Received 2002 July 22; accepted 2002 November 20

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1 The Astronomical Journal, 125: , 2003 March # The American Astronomical Society. All rights reserved. Printed in U.S.A. ESO LARGE PROGRAMME ON PHYSICAL STUDIES OF TRANS-NEPTUNIAN OBJECTS AND CENTAURS: VISIBLE SPECTROSCOPY 1 M. Lazzarin, 2 M. A. Barucci, 3 H. Boehnhardt, 4 G. P. Tozzi, 5 C. de Bergh, 3 and E. Dotto 6 Received 2002 July 22; accepted 2002 November 20 ABSTRACT We present the first results from a visible spectroscopic investigation of trans-neptunian objects (TNOs) and Centaurs, performed within an ESO Large Programme started in 2001 April to spectrophotometrically study these pristine objects in the visible and near-infrared. So far, spectra of 12 TNOs and Centaurs have been obtained using the FORS1 instrument at the Very Large Telescope. The principal preliminary results are differences in the spectral gradient the gradients obtained indicate the existence of a range of values from moderately red to very red and the presence of absorption features on two of the observed objects of as yet unexplained origin. The spectral gradients of the objects are also compared with photometric slopes obtained from quasi-simultaneous BVRI magnitudes of the objects (where available). An analysis of the spectral gradients with respect to the perihelion distance of the objects suggests that Centaurs (with the possible exception of 1999 OX 3 ) occupy a zone of lower reflectance slope compared with the TNOs, probably indicating stronger resurfacing effects from cometary activity and, though less likely, collisions. Key words: Kuiper belt methods: observational minor planets, asteroids techniques: spectroscopic 1. INTRODUCTION An ESO Large Programme for the study of the physical and compositional properties of trans-neptunian objects and Centaurs at the Very Large Telescope (Cerro Paranal) and New Technology Telescope (La Silla) has been accepted for the period from 2001 April to 2003 March. The investigation is based on visible and near-infrared spectroscopy and photometry, which are the best means for studying the physical and chemical properties of these distant and pristine objects. Located in the outer solar system, the trans- Neptunian objects (TNOs) have been divided into three dynamical classes: the classical Kuiper belt objects, also called Cubewanos, from the name of the first discovered object (1992 QB 1 ), which are characterized by elliptical orbits and semimajor axes between about 40 and 46 AU; the scattered-disk objects, with highly eccentric orbits and semimajor axes much larger than the Cubewanos ; and the Plutinos, in the 2 : 3 resonance with Neptune. The Centaurs, very likely objects in transit from the Kuiper belt to the inner solar system, have orbits between Jupiter and Neptune. At the time of this writing, several hundred TNOs have been discovered, and more than 50 Centaurs. Some of the known objects have only been photometrically investigated, and only a few of them have been spectroscopically observed in the visible, the near-infrared, or both. The main reason is that they usually are very faint (m v > 20 21), so they call for large telescopes. Only 2060 Chiron, the first discovered Centaur, has clearly shown cometary activity (Tholen, Hartmann, & Cruikshank 1988; Meech & Belton 1989, 1990; Hartmann et al. 1990; Bus et al. 1991); none of the other investigated TNOs or Centaurs have shown any kind of activity, and at best indirect indications exist (e.g., light-curve change in 1996 TO 66 ; Hainaut et al. 2000). Up to now, very little visible spectroscopic data has been available: five Centaurs were observed by Barucci, Lazzarin, & Tozzi (1999), the Centaur 5145 Pholus by Binzel (1992) and Fink et al. (1992), and five TNOs by Boehnhardt et al. (2001). The main result is a difference in the slopes of the spectra, which has been observed among both TNOs and the Centaurs. Barucci et al. (1999) also searched for the possible presence of CN on some Centaurs, but without a positive result. With this Large Programme we intend to investigate the spectral characteristics of as many TNOs and Centaurs as possible, in order to make a statistical analysis of their surface properties. In this paper, we report the first results obtained from the visible spectroscopic investigation, which has produced spectra of 12 TNOs and Centaurs so far. 1 Based on observations collected at the European Southern Observatory, Paranal, Chile (program 167.C-0340, principal investigator H. B.). 2 Dipartimento di Astronomia, Università di Padova, vicolo dell Osservatorio 2, I Padova, Italy; lazzarin@pd.astro.it. 3 Observatoire de Paris, Place Jules Janssen 5, F Meudon Cedex, France; antonella.barucci@obspm.fr, catherine.debergh@obspm.fr. 4 European Southern Observatory, Casilla 19001, Santiago 19, Chile; hboehnha@eso.org. 5 Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I Firenze, Italy; tozzi@arcetri.astro.it. 6 Osservatorio Astronomico di Torino, via Osservatorio 20, I Pino Torinese, Italy; and Osservatorio Astronomico di Roma, via Frascati 33, I Monteporzio Catone, Italy; dotto@to.astro.it OBSERVATIONS AND DATA REDUCTION The visible spectroscopic observations presented here were performed in 2001 April, 2001 October, and 2002 February, for a total of four observational runs performed with the VLT (see Table 1): Unit Telescope 1 (UT1, Antu ) was used in 2001 April, and Unit Telescope 3 (UT3, Melipal ) was used for the other runs. The FORS1 instrument, a focal reducer and spectrograph, was used for the observations. FORS1 is equipped with a TK2048EB4-1 CCD, with pixels and a pixel size of lm. We used grism 150I, which

2 SPECTROSCOPY OF TNOs AND CENTAURS 1555 TABLE 1 Aspect Data during the Observations Date (UT) Dynamical Class r D m v (mag) (38628) 2000 EB Apr 27 Plutino EC Apr 27 Centaur GN Apr 27 Plutino KR Apr 27 Cubewano OX Apr 27 Centaur PT Oct 10 Centaur QC Oct 15 Centaur TC Oct 15 Plutino SG Oct 15 Centaur BL Feb 17 Centaur (26375) 1999 DE Feb 17 Scattered GQ Feb 17 Scattered gives a dispersion of 230 Å mm 1 in the first order. The spectral range is about lm, and with a slit width of 1 00 the spectral resolution obtained is about 200. Most of the spectra were recorded through a slit oriented along the parallactic angle to reduce the effects of differential refraction and possible loss of signal, while others (April run) were recorded through a slit oriented in the east-west direction. Both spectrophotometric flux standards and solar-analog stars were observed during each night of the program. Each solar analog was observed at least twice in the course of one night, with and without a filter cutting off the signal below 5000 Å. In fact, we wanted to check possible overlap of the second spectral order on the spectrum. A comparison of the spectra obtained with and without the filter shows that the contribution of the second order is negligible for the solar analogs (less than 2%), so, owing to their faintness, this is also true for the target objects. In Table 1, we report the aspect data for the objects during the observations: the date of observation, the dynamical class, the geocentric and heliocentric distances, the phase angle, and the visual magnitude at the time of observation. For most of the observed objects, owing to the low signal received, we summed two individual spectra in order to obtain a signal-to-noise ratio above the noise level and diminish the influence of the background. The two spectra were always reduced separately and then summed. This method is also very useful to check the reduction procedure: for example, a feature present in both exposures of one object is very likely real. Bias, flat-field, and arc exposures for the calibrations of the science data were taken during daytime. In Table 2, we report the circumstances of the observations: the UT start of the observation, the instrument used, the total exposure time, the average air mass of the two observations per object, and the solar analog used. During the April run, the seeing varied from a minimum of about 0>6 to a maximum of 1>3, with an average value around 0>7. The object 2001 PT 13 was observed in October with an average seeing of 0>51. During the other October run the seeing otherwise ranged from 0>58 to 0>8, and during the February run from 0>5 to1>8. The spectra were reduced using the software package MIDAS. The reduction procedure includes subtraction of the bias from the raw data, flat-field correction, cosmic-ray removal, background subtraction, collapsing of the twodimensional spectra to one dimension, wavelength calibration using He, HgCd, and Ar lamp spectra, and atmospheric extinction correction. The residuals of the wavelength calibration were 2 Å. After these procedures, we normalized all of the spectra, both of the TNOs and Centaurs and of the solar analogs, to unity around 5500 Å. The reflectance of the objects was then obtained by dividing the spectra of the objects by a solar-analog spectrum. TABLE 2 Main Characteristics of the Observations Start Time (UT) Instrument Exp. Time (minutes) Air Mass Solar Analog (38628) 2000 EB Apr 27, 0213 UT1 + FORS , HD EC Apr 27, 0038 UT1 + FORS , HD GN Apr 27, 0351 UT1 + FORS , HD KR Apr 27, 0536 UT1 + FORS , HD OX Apr 27, 0921 UT1 + FORS HD PT Oct 10, 0047 UT3 + FORS , HD QC Oct 15, 0505 UT3 + FORS , HD TC Oct 15, 0644 UT3 + FORS , HD SG Oct 15, 0808 UT3 + FORS , HD BL Feb 17, 0402 UT3 + FORS , Landolt (26375) 1999 DE Feb 17, 0630 UT3 + FORS , Landolt GQ Feb 17, 0830 UT3 + FORS , Landolt

3 1556 LAZZARIN ET AL. Vol. 125 TABLE 3 Reflectance Slopes and Perihelion Distances Slope (% per 100 nm) Perihelion (38628) 2000 EB EC GN KR OX PT QC TC SG BL (26375) 1999 DE GQ Solar-analog stars are fundamental in the final step of the reduction procedure, to remove the solar contribution from the spectra of the objects and to obtain reflectivities. The solar analogs were chosen on the basis of their spectral similarity to the Sun (Hardorp 1978) and based on the observational period. In the first two runs, one analog was observed twice in the course of the night (with and without filter), and the two spectra, after air-mass correction, were indistinguishable. In the third and forth runs, we observed two solar analogs twice in the course of the night and found negligible differences among them. More precisely, the reduced solaranalog spectra from each night were compared with each other, and differences in the reflectivity gradient were found to be on the order of 1% per 10 3 Å. Thus, the compatibility of the observed solar analogs places an upper limit of a similar amount on the reflectivity gradient errors that could be due to division by different solar analogs. The spectrum of 1999 OX 3, because it was observed for a shorter time and so is noisier than the others, has been smoothed with a median filter technique, calculating the average over a 3 3 pixel window around each pixel and replacing the original value by the average value only if the average value differed by more than 10% from the original value. The small reduction in resolution affects neither the slope of the spectrum nor the possible presence of features. We computed the reflectance slope S 0 (as defined by Luu & Jewitt 1990) of the continuum for each spectrum using least-squares fitting to the data over a spectral range of about Å, which is almost all of the available spectrum. The values of S 0 are reported in Table 3, as well as the perihelion distances of the objects Fig. 1. Reflectance spectra of the TNOs. The spectra are normalized to 1 around 5500 Å and have been offset by 1.5 for clarity. The average broadband color data are also reported on the spectra of the objects for which they were available (circles). 3. RESULTS AND DISCUSSION The reflectance spectra of the 12 objects, six TNOs and six Centaurs, are shown in Figures 1 and 2, respectively. The spectrum of 2001 PT 13 begins around 4700 Å, a longer wavelength than the others. This is due to a problem connected to the flat fields of that night, which presented a spot just around this wavelength that resulted in strange behavior of the spectra below 4700 Å. When possible, we have superposed photometric data obtained in the course of the same Large Programme (Boehnhardt et al. 2002; Barucci et al for 2001 PT 13 ), with the same instrumentation, so that we compare homogeneous data. As can be seen from Figures 1 and 2 (circles), there is always a good match between the spectra and the reflectances obtained from the photometric colors. Small differences could be due to some systematic errors in the observations but are more likely attributable to surface differences due to the rotation of the objects. We do not report the B color obtained from the photometry, because of the low signal-to-noise ratio of the B-filter Fig. 2. Same as Fig. 1, but for the Centaurs

4 No. 3, 2003 SPECTROSCOPY OF TNOs AND CENTAURS 1557 photometry (Boehnhardt et al. 2002). The B color is reported only for 2001 PT 13, because the object was brighter than the others and its B photometric observation is more accurate. All of the objects presented here have been investigated spectroscopically in the visible region for the first time, although for some of them photometric investigations by several groups do exist (e.g., Sheppard & Jewitt 2002; Barucci et al. 2001; Tegler & Romanishin 2000; Ortiz et al. 2002; Peixinho, Doressoundiram, & Romon-Martin 2002). The best-investigated object is 2000 EB 173, which has been observed photometrically in the visible and near-infrared and spectroscopically in the near-infrared (1 2.5 lm). The infrared spectrum of 2000 EB 173 obtained by Jewitt & Luu (2001) appears featureless, as does that of Brown, Blake, & Kessler (2000). The infrared spectrum from Licandro, Oliva, & Di Martino (2001) shows a quite strong and broad absorption band in the K region. The infrared spectrum of 1999 DE 9 shows several absorption features, at 1.4, 1.6, 2.0, and 2.25 lm (Jewitt & Luu 2001), suggesting the presence of water ice or minerals that incorporate OH in their crystalline structure. Jewitt & Luu found a good agreement with a mixture of Mauna Kea red cinder and 1% (by mass) of water ice (see also Clark 1981). The infrared spectrum of 2001 PT 13 obtained in the course of this Large Programme (Barucci et al. 2002) shows the possible presence of signatures of water ice and an inhomogeneous surface. The visible spectra presented here show the classical grayto-red behavior of the continuum and are almost all featureless, with the exception of 2000 GN 171 and 2000 EB 173, which present absorption features. The former spectrum exhibits a feature around 7250 Å, and the other presents two absorption bands centered respectively around 6000 and 7300 Å. These kinds of features are somewhat analogous to spectral signatures found on main-belt asteroids that are typical of aqueous alteration processes. In the main belt, this process seems to be connected to the first stages of evolution of our Sun, the T Tauri phase: strong heating would have melted ice in the interiors of the objects, and the water so formed would have reached the surface and interacted with the surface materials, thus producing hydrated minerals. Clearly, this process cannot be invoked at the TNOs distances, so the presence of these bands must be due to some other process that caused the aqueous alteration. Obviously, one might first ask whether these features are real or an artifact due to some technical problem or the reduction procedure. First of all, during the same night, other TNOs were observed (see Table 1) and none shows the presence of absorption features; it is hard to believe that technical problems would have affected only two objects. Second, the reduction of the data from this run was performed independently by two members of our group, and with different methods: one obtains the reflectance using the real solar analog by dividing the spectra of the objects by that of the solar analog; the other flux-calibrates the spectra and then divides them by a high-resolution synthetic solar spectrum degraded to the resolution of our observations by convolving the solar spectrum with the instrument response of the observational night. The result is the same with the two methods, and the differences between the two solar analogs (the real and the synthetic) are negligible. Moreover, the features present in the final spectrum of 2000 EB 173 and 2000 GN 171 are present in both spectra obtained for each object (which were reduced separately), and the same solar analog was used to obtain the reflectance of the other objects observed that night. The reality of the absorption features must be addressed through additional observations of these two objects. 7 Lederer & Vilas (2002) have presented work at the 34th meeting of the AAS Division for Planetary Sciences in which they show the first results of a search for aqueous alteration features on TNOs and Centaurs. The initial analysis indicates an absorption feature near 0.7 lm, typical of aqueous alteration, in the grayer objects. They say also that this is consistent with the absorption band detected near 0.7 lm in our spectra of 2000 EB 173 and 2000 GN 171. This confirms the need for further observations of these objects. We have determined the reflectance slopes S 0 of the observed objects over a wavelength range covering almost all of the spectrum of each object (Table 3). We find a wide range of spectral gradients, although the Centaurs, with the exception of 1999 OX 3, seem to be distributed in a region of lower S 0 than those of the observed TNOs. This is evident from Figure 3, in which we report the colors of the observed objects (open circles) as a function of perihelion distance. From the figure, it can be noted that there seems to be a tendency toward S 0 20 for objects with q < 20 AU. This trend appears to be confirmed if one compares the present data with the few others in the literature (Barucci et al for five Centaurs, Binzel 1992 and Fink et al for 5145 Pholus, and Boehnhardt et al for the other TNOs; for Chiron, we also consider the data of Lazzaro et 7 We have obtained such data very recently; they are being reduced, and additional observational time has been requested from ESO to observe these objects in the near-infrared. These data, if time should be allocated, will be discussed in a future paper. Fig. 3. Reflectance slope of the observed objects and of Centaurs and TNOs spectroscopically observed in the visible range as a function of perihelion distance: Centaurs (C), Plutinos (P), Cubewanos (Q), and scattereddisk objects (S). Open circles correspond to the objects in this work, filled circles to those taken from the literature. The S 0 error bars are reported for every object.

5 1558 LAZZARIN ET AL. al and Fitzsimmons et al. 1994), as also shown in Figure 3 ( filled circles). It can be noted that the Centaurs are mostly located in a region of lower S 0 than the TNOs, while some others (Nessus, Pholus, and 1999 OX 3 ) are very red and the Cubewanos appear to be more randomly distributed. Tegler & Romanishin (2000), in a survey of 34 Centaurs and KBOs, found that the Centaurs and KBOs were evenly split between red surfaces and gray surfaces for perihelion distances between 6 and 40 AU. Beyond 40 AU, the KBOs were predominantly red. Doressoundiram et al. (2002), analyzing 52 Centaurs and TNOs, find that objects with perihelion distances around and beyond 40 AU are mostly very red, while the classical objects with high eccentricity and inclination are preferentially gray or slightly red. Boehnhardt et al. (2002) have defined a sorting criterion for the reflectance of the TNOs and Centaurs: Bluish to gray objects, spectral gradient less than 10% per 100 nm; Moderately red, spectral gradient 10% 25% per 100 nm; Red, spectral gradient 25% 40% per 100 nm; and Very red, spectral gradient over 40% per 100 nm. Following this classification, the Centaurs in this work (S 0 min = 10.1% per 100 nm; S0 max = 23.7) belong to the moderately red class if we exclude 1999 OX 3, which belongs to the very red class (Table 3). However, it is not surprising that objects of the same dynamical class show very different reflectance slopes. Other Centaurs in fact have very red slopes, such as Pholus and Nessus. The three Plutinos belong to the moderately red or red class, the Cubewano to the very red class, and the scattered objects to the red class. Overall, the objects observed have S 0 within 10% 55.6% per 100 nm, spanning from moderately red to very red reflectance slopes. However, this result is only indicative, owing to the small number of objects investigated thus far. So, similarities or differences among the four dynamical classes of objects are not easily drawn from analysis of the present data set, and more data are necessary. The possible similarity among Plutinos, Centaurs, and scattered-disk objects found by Boehnhardt et al. (2002) is only very weakly confirmed by the present data set of visible spectra, which is not statistically significant. Thus, a clear conclusion regarding the S 0 distribution among Centaurs, Cubewanos, etc., is not available for the time being. Red colors could be due to carbon-rich compounds (e.g., organics and tholins) on the surfaces, and the differences found among TNOs might be due to compositional variation in the TNO population or to time-dependent irradiation processes. Collisions could also be responsible for color diversity, depending on the quantity of fresh materials from the interior revealed by the collision. As a result, red objects probably have not recently undergone a collision. Among Centaurs, color diversity and the lower values of S 0 could also be explained by cometary activity causing resurfacing. 4. CONCLUSION We have obtained visible spectra of 12 TNOs and Centaurs in the lm range. The obtained spectra show a wide range of types, which implies a wide range of surface composition or, also, different evolutionary paths, including cosmic irradiation, cometary activity, etc. The reflectance slopes of the Centaurs seem to confirm a distribution in a zone of lower values than the TNOs, while others have very red slopes. However, the data now available are too few for a clear statistical analysis, and a conclusion on the distribution of S 0 will require more data. We have detected absorption features in the spectra of two TNOs, the Plutinos 2000 EB 173 and 2000 GN 171. The features are located where aqueous alteration bands are usually present in asteroid spectra, but a clear origin of the bands on the TNO spectra is not easily determined from the present data, and a deeper investigation is necessary. Barucci, M. A., et al. 2002, A&A, 392, 335 Barucci, M. A., Fulchignoni, M., Birlan, M., Doressoundiram, A., Romon, J., & Boehnhardt, H. 2001, A&A, 371, 1150 Barucci, M. A., Lazzarin, M., & Tozzi, G. P. 1999, AJ, 117, 1929 Binzel, R. P. 1992, Icarus, 99, 238 Boehnhardt, H., et al. 2002, A&A, 395, , A&A, 378, 653 Brown, M. E., Blake, G. A., & Kessler, J. E. 2000, ApJ, 543, L163 Bus, S. J., A Hearn, M. F., Schleicher, D. G., & Bowell, E. 1991, Science, 251, 774 Clark, R. N. 1981, J. Geophys. Res., 86, 3074 Doressoundiram, A., Peixinho, N., de Bergh, C., Fornasier, S., Thébault, P., Barucci, M. 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