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1997MNRAS.290..186G Mon. Not. R. Astron. Soc. 290,186-192 (1997) Surface reflectance properties of distant Solar system bodies s. F. Green,i N. McBride, 1 D. P. 6 Ceallaigh,2 A. Fitzsimmons,2 I. P. Williams 3 and M. J. Irwin4 Unit for Space Sciences and Astrophysics, Physics Laboratory, The University of Kent, Canterbury cn 7NR APS Division, Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN 3 Astronomical Unit, Queen Mary and Westfield College, Mile End Road, Landon E1 4NS 4 APM Unit, Royal Greenwich Observatory, Madingley Road, Cambridge CB3 OEZ 1 2 Accepted 1997 May 16. Received 1997 May 16; in original fonn 1996 September 18 ABSTRACT The recent discoveries of over 40 new objects with orbits beyond 30 au represent the first sampling of a reservoir of objects lying beyond Neptune, known as the Kuiper Belt (or Edgeworth-Kuiper Belt), which may be a source of short-period comets and Centaurs (objects whose orbits cross those of the giant planets). There are very few observations from which to derive physical properties of these Kuiper Belt objects (KBOs) due to their faintness and the concentration on discovery rather than follow-up, although optical and near-IR photometry of the few brightest KBOs and Centaurs shows a diversity from neutral to extremely red colours. We present new BVRI photometry of five KBOs (l994JQb 1995DC 2 , 1994JR b 1995DA2 and one undesignated new KBO) and the Centaur object 1995DW2. With the current small sample, we find no compelling evidence for a correlation of colours with orbital zones, and consequently no clear mechanism to explain this diversity in terms of the irradiation mantle model. Key words: comets: general - minor planets, asteroids - Solar system: general. 1 INTRODUCTION The recent discoveries of over 40 new objects with orbits beyond 30 au represent the first sampling of a reservoir of objects lying beyond Neptune. This reservoir, known as the Kuiper Belt (or more correctly the Edgeworth-Kuiper Belt) is one of the major discoveries of Solar system science in recent years. The existence of Kuiper Belt objects (KBOs) was originally postulated from considerations of the mass distribution in the early Solar system (see Edgeworth 1949; Kuiper 1951), but has since been strengthened by observations of extended discs around main-sequence stars (e.g. Aumann et al. 1984) and the need for a source of the short-period (SP) comets. The spherically symmetric 60rt cloud (source of the long-period comets) cannot produce the inclination distribution of the predominantly prograde orbits of SP comets (Duncan, Quinn & Tremaine 1988; Quinn, Tremaine & Duncan 1990), although evolution from the inner edge of a disc of icy planetesimals orbiting between ~40 and 100 au could provide this source, since dynamically chaotic regions exist inside 40 au (e.g. Morbidelli, Thomas & Moons 1995; Malhotra 1996). The Centaurs (minor bodies in planet-crossing orbits between Jupiter and Neptune), of which six are currently known, are believed to represent the largest members of a transitional group between the Kuiper Belt and SP comets. The current detection rate implies a population of a few X 104 KBOs with semi-major axes 30-:-50 au and diameters >100 kIn, with a few tens of Centaurs of this size awaiting discovery. By a statistical analysis of possible faint objects found on long-exposure frames taken with the Hubble Space Telescope (HST), Cochran et al. (1995) found that many of the possible objects identified were not just noise, but were likely to be Halley-sized (i.e. ~ 10 kIn) KBOs (although no one object could be confirmed as being 'real'). This suggests that there must be > 2 x 108 objects with diameters> 10 kIn in the Kuiper Belt with inclinations <120 and within ~40 au of the Sun. There are very few observations from which to constrain the physical properties of the trans-Neptunian objects, due to their faintness and the concentration on discovery rather than follow-up. However, the largest Centaurs (Chiron and Pholus) which presumably originated in the Kuiper Belt have low albedos and similar sizes, but very different spectra. Chiron, which has recently exhibited cometary activity in its approach to perihelion, has neutral colours similar to C-type asteroids and some cometary nuclei (Hartmann et al. 1990). However, Pholus was found to have an astonishing spectrum (Fink et al. 1992) which made it one of the reddest objects in the Solar system (Buie & Bus 1992). Objects 1993HA2 (Luu 1994) and 1992QB 1 (Jewitt & Luu 1993) also appear to have extreme red colours, whereas 1993FW is less red (Luu & Jewitt 1993). These extreme red optical and infrared colours (Davies, Sykes & Cruikshank 1993) have been interpreted as due to the presence of organic tholins which result from radiation bombardment of mixtures of methane and other organic molecules (Fink et al. 1992; Mueller et al. 1992; Binzel1992). One possible scenario is that icy © 1997 RAS © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G Distant Solar system bodies planetesimals containing simple volatiles (e.g. H 2 0, CO, CO2 , NH3 , CH4 , ~, N2 ) are irradiated by UV photons and cosmic rays over 4.6 x 109 yr in the Kuiper Belt (or bort cloud) to produce an 'irradiation mantle'. Collisions could also expose un-irradiated material. Luu & Jewitt (1996) estimate ~ 10 impacts of km-sized bodies on a 100-km KBO in the lifetime of the Solar system. This may result in resurfacing by both fresh subsurface ice and material from the impactor. The true colours of KBOs could therefore be an indicator of the degree of collisional resurfacing and may be correlated with location in the Kuiper Belt. Ideally, spectra of the objects are needed, although their extreme faintness makes this difficult. Luu & Jewitt (1996) obtained low-resolution optical spectroscopy of 1993SC (one of the brightest objects), in the wavelength range 400 to 800 nm. The relative reflectance spectrum of 1993SC (object spectrum divided by the spectrum of a solar-type star and normalized to 1 at A = 560 nm which provides a measure of the intrinsic colour of the object) exhibited a positive gradient (i.e. reddened object) intermediate in value between those of Chiron and Pholus. The overall effect of either of these two processes on the surfaces of KBOs is unknown at present. Application of resurfacing models requires the observation of colours of KBOs, of their probable dynamical cousins Centaurs, and of inactive SP comets. In this paper we present optical colours for bodies in all three populations. 2 OBSERVATIONS 2.1 Anglo-Australian Telescope Observations with the 3.9-m Anglo-Australian Telescope, New South Wales, were obtained on 1995 March 28, 29 and 30. Images of 1024 x 1024 pixels were obtained using the TEK CCD chip with Kitt Peak V, R and I filters used at prime focus, providing an image scale of 0.39 arcsec per pixel. Readout noise was 4.8 electrons with a gain of 2.74 electrons per ADU. Exposures of 1000 s in R separated by at least an hour were used to identify the targets before colour measurements were made. Exposures with the I filter were limited to 500 s to prevent sky background saturation. Guiding was at sidereal rate except for comet 109P/Swift- Tuttle which was tracked. Flat-fields were obtained from median-filtered twilight sky observations at the beginning and end of each night. On March 28, seeing deteriorated from 2 to 3 arcsec during the night, effectively preventing detection of any of the KBOs attempted, with only the Centaurs 1995DW2 and 1993HA2 being observed. Poor seeing also compromised observations on March 29, although 109P/Swift-Tuttle was identified in a crowded field. Several KBOs were not found, although 1994JS and 1994JV should have been sufficiently bright for detection even in 3-arcsec seeing. However, a pair of search frames adjacent to the predicted position for 1994JS revealed a new object at 7 arcmin from the expected 1994JS position which was confirmed the following night. Although the short arc could be fitted by a main belt asteroid close to its stationary point, a subsequent independent discovery at the European Southern Observatory (Lagerkvist, Lagerros & Magnusson 1995) resulted in its identification as a KBO designated 1995FB 21 • 1994JR1 was detected but was too faint for colours to be determined. Improved seeing of 1 to 1.5 arcsec on March 30 allowed colours to be determined for 1995DA2 and 1995DC2 , detection of 1995DB2 and 1994ES2 , and a search of 18 x 18 arcmin2 about the subsolar point (although no new slow-moving objects were discovered from this search), before patchy clouds prevented colour determination 187 for 1994JRI. During these observations calibration frames were taken of fields RUl49, PGl047, PG1323 and PGl633 (Landolt 1992), providing colours in the Kron-Cousins photometric system. 2.2 Isaac Newton Telescope Observations of 1993FW and 1994JQI were obtained with the 2.5-m Isaac Newton Telescope of the Observatorio del Roque de Los Muchachos, La Palma, on the nights of 1995 March 31 and April 1. Images of 1024 x 1024 pixels were obtained using the TEK3 thinned CCD chip at the prime focus, giving an effective image scale of 0.59 arc sec per pixel. Observations were made with 900-s exposures through broad-band Band R (Harris) filters during which the telescope was auto-guided at the sidereal rate. The CCD had a readout noise of 5-6 electrons and a gain of 0.76 electrons per ADU. Each KBO was easily detected in the centre of the CCD frame by visually blinking. Bias frames were recorded and flatfields of the evening twilight sky were obtained in both R and B filters. Images of the standard fields SAlOl, SA107 and SA110 (Landolt 1992) were measured through both B and R filters on both nights, allowing subsequent magnitudes in the Kitt Peak system to be derived. The observing conditions were photometric throughout the observing run, thus enabling absolute flux calibration. Unfortunately we could not obtain a value for the R-band extinction for the second night due to difficulties in reading out these particular standard field images. However, it was possible to obtain a photometric calibration for the second night via the large overlap with the first night's images. The nightly extinction values obtained were larger than expected from theoretical calculations (King 1985) due to the presence of Saharan dust above the observatory. 2.3 Data reduction The CCD images were processed in the usual manner. All the images were bias-subtracted and divided by a flat-field using the software package KAPPA (Currie 1992). A similar procedure was then used to reduce the observations from each site, combining aperture photometry using PHOTOM (Eaton 1989) and profile fitting from DAOPHOT II (Stetson 1987). Instrumental magnitudes of the Landolt standard stars were obtained using the aperture photometry package PHOTOM. At this point, PHOTOM could have been used to obtain the instrumental magnitudes of the KBOs, although on investigation it was found that aperture photometry of these extremely faint objects could result in large uncertainties due to the non-uniformity of the sky from faint background objects. Therefore the profile fitting from DAOPHOT II was used to obtain the instrumental magnitudes of the KBOs [this was reasonable as the motion of KBOs during each exposure was smaller than the seeing, and hence the KBO point spread functions (PSFs) were comparable to those offield stars]. However, the problem is further complicated, as calibration using profile fitting alone would be urrreliable due to the differences in PSF for exposures of a few seconds on bright standards and ~ 1000 s on faint objects. Thus a selection of 15-20 field stars in each KBO frame was used to determine the PSF and provide instrumental magnitudes using both aperture photometry and profile fitting. They could then be used to transfer the standard star calibration to the KBOs' profile fitted magnitudes. This procedure was not possible for 109P/Swift-Tuttle because of the trailed field star images, and so just aperture photometry using PHOTOM was employed on three frames taken in V, R and l. © 1997 RAS, MNRAS 290, 186-192 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G 188 S. F. Green et ai. Table 1. Aspect data and magnitudes. The dates are for mid-frame, and are light-time-corrected. The slow-moving object (SMOl) was discovered on the same frame as 1993FW. Object 1995DW2 1993HA2 I 994JR1 109P/Swift -Tuttle 1995FB21 1995DA2 1995Dy 1995DB2 1995ES2 1993FW SMOI 1994JQI Date r ..i [AU] Exp [sec] Magnitude [AU] ex [deg] Filter [UT] 19950328.5817 .5861 .6291 .6577 .6675 .6745 19950328.6214 .6830 .6901 .6972 .7919 19950329.7180 .7741 19950330.7437 .7601 19950329.4112 .4180 .4253 .4805 .4898 .4973 19950329.7412 .7814 19950330.7491 .7898 19950330.3992 .4652 .4917 .5018 .5086 19950330.3603 .5252 .5648 .5747 .5815 19950330.4127 19950330.4388 19950401.0245 .0841 .1373 19950401.0245 .0841 .1373 19950401.1229 .1765 18.892 17.896 0.2 R R R 100 300 300 1000 500 500 500 500 500 500 500 1000 1000 1000 1000 500 500 500 500 500' 500 1000 1000 600 300 1000 1000 1000 500 500 1000 1000 1000 500 500 1000 1000 900 900 900 900 900 900 900 900 21.69:<::: 0.12 21.85:<::: 0.10 21.65:<::: 0.08 21.96:<::: 0.06 21.08:<::: 0.08 21.07:<::: 0.08 19.88:<::: 0.02 V 12.509 11.848 R 3.5 R V 34.786 34.247 1.4 8.531 8.009 5.9 R R R R V R R R R V 42.426 41.782 1.0 34.000 33.533 1.5 R R R R R R V I 45.208 44.406 R R 0.8 V I I 40.575 45.805 42.094 40.090 44.943 41.095 1.2 0.6 0.0 43.131 42.190 0.5 Three additional R frames were available, but the object passed through the outer wings of a reasonably bright star image in these. However, by measuring the object on all three frames, and also measuring the sky contribution at the positions that the object would/did occupy, we obtained separate object and sky contributions for each frame. The R magnitudes derived with this method compare favourably (Table 1) with the R magnitude obtained when the object was away from the bright star. Subsequent inspection of the 1993FW frames revealed a slowmoving object with similar near-opposition motion, called here SM01. Unfortunately, attempts to recover this object about 1 month later failed, and it remains unrecovered. R R R B R R B R B R 3 Comment Blended with star Blended with star Blended with star 19.82:<::: 0.03 22.19:<::: 0.04 22.18:<::: 0.04 22.30:<::: 0.Q7 23.05:<::: 0.05 20.80:<::: 0.10 20.85:<::: 0.10 20.87:<::: 0.10 20.83:<::: 0.03 21.39:<::: 0.04 20.06:<::: 0.05 23.29:<::: 0.09 23.38:<::: 0.10 23.18:<::: 0.Q7 23.22:<::: 0.10 23.06:<::: 0.06 23.10:<::: 0.06 23.65:<::: 0.08 22.67:<::: 0.13 22.53:<::: 0.11 23.09:<::: 0.06 23.62:<::: 0.Q7 24.39:<::: 0.13 22.91 :<::: 0.11 23.17:<::: 0.13 24.07:<::: 0.12 24.07:<::: 0.13 22.44:<::: 0.17 24.12:<::: 0.24 22.30:<::: 0.16 22.07:<::: 0.11 24.00:<::: 0.19 22.21 :<::: 0.15 24.18:<::: 0.25 22.71:<::: 0.14 Near star Near star Near star Discovery frames RESULTS Aspect data and magnitudes are listed in Table 1. Quoted 1cr uncertainties are based on statistical errors from photon counting combined with calibration transfer. The statistical uncertainties are generally larger than might be expected from extrapolating the performance of the CCD cameras on these telescopes quoted in the user manuals, probably due to contamination by faint background objects. In addition, any unknown rotational light curve effects may contribute to temporal changes in brightness, and these have not as yet been fully investigated. Williams et al. (1995) suggested the possibility that 1993SC might have a © 1997 RAS, MNRAS 290, 186-192 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G Distant Solar system bodies 189 Table 2. Optical colours of outer Solar system objects. Object Group q e [AU] 1992QB! 1993FW 1994JQ! 1995DC2 1993SC 1994JR! 1995DA2 SM01 KBO KBO KBO KBO KBO KBO KBO KBO? CEN 95P/Chiron 5145 Pho1us CEN B-V a [AU] [deg] 40.0 41.6 43.0 40.1 32.1 34.7 32.9 0.07 0.05 0.03 0.09 0.19 0.13 0.09 44.2 43.8 44.1 44.0 39.8 39.8 36.3 2.2 7.7 3.7 2.3 5.1 3.8 6.6 8.5 0.38 13.7 6.9 B-R V-R R-J Diameter [km] 0.6:t0.1a 0.4:t0.1b 1.0:t0.2a 0.77:t0.16 0.54:t0.14c 0.75:t0.09 0.55:t0.11 0.58:t0.16 0.43:t0.14c 283 286 220 224 0.38:t0.03 e ! 0.39:t0.03 e2 0.34:t0.03 e3 0.34:t0.03 e4 0.34:t0.03 e ! 0.32:t0.04e2 0.28:t0.03 e3 168:t2of 0.76h ! 0.68h2 189:t26i 1.47:t0.28 0.50:t0.16 Albedo 214 145 1.88:t0.21 8.7 0.58 20.4 24.7 0.70:t0.02d 1.35h2 0.75 h ! 0.66h2 0.81O:t0.00&' 0.81 :to.02k 0.87:t0.05 k 0.78:t0.ozi 0.84:t0.03/ ~0.7m 1993HA2 1995DW2 1PIHalley 2PlEncke 10PfTempe12 CEN CEN COM COM COM 11.8 18.9 0.59 0.33 1.48 0.52 0.25 0.97 0.85 0.52 24.8 25.0 17.9 2.2 3.1 15.6 4.1 162 11.9 12.0 0.73:t0.03 n 0.81:t0.0]!' 0.94:t0.03 Q 26P/Gicobini-Zinner 28PINeujmin 1 COM COM 1.03 1.55 0.71 0.78 3.5 6.9 14.2 0.84:t0.04" 49PIArend-Rigaux COM 1.44 0.60 3.5 109P/Swifi-Tuttle COM 0.96 0.96 26.3 113 0.31:t0.1O 0.44:t0.03 n 0.47:t0.0]!' 0.56:t0.03 Q 0.53:t0.03' 0.49:t0.02' 0.47:t0.03" 0.50:t0.04' 0.47:t0.01' 0.56:t0.10 0.48:t0.07 n 0.56:t0.1O . 0.77:t0.1O 0.51:t0.02' ~O.1:Y 0.14~g:~g «l.044:t 0.0 13i 82 80 9.3:t0.3° 4.4-9.8P ~0.04° 1O.6~?:!r 0.03:t0.01r ~20" 0.02-0.03" 9.6:t0.8 v 9.0~?:6w 24 0.054:t0.01 v 0.06:t0.03 w Colours are derived from observations reported here, and diameters assuming an albedo of 0.04, except: a Jewitt & Luu (1993); b Luu & Jewitt (1993); c Davies et al. (1996); d Lebofsky et al. (1984); e Hartmann et al. (1990) [(1) 1988 February 20; (2) 1988 February 21; (3) 1988 February 22; (4) 1988 September 9]; f Altenhoff & Stumpff (1995); g Campins et al. (1994); h Mueller et al. (1992) [(1) 1992 January 9; (2) 1993 January 23]; i Davies et al. (1993); j Buie & Bus (1992); k Fink et al. (1992); deduced here from their reflectance spectrum; I Binzel (1992); deduced here from his reflectance spectrum; mLuu (1994); nThomas & Keller (1989); deduced here from their reflectivity gradients; ° Keller et al. (1994); P Luu & Jewitt (1990); deduced here from their reflectivity gradient; qJewitt & Luu (1989); deduced here from their reflectance spectrum; r A'Hearn et al. (1989); , Jewitt & Meech (1988); , Luu (1993); deduced here from her reflectivity gradient; U Campins et al. (1987); deduced here from their reflectance spectrum; v Tokunaga & Hanner (1985); albedo at 1.25 fLm; W Birkett et al. (1987); albedo at 1.25 fLm. O.5-mag light curve, but Davies, McBride & Green (1997) find no light curve behaviour greater than 0.2 mag (i.e. within the uncertainties in the photometry) deduced from 33 frames taken over five nights. Significant light curve behaviour, however, cannot be ruled out for other objects. We assume here that any variations due to varying phase angle are less than the formal errors in our photometry. The colours listed in Table 2 have been derived from the data with the shortest timebase. Where multiple exposures in the I filter were performed consecutively, the results have been combined. Quoted errors are simply the combined errors from each magnitude, although the unknown effects of background contamination and light curves may result in rather larger errors. Table 2 also lists derived diameters assuming an albedo of 0.04 unless otherwise stated. The only objects for which radiometric data are available are Pholus, with a low geometric albedo (:50.08, Howell et al. 1992; :50.044, Davies et al. 1993), and Chiron, which has a surprisingly high albedo of -0.12 (Lebofsky et al. 1984; Campins et al. 1994; Altenhoff & Stumpf 1995). Cometary nuclei close to the Sun also have low albedos in the range 0.03-0.05 (see the review by A'Hearn 1988 and references therein). Since the calculated diameter is proportional to the inverse square root of the albedo and we assume a most likely value near the bottom of the possible range (0.03 to -0.2), true diameters might be much smaller than the values derived. In this case light curve amplitudes might also be expected to be larger due to the lower gravity, allowing greater deviations from spheroidal shapes. In addition, Centaurs or KBOs may have significant albedo variegation such as that detected on Pluto (Binzel 1988; and with the HST: Stem, Buie & Trafton 1997). It is clear that conclusive data can only be obtained by repeated observation to identify the magnitude of such effects, as has been performed for 1993SC (Davies et al. 1997), and careful removal of potential background sources. Davies et al. (1997) comment on the potential hazards in photometry of such faint moving objects. Table 2 also contains all the measured optical colours to date, including some derived from published spectroscopy. In most cases, the data have been obtained in the Kron-Cousins system and we have used appropriate wavelengths given by AB = 440 nm, AV = 550 nm, AR = 650 nm and Al = 830 nm, and solar colours given by (B-V)o = 0.67, (V-R)o = 0.36, (R-l)o = 0.35 (Meech, Knopp & Farnham 1995). The quoted spectral indices S' © 1997 RAS, MNRAS 290,186-192 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G 190 S. F. Green et al. (100run)-1 are converted to relative reflectance X(A) using X(A) = 1 + S (A ~~v), (1) where Aand AV are in run, and colours using mv - mA 4 = 2.5 log X(A) + (mv - mA)o. (2) DISCUSSION 4.1 Asteroids and comets Use of colorimetry usually represents the first attempt in classification of unresolved objects. In the case of asteroids, taxonomies are based on spectrophotometry in the 0.3-1.1 f.Lm range augmented by radiometric or polarimetric albedos. Photometry in standard broad-band or specially designed filters at diagnostic wavelengths is used for fainter targets. Taxonomies of asteroids (see e.g. reviews by Tholen & Barucci 1989; Gradie, Chapman & Tedesco 1989) are related to mineralogic interpretation of spectral features which provide insight into the formation conditions and processes as a function of heliocentric distance. In the outer asteroid belt, objects are characterized by low albedos and neutral colours associated with carbonaceous minerals (C type). With increasing heliocentric distance, D types (suggested carbon/organic-rich silicate composition), with low albedo and reddened spectra longward of 0.55 f.Lm (some flat longward of 0.95 f.Lm), are prevalent. Most comet photometry is dominated by gaseous emissions and dust particle scattering from the coma, preventing direct photometry of the nucleus. The few nuclear spectra or colours available (listed in Table 2) are obtained at large heliocentric distance where there is no significant volatile emission, or from observations of low-activity comets at small geocentric distances. These appear to be similar to D-type asteroids (with similar low albedos), although Neujmin 1 is considerably redder, as is Swift-Tuttle from data reported here. 4.2 The Centaurs There are currently seven known Centaur objects (2060 Chiron, 5145 Pholus, 1993HA2 , 1994TA, 1995DWz, 199500 and 1997CU26 ). After comet IPIHalley, Chiron is perhaps one of the most studied comets of all. Discovered in 1977 (Kowal 1977), its orbit ranges from 8.5 au (inside Saturn's orbit) to almost 19 au (Uranus' orbit). Although originally classified as an asteroid, it has since exhibited cometary behaviour in the form of secular brightness fluctuations as well as short-term impUlsive photometric variations. Between mid-1987 and the end of 1988, Chiron began to exhibit unequivocal cometary behaviour, brightening by 1.3 mag by 1990 (Tholen, Hartmann & Cruikshank 1988; Hartmann et al. 1990). Meech & Belton (1990) detected a low surface brightness coma around Chiron at a heliocentric distance of 11.8 au. In addition to these long-term changes in brightness which occur over a period of years, Chiron also exhibits short photometric variations (-0.1-0.2 mag) which occur over periods as short as a few hours (Luu & Jewitt 1990; Buratti & Dunbar 1991; Bus et al. 1991). Spectrally, Chiron is neutral in the near-infrared and nearneutral in the visible with no evidence for strong H2 0 absorption bands (Lebofsky et al. 1984; Hartmann et al. 1990; Luu & Jewitt 1990). Although spectrally similar to C-type asteroids, it has an unusually high albedo of 0.1-0.13, similar to 2 Pallas. The colours listed in Table 2 are for times when the coma intensity contribution is less than that of the nucleus, at heliocentric distances> 12 au. 5145 Pholus is the reddest Centaur observed to date. Its extreme V-R colour of 0.810 ± 0.006 (Buie & Bus 1992) and maximum albedo of 0.044 ± 0.013 (Davies et al. 1993) are believed to be due to the presence of tholins on its surface as a result of long-term exposure of organic molecules to energetic radiation. Deep CCD imaging shows an object which is stellar in appearance with no evidence of a coma (Hainaut & Smette 1992). Fink et al. (1992) obtained a visible spectrum of Pholus which displayed neither absorption nor emission features (although some structure has been determined in the near-infrared: Davies, Sykes & Cruikshank 1993; Luu, Jewitt & Ooutis 1994). They compared its spectrum with those of several terrestrial and meteoritic spectra (see fig. 3 of Fink et al. 1992). The only class of compounds that seemed to match the steep spectrum ofPholus were the organic residues, tholins. Fink et al. (1992) compared tholins produced by UV irradiation and those produced by a spark discharge. A combination of both of these organics could adequately match the spectrum of Pholus. 1993HA2 has a V-R-0.7, approaching that ofPholus, although the uncertainty in this value may be large. Davies, Tholen & Ballentyne (1996) find a V-J intermediate in value between those of Chiron and Pholus. 1995DW2 has similar colours to Chiron but no activity has been reported. 4.3 Kuiper Belt objects The known KBOs appear to be divided into two dynamical groups. About half of them appear to have nearly circular, low-inclination (and generally stable) orbits with perihelion distances 40-45 au. The rest have perihelion distances of around 30-36 au. Marsden (personal communication) speculated that these were in 2:3 resonance with Neptune (like Pluto), allowing several objects to be recovered. Many of the individual object orbits are so poorly determined that general solutions allow perihelia of around 30 au (right at Neptune), yielding unstable orhits which could indicate transitional objects. Our data, combined with previously published colours as listed in Table 2, are presented as a colour diagram in Fig. 1. The VRI colours of the KBOs range from the extreme Pholus-like (V-R and/or R-I> 0.7) red values for 1992QB 1, 1995DCz and 1994JR1, through intermediate values of 1993SC and 1995DA2 , to the Chiron-like (V-R-0.36, R-I-0.31) neutral colour of 1993FW. From inspection of Table 2, there appears to be no clear correlation of colours with orbital semi-major axis. Such a correlation might be expected if surface processes such as irradiation mantle creation are controlled by heliocentric distance, while eccentricity would affect the impact velocity of collisions between KBOs and hence might affect subsequent reflectance properties. The lack of such correlation in our small sample implies either that these processes do not affect surface colours on time-scales less than that for dynamical perturbations for these orbits (essentially stable for r > 42 au and highly variable between 34 and 43 au), or that perhaps other mechanisms are responsible for producing the observed range of colours. 4.4 Interrelations Although all KBO, Centaur and cometary populations discussed here are presumed to be composed of physically similar bodies, care must be taken when looking at interrelations between them. Because of observational selection effects and popUlation densities, the sampled size distribution is different for comets, Centaurs and KBOs. Therefore if the mechanisms producing the observed red colours on primitive objects are dependent on size or mass of the © 1997 RAS, MNRAS 290,186-192 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G Distant Solar system bodies 191 1.0 1994JRl (KBO) 0.8 1993HA2 (CEN) Tempe12 ~0.6 ~ (COM) G-Z(COM) >- ~ Encke 0.4 Neujmin 1 (COM) (COM) • 1992QBl (KBO) • 1995DC2 (KBO) ... 1993SC (KBO) ~ 1993FW (KBO) • 1995DA2 (KBO) Chiron (CEN) Pholus (CEN) 1995DW2 (CEN) ® Halley (COM) EB Swift-Tuttle (COM) Arend-Rigaux (COM) Sun o o o 0.2 e o 0.0 0.2 0.4 0.6 R- I 0.8 1.0 1.2 Figure 1. VRI colour-colour plot of distant Solar system bodies. Objects with no I magnitude are shown on the left with arrows. We assigned an error of ±O.15 mag to the approximate V-R colour for 1993HA2 reported by Luu (1994). body, (e.g. retention of particles ejected by volatile outgassing or collisional ejecta), then comparisons between the different populations may be misleading. That said, it is clear from the data presented here that all three populations share a wide range of colours, from neutral to extremely red. The presence of low-albedo, red surfaces on outer Solar system objects may be expected if their surfaces have not been altered since formation. Their major constituents, volatile ices, when exposed to high-energy photon irradiation and cosmic rays, are converted to carbon-rich compounds and radicals to a depth of metres: an 'irradiation mantle' (e.g. Moore et al. 1983; Johnson 1991). This mantle may be sufficiently cohesive to inhibit activity if an object is perturbed closer to the Sun (Strazul1a et al. 1991). Short-period comets, which have undergone many perihelion passages which would have stripped away the irradiation mantle, will also have a predominantly inactive surface (calculated active areas of SP comets with known nuclear sizes are generally <10 per cent). Indeed, there is dynamical and physical evidence for inactive cometary nuclei observed as part of the planet-crossing asteroid population. In this scenario, Pholus retains its irradiation mantle while the cometary activity of Chiron has removed it. If this view is correct then we require an explanation for (i) the apparent diversity of colours in the KBO population, (ii) the lack of activity of Pholus while at a similar heliocentric distance to Chiron, (iii) the difference in nuclear colours for Chiron compared with SP comets. The apparent diversity of colours on KBOs could result from real differences in composition and/or surface scattering properties, or from some process that has altered the surface of only part of the popUlation. One possibility is that the surfaces of cometary bodies (initially with irradiation mantles) become neutral by the removal of the surface layers due to outgassing; such a conclusion is certainly consistent with the behaviour and colours of 2060 Chiron and 5145 Pholus. However, while the nucleus of 49P/Arend-Rigaux (Millis, A'Hearn & Campins 1988) also has a neutral or bluish colour, other currently active comets such as 1PIHalley (Thomas & Keller 1989), PfNeujmin 1 (Campins, A'Hearn & McFadden 1987) and Ptremple 2 (Jewitt & Luu 1989) have reddish surfaces. In addition, any mechanism for activity must be possible at 40 au, but not be universal, even at 8 au. Thus outgassing alone probably cannot explain the colour variation in KBOs. However, a possible consequence of activity on large bodies at large heliocentric distances is selective resurfacing due to a proportion of ejected particles not attaining escape velocity and returning to the surface (clearly dependent on both the particle size distribution and composition, and the size of the parent body). Recent HST observations of the inner coma of Chiron (Meech et al. 1997) have been interpreted as due to a bound population of particles on ballistic trajectories. Alternatively, resurfacing may be a consequence of impact processes at a range of sizes. If regolith 'gardening' of the cometary surface due to (small) meteoroid impacts were an important process for modifying colours, one would expect objects in similar orbits to be equally exposed. However, individual impacts of larger meteoroids could modify the surface (or part of the surface) by exposure of, or resurfacing by, 'pristine' subsurface material. Such random events could lead to a diversity of colours, without showing a strong dependence on orbital parameters. Additional evidence for this mechanism might be apparent from rotational light curve variations due to colour/albedo variegation. Another possibility is that KBOs originally possessed an intrinsic spread in compositions, in which case the original bodies may have had a range of colours, or have subsequently been processed by different degrees (or at different rates). If this is the case, then the observations presented in this paper imply that such primordial variations were not a strong function of heliocentric distance, © 1997 RAS, MNRAS 290,186-192 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.290..186G 192 S. F. Green et al. assuming that the observed KBOs have survived in or near their present orbits for the lifetime of the Solar system. 4.5 Conclusions We have presented optical colours for five KBOs, one Centaur and one comet. Combining these with previously published data for these classes of bodies and cometary nuclei, we find that these populations share a wide range of colours, from solar to extremely red. While dynamical studies may point to an evolutionary path from KBOs to Centaurs to comets, the cause of the range of colours in populations supposedly composed of similar bodies remains unclear. The small data set available shows that at present it is not possible to differentiate observationally between the mechanisms most likely to affect the surface colours, whether impact- or outgassing-related. To do so in the future will require significant enhancement in our knowledge of how these processes affect the surface colours of primitive bodies. The optical/near-infrared, the thermal infrared (from ISO) or spectra from a larger sample of objects with a diversity of orbits and dynamical histories may provide the only clues possible with current instrumentation. ACKNOWLEDGMENTS The Isaac Newton Telescope is operated on the Island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias. The Anglo-Australian Telescope is operated by staff of the Anglo-Australian Observatory on behalf of the UK Particle Physics and Astronomy Research Council and the Australian Department of Employment, Education and Training. Image processing and data reduction were performed using the Starlink network and software. NM acknowledges the financial support of the UK Particle Physics and Astronomy Research Council. DPOC acknowledges support from the Gaeltacht Scholarship. 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