A&A 541, A94 (2012) Astronomy DOI: 10.1051/0004-6361/201118743 & c ESO 2012 Astrophysics “TNOs are Cool”: A survey of the trans-Neptunian region VI. Herschel/PACS observations and thermal modeling of 19 classical Kuiper belt objects E. Vilenius1,C.Kiss2, M. Mommert3,T.Müller1, P. Santos-Sanz4,A.Pal2, J. Stansberry5, M. Mueller6,7, N. Peixinho8,9, S. Fornasier4,10,E.Lellouch4, A. Delsanti11, A. Thirouin12, J. L. Ortiz12,R.Duffard12,D.Perna13,14, N. Szalai2, S. Protopapa15,F.Henry4,D.Hestroffer16,M.Rengel17, E. Dotto13, and P. Hartogh17 1 Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany e-mail: [email protected] 2 Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, PO Box 67, Hungary 3 Deutsches Zentrum für Luft- und Raumfahrt e.V., Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany 4 LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, France 5 Stewart Observatory, The University of Arizona, Tucson AZ 85721, USA 6 SRON LEA/HIFI ICC, Postbus 800, 9700AV Groningen, The Netherlands 7 UNS-CNRS-Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 04, France 8 Center for Geophysics of the University of Coimbra, Av. Dr. Dias da Silva, 3000-134 Coimbra, Portugal 9 Astronomical Observatory of the University of Coimbra, Almas de Freire, 3040-04 Coimbra, Portugal 10 Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France 11 Laboratoire d’Astrophysique de Marseille, CNRS & Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France 12 Instituto de Astrofísica de Andalucía (CSIC), Camino Bajo de Huétor 50, 18008 Granada, Spain 13 INAF – Osservatorio Astronomico di Roma, via di Frascati, 33, 00040 Monte Porzio Catone, Italy 14 INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy 15 University of Maryland, College Park, MD 20742, USA 16 IMCCE, Observatoire de Paris, 77 Av. Denfert-Rochereau, 75014 Paris, France 17 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany Received 25 December 2011 / Accepted 6 March 2012 ABSTRACT Context. Trans-Neptunian objects (TNO) represent the leftovers of the formation of the solar system. Their physical properties provide constraints to the models of formation and evolution of the various dynamical classes of objects in the outer solar system. Aims. Based on a sample of 19 classical TNOs we determine radiometric sizes, geometric albedos and beaming parameters. Our sample is composed of both dynamically hot and cold classicals. We study the correlations of diameter and albedo of these two subsamples with each other and with orbital parameters, spectral slopes and colors. Methods. We have done three-band photometric observations with Herschel/PACS and we use a consistent method for data reduction and aperture photometry of this sample to obtain monochromatic flux densities at 70.0, 100.0 and 160.0 μm. Additionally, we use Spitzer/MIPS flux densities at 23.68 and 71.42 μm when available, and we present new Spitzer flux densities of eight targets. We derive diameters and albedos with the near-Earth asteroid thermal model (NEATM). As auxiliary data we use reexamined absolute visual magnitudes from the literature and data bases, part of which have been obtained by ground based programs in support of our Herschel key program. Results. We have determined for the first time radiometric sizes and albedos of eight classical TNOs, and refined previous size and albedo estimates or limits of 11 other classicals. The new size estimates of 2002 MS4 and 120347 Salacia indicate that they are among the 10 largest TNOs known. Our new results confirm the recent findings that there are very diverse albedos among the classical TNOs and that cold classicals possess a high average albedo (0.17 ± 0.04). Diameters of classical TNOs strongly correlate with orbital inclination in our sample. We also determine the bulk densities of six binary TNOs. Key words. Kuiper belt: general – infrared: planetary systems – techniques: photometric 1. Introduction belt objects (KBO), represent the leftovers from the formation ff period of the outer solar system (Morbidelli et al. 2008), and The physical properties of small solar system bodies o er con- they are analogues to the parent bodies of dust in debris disks straints on theories of the formation and evolution of the plan- around other stars (Wyatt 2008; Moro-Martín et al. 2008,and ets. Trans-Neptunian objects (TNO), also known as Kuiper references therein). Herschel is an ESA space observatory with science instruments In addition to Pluto more than 1400 TNOs have been discov- provided by European-led Principal Investigator consortia and with im- ered since the first KBO in 1992 (Jewitt & Luu 1993), and the portant participation from NASA. current discovery rate is 10 to 40 new TNOs/year. The dynamical Article published by EDP Sciences A94, page 1 of 17 A&A 541, A94 (2012) classification is based on the current short-term dynamics. We distributions (SEDs) close to the thermal peak. The new esti- use the classification of Gladman (Gladman et al. 2008,10Myr mates of the 19 targets are based on observations performed time-scale): classical TNOs are those non-resonant TNOs which with the ESA Herschel Space Observatory (Pilbratt et al. 2010) do not belong to any other TNO class. The classical TNOs and its Photodetector Array Camera and Spectrometer (PACS; are further divided into the main classical belt, the inner belt Poglitsch et al. 2010). Other Herschel results for TNOs have (a < 39.4 AU) and the outer belt (a > 48.4 AU). The eccentricity been presented by Müller et al. (2010), Lellouch et al. (2010) limit for classicals is e < 0.24, beyond which targets are classi- and Lim et al. (2010). New estimates of 18 Plutinos are pre- fied as detached or scattered objects. The classification scheme sented in Mommert et al. (2012) and of 15 scattered disc and of the Deep Eplictic Survey team (DES, Elliot et al. 2005) differs detached objects in Santos-Sanz et al. (2012). from the Gladman system in terms of the boundary of classical This paper is organized in the following way. We describe and scattered objects, which do not show a clear demarcation in our target sample in Sect. 2.1, Herschel observations in Sect. 2.2 their orbital parameters. Some of the classicals in the Gladman and Herschel data reduction in Sect. 2.3. New or re-analyzed system are scattered-near or scattered-extended in the DES sys- flux densities from Spitzer are presented in Sect. 2.4. As auxil- tem. Another division is made in the inclination/eccentricity iary data we use absolute V-band magnitudes (Sect. 2.5), which space. Although there is no dynamical separation, there seems we have adopted from other works or data bases taking into ac- to be two distinct but partly overlapping inclination distribu- count the factors relevant to their uncertainty estimates. Thermal tions with the low-i “cold” classicals, limited to the main classi- modeling is described in Sect. 3 and the results for individual cal belt, showing different average albedo (Grundy et al. 2005; targets in Sect. 4. In Sect. 5 we discuss sample properties of our Brucker et al. 2009), color (Trujillo & Brown 2002), luminos- sample and of all classicals with radiometric diameters and ge- ity function (Fraser et al. 2010), and frequency of binary sys- ometric albedos as well as correlations (Sect. 5.1) and the bulk tems (Noll et al. 2008) than the high-i “hot” classicals, which densities of binaries (Sect. 5.2). Finally, the conclusions are in has a wider inclination distribution. Furthermore, models based Sect. 6. on recent surveys suggest that there is considerable sub-structure within the main classical belt (Petit et al. 2011). To explain these differences more quantitative data on physical size and surface 2. Observations and data reduction composition are needed. The physical characterization of TNOs has been limited by Our sample of 19 TNOs has been observed as part of the their large distance and relatively small sizes. Accurate albedos Herschel key program “TNOs are Cool” (Müller et al. 2009) help to correctly interpret spectra and are needed to find correla- mainly between February and November 2010 by the photome- tions in the albedo-size-color-orbital parameters space that trace try sub-instrument of PACS in the wavelength range 60–210 μm. dynamical and collisional history. The determination of the size frequency distribution (SFD) of TNOs provides one constraint 2.1. Target sample to formation models and gives the total mass. The SFD of large The target sample consists of both dynamically cold and hot bodies is dominated by accretion processes and they hold in- ◦ formation about the angular momentum of the pre-solar nebula classicals (Table 1). We use a cut-off limit of i = 4.5 in illustrat- ing the two subsamples. Another typical value used in the liter- whereas bodies smaller than 50 to 100 km are the result of col- ◦ lisional evolution (Petit et al. 2008). The SFD can be estimated ature is i = 5 . The inclination limit is lower for large objects via the luminosity function (LF), but this size distribution also (Petit et al. 2011) which have a higher probability of belong- depends on assumptions made about surface properties such as ing to the hot population. Targets 119951 (2002 KX14), 120181 albedo. Consequently, ambiguities in the size distributions de- (2003 UR292) and 78799 (2002 XW93) are in the inner classical rived from the LFs of various dynamical classes are one sig- belt and are therefore considered to belong to the low-inclination tail of the hot population.