instru-ments under Spitzer Program P40601. MIPS data were ob-tained in scan mapping mode centered on the cluster cen-tre of 17h46m16s +5d41 ′ ′′ . A medium scan rate, scan leglength of 0.5 ◦ and 20 scan legs were used to cover an area of50 ′ × ′ with an exposure of ∼
80s at each sky position wereused in each AOR. This AOR was performed four times toachieve the required sensitivity.For IRAC the observations followed the Spitzer observ-ing manual instructions for a rapid shallow survey. A 12 × ′′ map steps, array orientationand a 3-point cycling dither with medium scale factor. Highdynamic range mode with 12 second exposures gave an ef-fective on-sky exposure of 36 seconds and avoids saturationin the range 7 < K < Identifier RA Dec
V V − K s T eff , KTYC424-75-1 266.838 5.59961 10.739 0.974 6910TYC428-1339-1 266.759 5.69184 7.993 0.127 8740TYC428-1483-1 266.762 5.69863 10.059 0.579 7480TYC424-55-1 266.695 5.56493 8.263 0.402 8483TYC428-969-1 266.840 5.75952 9.947 0.772 8208TYC424-473-1 266.741 5.42569 8.874 0.410 8420TYC428-737-1 266.671 5.77427 7.137 0.074 9017TYC428-1685-1 266.546 5.65822 7.339 0.046 8927TYC424-1087-1 266.531 5.53021 6.843 0.025 8960TYC428-1300-1 267.181 5.70126 7.508 -0.011 8979TYC428-1571-1 266.488 5.69444 7.567 -0.020 8941TYC424-174-1 266.786 5.22533 10.686 0.899 7115TYC424-292-1 266.969 5.23321 11.478 1.464 6863TYC428-1938-1 266.485 5.81237 11.200 1.253 7842TYC428-1755-1 266.579 5.93536 10.601 1.112 6630TYC428-675-1 266.390 5.71569 7.707 0.131 9013TYC424-309-1 266.395 5.42651 9.087 0.715 7628TYC428-691-1 266.539 5.98235 10.531 0.939 7253TYC424-223-1 266.913 5.11635 10.240 0.780 7808TYC428-1910-1 266.322 5.66767 9.070 0.513 7968TYC428-215-1 266.652 6.12062 7.748 -0.041 9056TYC424-1396-1 266.415 5.19808 9.820 1.000 7032TYC424-128-1 266.254 5.52291 9.422 0.840 7464TYC424-256-1 267.373 5.24004 10.299 1.020 7468TYC428-1445-1 267.424 5.92507 10.800 0.974 7373TYC428-1211-1 266.945 6.29000 10.578 0.570 6542TYC428-847-1 267.043 6.27151 9.256 0.434 8239TYC428-840-1 266.838 6.37147 11.333 0.462 7303TYC427-1623-1 266.066 5.71430 8.308 0.266 8842TYC423-66-1 265.983 5.41341 10.205 0.570 7596TYC423-369-1 265.958 5.42050 8.730 0.260 8601TYC427-1661-1 266.175 6.23509 10.541 0.706 7551TYC428-1933-1 266.275 6.36365 10.541 1.177 7138P39 266.609 5.82858 12.9300 2.011 5574P155 266.934 5.36664 13.5200 2.294 5043 Table 2.
Target sources in this study. These targets from Tychoand Prosser (P39 and P155) are likely members of IC 4665 basedon proper motion and colour. See text for details.
The data were extracted as BCD (basic calibrateddata) files from the Spitzer archive. These data areindividually flux-calibrated array images. The SpitzerScience Center MOPEX package (Makovoz & Marleau2005) was used to produce the final mosaics. We usedstandard MOPEX modules. The individual 24 µ m MIPSframes were flat-fielded using the flatfield module inMOPEX. Overlap correction was determined using thedefault settings in the overlap module and the final im-age mosaic consisting of all four repetitions of the AORwas constructed using the mosaic module. Mosaics werecreated for each of the IRAC channels using the overlapand mosaic modules in MOPEX under default settings.For details of these modules see Makovoz & Marleau(2005) or the on-line MOPEX user’s guide athttp://ssc.spitzer.caltech.edu/dataanalysistools/tools/mopex/mopexusersguide/.Photometry was extracted using the APEX packagefrom MOPEX. For the IRAC channels the PSF is under-sampled and thus photometry was extracted in a circularaperture of radius 3 pixels ( ∼ . ′′
6) with background deter-mined in an annulus of inner radius 12 pixels and outer c (cid:13) , 1–14 R. Smith, R. D. Jeffries and J. M. Oliviera radius 20 pixels ( ∼ . ′′ ′′ ). Apertures were centered onthe location of each source as listed in Tables 1 and 2. Of thetargets in Table 2, 12 fell outside the image mosaics. Pho-tometry was corrected for the array-location using correc-tion images available online. These correction images weremosaiced in the same way as the data frames to producea correction mosaic as described in the IRAC data hand-book. Aperture corrections were taken from tabulated val-ues in the data handbook. Colour corrections were appliedby interpolation from tabulated values using the effectivetemperatures listed in Tables 1 and 2. We used the tabu-lated values in the IRAC data handbook to convert the fluxin Jy to magnitude. Specifically, the zero points used were280.9Jy at 3.6 µ m, 179.7Jy at 4.5 µ m, 115.0Jy at 5.8 µ m, and64.1Jy at 8 µ m. Absolute calibration of IRAC is stable to1–3% (Reach et al. 2005). We add this 3% error in quadra-ture to statistical background errors determined from pixelto pixel variation in the aperture module to give a final erroron the IRAC photometry. The final photometry is listed inTable 3.For the MIPS data the PSF is not undersampled. Weused the APEX PRF(Point Response Function) fitting mod-ule to determine a PRF model for the final mosaic. As thePRF can vary for source colour we grouped the list of targetsby V − K s magnitude (from 0-6 for our targets, grouped so δ ( V − K s )=1) and used the brightest and cleanest (no nearneighbours, no bad pixels) sources in each group as the basisfor the PRF model. The prf estimate module was used todetermine the PRF models for each source. These modelswere used to fit the target stars to determine the source po-sition and flux in the PRF photometry module in APEX.If the source could not be well fit (according to a χ anal-ysis) with either a single or multiple point sources (activedeblend), then the PRF photometry was determined to havefailed. For sources which were not fit in the PRF photom-etry, or where these fits were sufficiently distant ( > . ′′ . ′′
37) were used with annuli of inner radius 8.16 pixels andouter radius 13.06 pixels (20 ′′ – 32 ′′ ) to determine the back-ground.Some of our target stars are close enough to otherobjects for contamination of aperture photometry to be-come an issue. To determine a correction for contaminatingsources we used the APEX module in MOPEX to createa list of all source detections ( > σ ) in the MIPS 24 µ mimage. We determined the sources that were most isolated(no other detection within >
10 pixels, 24 . ′′ . ′′ <
5% of the flux measured in an on-source aper-ture at > . ′′ <
1% of the on-source flux at > . ′′
9. Foreach of our target sources the area within 20 ′′ was checkedfor detected sources in the 24 µ m image, and any possiblecontamination calculated by multiplying the aperture flux by the value of the Gaussian function described above atappropriate distance. In order to account for the possibil-ity that sources below the 3 σ detection threshold may becontaminating the source we also considered the detectionsrecorded by the APEX module in the IRAC 8 µ m image.The relation between 8 µ m and 24 µ m flux was calculatedby comparing the values determined for sources detected inboth images. The median and median absolute deviation forthe ratio of measured fluxes was F µ m /F µ m = 1 . ± . ′′ of our target detected at 24 µ m hadtheir contamination included, the source was removed fromthe list of detected sources at 8 µ m and then the 8 µ m listof detections was checked for any additional sources within20 ′′ . The flux of these sources was divided by 1.32 to get a24 µ m flux and the contamination in the aperture around ourtarget calculated as above. The median and median absolutedeviation of the contamination for our targets is 3% ±
3% ofthe aperture flux. Sources with high levels of contaminationor other issues (e.g. close to edge of array) are noted in Ta-ble 3. PRF photometry should mitigate against the effects ofnearby contaminants through the active deblend algorithm.We used the above steps to determine the levels of contami-nation that would be seen in aperture photometry for thosesources with PRF photometry. This was used as a methodfor checking that there were no nearby sources in the IRAC8 µ m mosaic that could have been blended with the targetsource in the MIPS 24 µ m mosaic (as this has lower resolu-tion). In all cases the possible contamination was µ m observations is 4%(Engelbracht et al. 2007). We add this in quadrature to sta-tistical error returned from the aperture/APEX PRF pho-tometry modules arising from pixel to pixel variations togive a final error on the flux. The final MIPS photometry islisted in Table 3. For 4 of our targets (JC04 053, TYC424-174-1 TYC424-292-1 and TYC424-1396-1) the source fallsoutside the MIPS 24 µ m mosaic and therefore it is not listedin Table 3. µ M EXCESS
We follow the example of recent authors (Rebull et al. 2008;Stauffer et al. 2010) in using the K s − [24] colour of oursources to determine which of our targets exhibits 24 µ mexcess emission. This requires a well defined model forphotospheric colours. Recently Stauffer et al. (2010) usedSpitzer observations of the Hyades cluster to determine anempirical relation for K s - [24] from V − K s , defined as K s − [24] = 0 . − . × ( V − K s ) + 0 . × ( V − K s ) . They found that this relation was very similar to that pro-posed by Gorlova et al. (2006), but differed from the rela-tion given by Plavchan et al. (2009) that included M dwarfstars. The Stauffer et al. relation is only valid for sourceswith V − K s < K s − [24] vs V − K s colours c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Table 3.
Spitzer IRAC and MIPS 24 µ m data on confirmed or suspected members of IC 4665.Source [3.6] [4.5] [5.8] [8.0] [24] K s − [24] F /F phot CommentsJCO1 427 10.802 [0.033] 10.823 [0.033] 10.804 [0.034] 10.806 [0.035] 10.725 [0.100] 0.144 1.093JCO1 530 10.877 [0.033] 10.860 [0.033] 10.849 [0.034] 10.890 [0.035] 11.060 [0.200] l -0.197 l l EdgeJCO2 145 10.615 [0.033] 10.602 [0.033] 10.608 [0.033] 10.612 [0.034] 10.458 [0.106] 0.176 1.139JCO2 213 11.147 [0.033] 11.165 [0.033] 11.134 [0.034] 11.207 [0.036] 10.802 [0.174] 0.379 1.374 XSJCO2 220 12.227 [0.033] 12.225 [0.034] 12.194 [0.036] 12.253 [0.051] 12.092 [0.667] l l l JCO2 373 10.363 [0.033] 10.349 [0.033] 10.324 [0.034] 10.348 [0.034] 9.882 [0.085] 0.555 1.616 XSJCO2 637 11.821 [0.034] 11.793 [0.033] 11.786 [0.034] 11.731 [0.042] 11.551 [0.326] l l l JCO3 065 11.815 [0.033] 11.568 [0.033] 11.548 [0.038] 11.546 [0.038] 11.234 [0.318] 0.434 1.210JCO3 285 11.569 [0.033] 11.569 [0.033] 11.569 [0.035] 11.717 [0.041] 11.827 [0.454] l -0.212 l l Contam.JCO3 357 10.972 [0.033] 10.954 [0.033] 10.814 [0.034] 10.889 [0.035] 10.898 [0.112] 0.143 1.058JCO3 395 11.443 [0.033] 11.412 [0.033] 11.433 [0.035] 11.425 [0.038] 11.049 [0.122] 0.387 1.354 XSJCO3 396 11.281 [0.033] 11.277 [0.033] 11.265 [0.034] 11.261 [0.035] 10.784 [0.104] 0.556 1.616 XSJCO3 770 12.210 [0.034] 12.205 [0.034] 12.133 [0.036] 12.148 [0.049] 12.450 [0.935] l l l JCO4 226 11.462 [0.033] 11.377 [0.033] 11.397 [0.035] 11.390 [0.038] 12.114 [0.605] l -0.692 l l Contam.JCO4 337 10.780 [0.033] 10.902 [0.033] 10.753 [0.034] 10.721 [0.034] 11.817 [0.483] l -0.956 l l Contam.JCO4 437 12.228 [0.033] 12.200 [0.034] 12.160 [0.036] 12.148 [0.049] 11.523 [0.312] 0.829 1.545JCO4 459 12.740 [0.034] 12.730 [0.034] 12.676 [0.040] 12.668 [0.064] 11.911 [0.479] l l l JCO4 591 12.249 [0.033] 12.307 [0.033] 12.256 [0.037] 12.246 [0.045] 11.797 [0.442] l l l l JCO5 280 11.711 [0.033] 11.756 [0.033] 11.771 [0.035] 11.750 [0.042] 11.859 [0.451] l -0.095 l l Contam.JCO5 282 10.518 [0.033] 10.532 [0.033] 10.528 [0.034] 10.532 [0.034] 10.221 [0.104] 0.299 1.276 XSJCO5 296 11.368 [0.033] 11.459 [0.033] 11.305 [0.034] 11.336 [0.038] 10.688 [0.152] 0.805 1.952 XSJCO5 472 12.386 [0.034] 12.411 [0.034] 12.348 [0.038] 11.957 [0.044] 11.596 [0.343] l l l JCO5 515 12.505 [0.033] 12.465 [0.034] 12.377 [0.038] 12.515 [0.054] 11.647 [0.378] l l l JCO5 521 12.318 [0.034] 12.281 [0.034] 12.336 [0.040] 12.293 [0.053] 11.808 [0.446] l l l JCO6 088 11.414 [0.033] 11.423 [0.033] 11.386 [0.034] 11.442 [0.036] 10.895 [0.115] 0.588 1.638 XSJCO6 095 11.240 [0.033] 11.239 [0.033] 11.236 [0.034] 11.251 [0.037] 11.248 [0.260] 0.019 0.968JCO6 111 12.132 [0.033] 12.156 [0.033] 12.077 [0.036] 12.048 [0.040] 11.755 [0.433] l l l Contam.JCO6 240 10.410 [0.033] 10.403 [0.033] 10.282 [0.033] 10.431 [0.034] 9.650 [0.076] 0.794 2.014 XSJCO7 021 10.688 [0.033] 10.686 [0.033] 10.676 [0.034] 10.717 [0.034] 10.675 [0.100] 0.135 1.069JCO7 079 11.106 [0.033] 11.091 [0.033] 11.095 [0.034] 11.109 [0.036] 10.658 [0.146] 0.501 1.537 XSJCO7 088 11.257 [0.033] 11.269 [0.033] 11.252 [0.035] 11.230 [0.037] 11.120 [0.222] 0.320 1.005JCO7 670 12.068 [0.033] 12.053 [0.033] 11.812 [0.036] 12.043 [0.044] – - – EdgeJCO8 257 10.691 [0.033] 10.699 [0.033] 10.684 [0.034] 10.722 [0.035] 9.857 [0.143] 0.955 2.331 XSJCO8 364 12.465 [0.034] 12.449 [0.033] 12.337 [0.038] 12.337 [0.043] 12.383 [0.883] l l l JCO8 395 12.665 [0.034] 12.656 [0.034] 12.567 [0.040] 12.614 [0.065] 14.234 [0.853] l -1.293 l l JCO8 550 12.297 [0.033] 12.246 [0.034] 12.210 [0.037] 12.230 [0.051] 12.183 [0.749] l l l JCO9 120 11.999 [0.033] 12.062 [0.034] 12.038 [0.036] 12.012 [0.046] 11.645 [0.379] l l l JCO9 281 11.740 [0.033] 11.808 [0.034] 11.742 [0.035] 11.765 [0.042] 12.332 [0.836] l -0.472 0.543TYC424-75-1 9.855 [0.034] 9.734 [0.033] 9.643 [0.034] 9.777 [0.034] 9.677 [0.074] 0.088 1.050TYC428-1339-1 8.444 [0.033] ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ -0.191 ∗ ∗ TYC428-1685-1 9.383 [0.033] ∗ ∗ ∗ ∗ ∗ -0.252 ∗ ∗ TYC428-1571-1 9.550 [0.033] ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ = Saturated photometry. l = Signal to noise <
5. XS = Significant 24 µ m excess emission. Contam. = High level of contamination in24 µ m photometry. Edge = Source falls on edge of 24 µ m mosaic.c (cid:13) , 1–14 R. Smith, R. D. Jeffries and J. M. Oliviera
Figure 2.
A colour-colour plot of the IC4665 target stars used to determine 24 µ m excess. The solid black line is the expected photosphericrelation from Plavchan et al. (2009), with dotted black lines showing 3 σ limits from errors on the 24 µ m photometry (see text). Alsoshown are the photospheric relations from Gorlova et al. (2006) (dashed grey line) and Stauffer et al. (2010) (solid grey line) which donot cover the full range of V − K needed for this study. Diamond symbols show the targets taken from Jeffries et al. (2009) and crossesmark the sources listed in Table 2. Grey symbols mark the 5 σ upper limits for sources with low signal to noise ( < of our target sources. The diamonds and crosses mark thetarget sources from Jeffries et al. (2009) as listed in Table1 and the brighter targets as listed in Table 2 respectively.Colours are shown in grey as upper limits if the target hasa signal to noise of less than 5 σ in the 24 µ m photome-try. Overplotted as a solid line is the expected photosphericcolour from Plavchan et al. (2009). We also show the re-lations from Gorlova et al. (2006) (dashed grey line) andfrom Stauffer et al. (2010) (solid grey line). As our targetsources cover the range 0 < V − K < σ errors on this relation are taken from the errors on the24 µ m photometry. We used all significant ( > σ ) detec-tions to calculate a quadratic relation between error (asgiven in Table 3) and V − K s which was determined to be0 .
045 + 0 . × ( V − K s ) + 0 . × ( V − K s ) . The scatteraround this fit is low, with a median difference between thefitted error and measured error of 2%.From the figure we can identify 10 of the sources fromJeffries et al. (JCO2 213, JCO2 373, JCO3 395, JCO3 396,JCO5 282, JCO5 296, JCO6 088, JCO6 240, JCO7 079 andJCO8 257) and 7 of the additional targets (TYC428-1339-1,TYC428-969-1, TYC424-473-1, TYC428-1938-1, TYC428-1755-1, TYC424-128-1, P39) that have K s − [24] greater thanthe 3 σ limit on the photospheric colours. Excluding the tar-gets which do not fall within the MIPS 24 µ m mosaic (see sec-tion 3) this gives a total of 16/59 sources or 27 +9 − %. If we con-sider only the sources with confirmed membership, i.e. thosefrom Jeffries et al., we have an excess detection frequencyof 10/39 or 26 +11 − %. Note that 2 targets appear to have 3 σ negative detections. These targets have very low V − K s ,in the range at which the models of Stauffer et al. (2010),Gorlova et al. (2006), and Plavchan et al. (2009) most dif-fer. If we were to use the Gorlova et al. (2006) models forthe photospheric colours these targets would be within 3 σ of the expected colour, however this model does not coverthe full V − K s range of interest in this study. Using anyof the considered models does not change the conclusionsregarding which sources have 24 µ m excess. In addition to the MIPS 24 µ m observations we have IRACphotometry across 4 channels which allows us to search forevidence of near-infrared excess. If any stars exhibit near-infrared excess we could be seeing the remnants of an opti-cally thick primordial disc rather than a true debris disc. Atthe 27Myr age of this cluster such primordial discs wouldbe rare as they are expected to dissipate on a timescale of afew Myr (see e.g. Wyatt 2008 and references therein). Tran-sition discs would also be unexpected, as the lifetime of suchdiscs is expected to be very short, < K s − [3 . K s − [4 . K s − [5 . K s − [8 .
0] colours against V − K s . These plots can be seenin Figure 3. In all panels the diamonds and crosses mark thetargets from Jeffries et al. (2009) and those listed in Table2 respectively. Those sources marked with square boxes arethose which are saturated in the IRAC image and there-fore have very unreliable photometry in this band (these c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Figure 3.
A colour-colour plot of the IC4665 target stars for each of the IRAC channels. Diamond symbols show the targets takenfrom Jeffries et al. (2009) and crosses mark the sources listed in Table 2. Sources are shown with 1 σ errors from the IRAC photometry.Sources that appear saturated in the IRAC image are indicated by a surrounding square box. Labelled sources have colours that mayindicate excess in this IRAC band. See text for discussion. are marked in Table 3 with an asterisk). Overplotted arethe errors from the IRAC photometry. Sources with coloursthat may indicate excess are labelled, and we discuss theseindividually below. K s photometry The source JC02 637 has apparent excess in all IRAC chan-nels. The 2MASS catalogue record for this target from whichwe obtain K s flags the JHK s results as poor due to is-sues in the PSF fit photometry. Furthermore if we producean SED fit to the temperature of the target as listed inJeffries et al. (2009) and scaled to the IRAC 3.6 µ m flux theresults from the remaining channels are consistent with pho-tospheric emission alone. Thus we do not believe we haveevidence for a near-infrared excess around this target. Notethat as one of our coolest targets the 24 µ m MIPS detectionof this source is below a signal to noise threshold of 5, andthus we have only upper limits on the 24 µ m flux. Two further sources, JC05 515 and JC07 670, with simi-lar V − K s have possible evidence for excess at 5.8 µ m.For targets in the range 4 . < V − K s < . K s − [5 .
8] of 0.346 ± K s − [5 .
8] = 0 . ± . K s − [5 .
8] = 0 . ± .
048 is closer toindicative of a significant excess. As there is no evidencefor excess in the other IRAC bands, there is no significantevidence for a near-infrared excess which could indicate aprimordial disc remnant.The source JC06 240 also has apparent 5.8 µ m ex-cess. In this case comparison with targets of similar colour(JC06 240 has V − K s = 1 . . < V − K s < .
7) suggests we may have a significant5.8 µ m excess; JC06 240 has K s − [5 .
8] = 0 . ± . ± µ m. This source lies on the edgesof different tiles in the IRAC channel 3 imaging and pixels inthe source location have a high level of standard deviationacross different exposures, and so the error (taken from theSpitzer uncertainty images) is underestimated. Adopting theuncertainty from the standard deviation between exposuresinstead gives K s − [5 .
8] = 0 . ± . . < V − K s < . c (cid:13) , 1–14 R. Smith, R. D. Jeffries and J. M. Oliviera
The target TYC424-75-1 has an apparent excess at 5.8 µ m,however examination of the 5.8 µ m image shows that it isclose to a very bright target which contaminates the 5.8 µ maperture and which is responsible for the high K s − [5 . µ m, and the lower brightness at 8.0 µ m meansthat it does not spread to contaminate the flux of TYC424-75-1 in this band. For TYC424-473-1 the difference between this and other tar-gets of similar colour in V − K s is highly significant. Thissource is also reddened in the K s − [5 .
8] plot. This near-infrared excess could indicate the presence of a remnantprimordial disc. We construct an SED for this object us-ing a Kurucz profile for a temperature of 8337K (Figure 4).The photospheric model is scaled to a best fit to the
JHK s χ analysis. From this plot it isclear that the near infrared slope of the target is not consis-tent with photospheric emission alone unless the target hasbeen misidentified. However, a single temperature blackbodyadded to the photospheric emission can be shown to fit theIRAC and MIPS data. Overplotted on the SED is a 500Kblackbody emission profile (dotted line) and the total emis-sion from this plus the photospheric profile (dashed line).This total emission profile fits the Spitzer data within theerrors (note that this target is saturated in the IRAC 1 chan-nel, and so the [3.6] data is ignored). The fractional luminos-ity of this fit is f = L IR /L ⋆ = 2 . × − . This is within therange expected for debris discs ( f < − , Lagrange et al.2000) and lower than a primordial disc. Thus we can assumethe dust is optically thin, and thus this temperature impliesa distance of 1.7AU from the central star assuming that theemitting material behaves like a blackbody. If small grainsdominate the emission then the offset is likely to be greateras such grains are inefficient emitters of radiation and thusheat up to higher temperatures at greater distances fromthe star than would be assumed from a blackbody approx-imation (see e.g. discussion in Section 3 of Smith & Wyatt2010).For two sources not yet discussed we have appar-ent excess at 8 µ m with no excess indicated in the otherIRAC bands. JC05 472 has a significant excess at 8 µ m,with K s − [8 .
0] = 0 . ± . . < V − K s < .
8) have a much lower average K s − [8 .
0] = 0 . ± . µ m excess around this target. Un-fortunately this source has a low level detection at 24 µ m( < σ ). If we accept the low signal to noise detection as atrue reflection of the 24 µ m flux of the target then the flux(164 ±
43 mJy) is close to the expected level of flux fromthis source at 24 µ m from an SED fit using a Kurucz pro-file at the temperature taken from Jeffries et al. (2009) andscaled to the 2MASS JHK s photometry (expected 116mJyfrom the photosphere). Similarly P155 has a large 8 µ m ex- Figure 4.
The SED of source TYC424-473-1. A Kurucz profileof 8337K has been scaled to the 2MASS JHK photometry. Over-plotted is a blackbody at 500K (dotted line) and the total fluxfrom the photospheric model and blackbody (dashed line). Thisprofile fits the IRAC (asterisks) and MIPS (square) data we haveobtained. cess ( K s − [8 .
0] = 0 . ± .
043 compared to an average of0.055 ± V − K s = 2 . ± . µ m imagewhich is dominated by noise and therefore we cannot de-tect this target nor can we determine limits for its 24 µ mflux. There are no sources near P155 that could be contam-inating the 8 µ m flux nor are there any indications of highvariation between different exposures including this target.For both targets, with no excess at shorter wavelengths andno significant detection at 24 µ m the nature of the excessemission at 8 µ m remains currently unresolved. SED plots are constructed for the other targets in our sur-vey in the same way, adopting a temperature as listed inTables 1 and 2 scaled to the 2MASS JHK photometry of thetargets. These were examined for evidence of near-infraredexcess and with the exception of the unresolved 8 µ m issuesfor JC05 472 and P155 (discussed above), and the excessaround TYC424-473-1, none was found. We also comparethe targets that appear to have 24 µ m excess in the SED fitto those that appear to have an excess in the K s − [24] colourplot (Figure 2). For all such targets the SED is consistentwith there being excess emission at 24 µ m.In summary one target, TYC424-473-1, has strong evi-dence of near-infrared excess in all unsaturated observations.For the solar-type JC05 472 and P155 there is some evidenceof an 8 µ m excess (at 5 and 4 σ significance respectively) butnot of an excess at 24 µ m or in other IRAC channels. For allother targets with uncontaminated photometry the IRACphotometry is consistent with photospheric emission withinthe errors (any excess has < σ significance). The lack ofIRAC excess indicates we do not have a population of pri-mordial discs in the sample, as expected at an age of 27Myr.These discs are also unlikely to be transition discs, as thelifetime of such discs is expected to be very short ( ∼ c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Figure 5.
The K s − [24] excess versus projected rotational ve-locity for the targets from Jeffries et al. (2009). Here excess is de-fined by by difference from the photospheric colours determinedby Plavchan et al. (2009) as shown in Figure 2. We find no evi-dence for a relationship between excess and rotational velocity. dial disc population. Thus we consider that the 24 µ m excessis indicative of a debris disc population. Following the study by Stauffer et al. (2010) we search for alink between rotational velocity and the 24 µ m excess. Stel-lar winds from low mass stars are thought to be powered bydynamo activity driven by differential rotation. Thus rota-tional velocity could be used as a proxy for stellar wind. Stel-lar winds are expected to remove small particles from discs(Chen et al. 2005; Plavchan et al. 2009) and so we mightexpect a correlation between rotational velocity and 24 µ mexcess. This would not be a linear relation as the stellarwind is believed to saturate above some rotational velocity.Figure 5 shows the rotational velocity of the targets fromJeffries et al. (2009) versus infrared excess as measured bythe difference between the photospheric relation given byPlavchan et al. (2009) (solid line, Figure 2) and the mea-sured K s − [24]. This can be compared directly with Figure7 of Stauffer et al. (2010). We have no rotational velocityinformation for the Tycho targets in our survey, and thetwo sources included from Prosser & Giampapa (1994) havebadly contaminated and thus unreliable 24 µ m photometry.For many of our targets the measurements ofJeffries et al. (2009) provide only upper limits to the rota-tional velocity of the targets (indicated by arrows in Figure5). Accepting this limitation, there is no evidence in thefigure for a decrease in excess with increased rotational ve-locity. A similar null result was reported in Stauffer et al.(2010), who argued that this result does not suggest that thewind scouring model is incorrect as the sample mostly showsthe expected relation between rotational velocity and colour(lower mass stars such as late F and G dwarfs are expectedto be slow rotators, early F type stars are rapid rotators).Within their sample only one rapidly rotating lower masstarget was found which has no excess emission, and there-fore the Stauffer et al. (2010) sample could not be used toconstrain the wind scouring model. If we examine our tar- Figure 6.
The rotational velocity for the targets fromJeffries et al. (2009) shown by colour. Squares indicate sourceswhich have been judged to have 24 µ m excess using Figure 2. OneK dwarf, JC05 296 (labelled), has a high rotational velocity forits spectral type and apparent 24 µ m excess. This contradicts theexpectations of the wind scouring model of Chen et al. (2005) andPlavchan et al. (2009). gets in a similar way we find one rapidly rotating K dwarfwith an excess, as shown in Figure 6. This star is JC05 296, atarget with an excess in K s − [24] of 0.794 ± Cieza et al. (2009) presented evidence for the impact of stel-lar multiplicity on the evolution of circumstellar discs. Us-ing the IRAC data from several Spitzer legacy surveys theyfound that for projected separations of < > µ m, and so on discs thatwere further from their host stars. Plavchan et al. (2009)found no evidence of the trend suggested by Trilling et al.(2007), and Duchˆene (2010) found no significant dependenceof debris disc incidence with binarity or binary separation.Stauffer et al. (2010) showed tentative evidence that in the100Myr Blanco 1 cluster there is a link between 24 µ m excessand binarity (or rather with a binarity proxy). They com-bine their results with data from the Pleiades ( ∼ ∼ K s − [24] excess. The isochrone was tuned from a fit to c (cid:13) , 1–14 R. Smith, R. D. Jeffries and J. M. Oliviera
Figure 7.
Dependence of excess on a proxy for multiplicity. We use height above the single star isochrone (left) as a proxy for binarityfollowing Stauffer et al. (2010). The diamonds and crosses in both plots indicate targets from Jeffries et al. (2009) and those listed inTable 2 respectively. Stars with good (signal to noise >
5) 24 µ m detections are indicated by circles. We find no evidence for a link betweenexcess and binarity (right). the Pleiades following Stauffer et al. (2007). The results areshown in Figure 7.Four targets have V − V pred much higher than would in-dicate a binary system (0.75 for an equal mass binary, where V pred is the predicted value of V for a given value of V − K s using the single star isochrone). If these stars have highermultiplicity (triple or higher order systems) then of our 35high signal to noise detections we would have a 11 +9 − % de-tection of triple or higher order systems. This is compatiblewith previous studies (see e.g. Abt 1983, Abt & Levy 1976).One of these systems, TYC424-128-1, has significant excessat 24 µ m. In general we find no significant evidence for adependence of excess on height above the isochrone. Sepa-rating the sample into those with a height above the singlestar isochrone of > . V ) and those below this level aK-S test returns a 32% probability that the two populationsare drawn from the same parent distribution. The star withthe highest K s − [24] is TYC424-473-1, the star which alsohas a near-infrared excess. The value of V − V pred foundfor this target (0.59) suggests that this star is a binary andthe two components are likely to be close in mass. Usingthis value to constrain our SED fitting we find a fit witha secondary component could have a temperature of 7590K(suggesting spectral types of A4V for the major componentand A8V for the companion). This does not affect the fit tothe excess emission presented in section 4.1.Amongst the sources taken from Jeffries et al. (2009)there are two targets with significant excess in K s − [24]that are likely binaries (JC02 373, V − V pred = 0 . K s − [24] = 0 . ± .
09; JC08 257, V − V pred = 0 . K s − [24] = 0 . ± . Following the examples of Siegler et al. (2007) andMeyer et al. (2008) we add the results from the IC 4665cluster to the growing sample of solar-type stars studiedwith MIPS. These studies and references are listed in Table4. Sources included in the table are of spectral type F5-K5.This range covers the solar-type stars that are bright enoughto have their photospheres detected at a signal to noise of atleast 3 in our MIPS 24 µ m imaging. For a 3 σ detection in theMIPS 24 µ m image a source is required to have a magnitudeof [24] ∼ K5. Concen-trating on F5-K5 type stars (1 . > V − K s .
05) gives us asample of 25 stars in the MIPS field. If we consider only thesample restricted to this limit our excess detection rate be-comes 10/24 or 42 +18 − %. This is somewhat higher than othersamples of a similar age, but not significantly so (see Table4). Compared to the FEPS targets presented in Meyer et al.(2008), in which the frequency of 24 µ m excess was found tobe 0.19 +0 . − . for stars aged 10-30Myr and 0.08 +0 . − . for starsaged 30-100Myr, IC 4665 has a reasonably large fractionof sources with debris disc emission at 24um. We show thefrequency of 24 µ m excess in the samples listed in Table 4and the Meyer et al. (2008) sample in Figure 9. Note thata uniform detection threshold ( F /F phot = 1 .
15) is used inall these surveys with the exception of the Pleiades cluster(Sierchio et al. 2010). For this cluster we also show the fre-quency in the case that the higher threshold is used. Thedetection threshold used for the IC4665 cluster (as givenby the error calculation in section 4) is higher than thislimit for the lower mass stars considered, however examina-tion of Figure 2 indicates that there are no objects in theregion between the statistical errors shown and a limit of F /F phot = 1 .
15, and thus we consider that these resultsare compatible with the other cluster samples. The samespectral range (F5-K5) is considered for all clusters (and sothe excess frequency may differ to those quoted in the pa-pers, and in Siegler et al. 2007, where they consider a differ-ent spectral range. ) In this plot we can see that the overallfrequency of excess emission in IC 4665 is the highest of the c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Figure 8.
The 24 µ m excess emission around stars in the stud-ies listed in Table 4. The sources from this paper are markedwith diamonds. Sources with high excess compared to typicalranges for their age are named. In addition to the cluster datalisted in Table 4 we show BD+20307 (Song et al. 2005, age sev-eral Gyr from Zuckerman et al. 2008 ) and P1121 in M47(Gorlova et al. 2006). Overplotted are an inverse time dependence(dotted line) and an inverse time-squared dependence (dashedline). Figure 9.
The frequency of 24 µ m excess emission in the sampleslisted in Table 4 (squares) and in the Meyer et al. (2008) sam-ple (crosses). The IC 4665 cluster data presented in this paper ismarked with a diamond. The grey square marks the excess frac-tion seen in the Pleiades if we adopt an excess detection level of F/F ∗ = 1 .
15 as is typical for the other samples in the plot (seeTable 4. cluster samples listed in Table 4 but within the errors is con-sistent with the frequencies seen in other clusters of similarages.In Figure 8 we show the level of 24 µ m excess (expressedas measured flux over that expected from the photosphere)seen in studies of different clusters. Overplotted are an in-verse time and an inverse time-squared dependence. As firstsuggested by Siegler et al. (2007), the inverse time-squareddependence seems to offer the best fit to the upper enve-lope of excess for the Upper Scorpious and Lower CentaurusCrux members of the Scorpius-Centaurus association (datafrom Chen et al. 2005), but the inverse time dependence is a better fit to the rest of the data. An inverse time depen-dence has also been shown to fit A star debris disc statistics(Rieke et al. 2005). This agrees with predictions for steady-state evolution of debris discs, in which the excess emissionarising from the debris discs falls inversely in proportionto time. This time − fall-off arises once the debris reaches acollisional equilibrium in which the largest bodies in the pop-ulation are colliding and producing smaller material whichis eventually collisionally processed into dust small enoughto be removed from the system (Dominik & Decin 2003;Wyatt et al. 2007).Compared to the 30Myr old cluster NGC 2547 the ex-cess levels seen in IC 4665 show a similar spread apart fromthe presence of one high excess source in NGC 2547, 2MASS08090250-4858172 (labelled ID 8 in Gorlova et al. 2006; thissource has excess emission from 3 µ m and is possibly a pri-mordial optically thick disc). Our results are consistent withthe inverse time dependence. We find no sources that ex-ceed this envelope and could be considered to have dustresulting from a transient event as has been postulated forHD69830 and BD+20307 (Wyatt et al. 2007). Extreme lev-els of excess could be interpreted as evidence for a recentmassive collision such as those expected between proto-planets in the final stages of terrestrial planet accretion(Weidenschilling 1977). Simulations by Kenyon & Bromley(2005) have shown that at an age of 27Myr a collisional cas-cade in a minimum-mass solar nebula around a solar-typestar at 0.4–2AU would be above our detection threshold,and so could be a valid model for the emission we detect.Similarly around higher mass stars the emission could arisefrom a collisional cascade at 3–20AU or 30–150AU. Witha single photometric detection for most of these targets wecannot constrain either the location or the mass of the dust,however these models show that a collisional cascade pro-duced by catastrophic collisions in a disc is a possible modelfor the emission seen here.We can also consider the relative proportions of differentlevels of excess in the clusters studied. We follow the exampleset by Rieke et al. (2005) who showed the rates of low, in-termediate and high excess fractions as a function of time toexplore the decay of debris discs around A stars. We split thesamples for each cluster into small or no excess ( F /F phot < .
25, where F phot is the expected flux from the stellar pho-tosphere), intermediate excess (1 . < F /F phot < . ), andlarge excess ( F /F phot > F /F phot < .
25) increasing withage, and the proportion of sources with intermediate excessbeing higher than those with large excess ( F /F phot > . < F /F phot <
2) than for any other cluster (although not significantlyso). Consequently the proportion of sources with no or low c (cid:13) , 1–14 R. Smith, R. D. Jeffries and J. M. Oliviera
Table 4.
Frequency of 24 µ m excess from cluster data (after Siegler et al. 2007)Name Age, Myr Number of sources Excess frequency ReferenceSco Cen 16 ± +0 . − . Chen et al. (2005)
IC 4665 27 ± +0 . − . This paper
NGC 2547 30 ± +0 . − . Gorlova et al. (2007)IC 2391 50 ± +0 . − . Siegler et al. (2007)Blanco 1 100 ±
20 26 0.19 +0 . − . Stauffer et al. (2010)Pleiades 115 ±
20 71 0.32 +0 . − . a Sierchio et al. (2010)Hyades 625 ±
50 63 0.00 ± ∼ b
69 0.03 +0 . − . Bryden et al. (2006) a Uses an excess detection threshold of 1.10. Majority of other surveys use a threshold of 1.15 (if adopting this higher threshold excessfrequency becomes 0.21 +0 . − . ). b Median age of the sample
Figure 10.
Fraction of stars with low or no excess ( F /F phot < . . < F /F phot <
2) and high ex-cess ( F /F phot >
2) for each of the cluster samples listed in Table4. The overall trend agrees well with the A stars as shown in Fig-ure 3 of Rieke et al. (2005) and Figure 9 of Su et al. (2006). Thedistribution of excess for the IC 4665 cluster presented in thispaper (shown at 27 Myr) is somewhat different from the othercluster of a similar age, NGC 2547. excess ( F /F phot < .
25) is lower than the other pub-lished clusters (again not significantly so). If assume thatthe minimum mass solar nebula (MMSN) predictions fromKenyon & Bromley (2005) for the evolution of debris in thefinal stages of terrestrial planet formation can be applied toour cluster data, then the frequency of discs with intermedi-ate excess in IC 4665 would suggest that many stars in thecluster have recently experienced a large collision. However,as the initial planetesimal discs may have differed from aMMSN then we cannot confirm this is the case.To further test whether the distribution of excess emis-sion in IC 4665 is different to other sources we examinethe cumulative distribution functions of the excess emis-sion of each cluster. The resulting normalised distributionsare shown in Figure 11. As we can see here the older clus-ters and the field have very similar distribution functions,which are somewhat different to the younger sources shownin the left-hand plot. This is to be expected if the evolution-ary timescale for debris discs is of the order of 10-100Myr. The cumulative frequency (CF) distribution for IC 4665 hassome differences to the CF distribution of NGC 2547, how-ever a two-sample Kolmogorov-Smirnov (K-S) test indicatesthat the probability that the two cluster samples are drawnfrom the same underlying probability distribution is still76%. The probability that the IC 4665 and Sco Cen sam-ples are drawn from the same distribution is 26%. This isnot low enough to confirm a significant difference betweenthe clusters, although the CF distribution looks quite dif-ferent. There is evidence that the underlying distributioncould be different for the older samples. A K-S test com-paring IC 4665 to these samples returns a probability thatthe underlying distributions are the same of 8% (Blanco 1,100Myr), 1% (Pleiades, 115Myr), 0.04% (Hyades, 625Myr)and 0.02% (field sample, average age 4000Myr). This pro-vides further support for evolution of debris discs on 10-100Myr timescales.We find no evidence of a link between binarity and ex-cess seen in IC 4665. A binary companion limits the size ofa stable region for a circumbinary disc to a crit , a func-tion of the binary’s configuration (e.g. Holman & Wiegert1999). This limit also approximately corresponds to theregion in which the final chaotic stages of planet forma-tion from lunar-sized embryos can proceed (Quintana et al.2007). This truncation of planet formation in binaries is ofparticular interest given the recent discoveries of ∼
80 exo-planets in binary systems (see e.g. Desidera & Barbieri 2007,Mugrauer & Neuh¨user 2009). The presence of dust grains inso-called forbidden regions ( < a crit ) found for several binarysystems by Trilling et al. (2007) may be explained by recentnumerical simulations in Th`ebault et al. (2010) who foundthat small grains can populate the forbidden region. Theamount of dust in unstable regions depends on the balancebetween the rate of small grain production through colli-sions and removal by the perturbations of the binary. Asdiscussed in section 4.3 several studies have offered contra-dictory evidence for a link between multiplicity and the pres-ence of a debris disc (Cieza et al. 2009; Trilling et al. 2007;Stauffer et al. 2010). With the wealth of seemingly contra-dictory conclusions about the relationship between binarityand debris disc incidence further studies, in particular ex-ploring the geometry of the binary system and the true dustdistributions to remove degeneracies from SED fitting, areneeded. For example the two likely binary sources in IC4665 c (cid:13) , 1–14 ebris discs in the 27 Myr old open cluster IC4665 Figure 11.
The normalised cumulative frequency distributions of the excess emission for the clusters listed in Table 4. The youngerclusters are shown in the left-hand plot and the older clusters ( > with significant excess (JC02 373 and JC08 257) could bewide binaries (several hundered AU) which would not be re-solved in the Spitzer observations and would not be expectedto affect the presence of 24 µ m excess. In this paper we have presented a study of the cluster IC4665 using Spitzer IRAC and MIPS data. These data havebeen used to search for debris discs in the cluster. Our con-clusions are: • the cluster IC 4665 has the highest incidence of 24 µ mexcess in the spectral range F5-K5 of all the clusters stud-ied with Spitzer to date, although the rate (42 +18 − %) isnot significantly higher than the similarly aged NGC 2547(33 +13 − %). The majority of the sources in the cluster have lowor intermediate levels of excess F /F phot <
2. No sourcesin IC 4665 have excess above the levels expected for an in-verse time decay of debris predicted by collisional evolutionmodels; • the source TYC424-473-1, which may be a binary, hasexcess in all unsaturated observations indicative of excessemission from the near to mid-infrared. Such near-infraredexcess could indicate the presence of a remnant primordialdisc. This excess can be fit by a simple blackbody curve ata temperature of 500K suggesting a radial offset of ∼ • there is no evidence of a dependence of excess at 24 µ mon multiplicity of the star in this cluster. We find severalsources which are suspected multiples that have significant24 µ m excess, but as the numbers are small these do notprovide contradictory evidence against the recent work byStauffer et al. (2010) who found evidence that 24 µ m excessis reduced around multiple stars in other clusters. More workto explore the nature of the multiple stars, in particular todetermine the system geometry and the location and dis-tribution of dusty debris in these systems is necessary todetermine the nature of any link between excess and multi-plicity. ACKNOWLEDGMENTSREFERENCES
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