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Photometric and Spectroscopic Studies of Small Solar System Bodies and the IMPACTON Project

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Photometric and Spectroscopic Studies of Small Solar System Bodies and the IMPACTON Project Asociaci´on Argentina de Astronom´ıa BAAA, Vol. 53, 2010 J.J. Clari´a, M.V. Alonso, A.E. Piatti & F.A. Bareilles, eds. TRABAJO INVITADO Photometric and spectroscopic studies of small Solar System bodies and the IMPACTON project D. Lazzaro1 (1) Observat´orio Nacional, MCT, Rio de Janeiro, Brasil Abstract. The correct physical characterization of small Solar System bodies is of outmost importance in trying to set constraints on their for- mation, origin and evolution. This characterization, however, depends on several observational parameters, which are not easy to be obtained, nor to be interpreted. In this paper we will describe recent results obtained by our group using detectors in diverse wavelengths at small to large sized telescopes. We will also describe the IMPACTON project which is devoted to install and operate a 1-meter telescope dedicated to perform the monitoring of the orbit and to study the physical properties of small bodies in near-Earth orbits. Resumen. Una caracterizaci´on f´ısica detallada de los peque˜nos cuer- pos del Sistema Solar es de suma importancia para tratar de establecer restricciones en su formaci´on, origen y evoluci´on. No obstante, esta car- acterizaci´on depende de varios par´ametros observacionales, que no son f´aciles de obtener ni de interpretar. En este trabajo describimos los resul- tados recientes obtenidos por nuestro grupo, en el cual se usaron detec- tores en diferentes longitudes de onda operando en grandes y peque˜nos telescopios. Tambi´en describimos el proyecto IMPACTON, cuyo objetivo es llevar a cabo la instalaci´on y operaci´on de un telescopio de 1-m, el cual est´adedicado tanto al monitoreo como al estudio de las orbitas y de las propiedades f´ısicas de peque˜nos cuerpos en ´orbitas pr´oximas a la de la Tierra. 1. Introduction The asteroids and the comets are classically considered to be the small bodies of the Solar System. The former are mostly located in the region between Mars and the Earth, although there exist many objects which come close and even intersect the inner planets orbits. The comets, on the other hand, are mostly located in the outer part of the Solar System, having very eccentric orbits which can bring them close to the Sun. It is believed that both populations lie close to their formation location being this the cause for their very diverse composition: silicate and iron-rich the asteroids, and icy-rich the comets. In-between these two populations, in terms of physical properties and location, many and diverse populations of small bodies are being discovered in the last years, among which 315 316 D. Lazzaro we can mention the main belt comets (MBC), the asteroids in cometary orbits (ACOs), the Centaurs and the Trans-Neptunian objects (TNOs). It is believed that most of these small bodies are remnants of the Solar System formation. We recall that the most accepted model considers that the Solar System formed from the collapse of a rotating nebula of gas and dust. As the material was accreting in the center, forming the pro-Sun, the dust material settled in the disk. In this protoplanetary disc the material in the inner part is heated by the proto-Sun, therefore, ices and gas cannot condense, just silicates and iron compounds. The external part of the disc, on the other hand, remains cold and ices and gases condense, along with silicates and iron compounds. This segregation of material is easily recognized among the planets: rocky and gas giant, respectively, in the inner and external part of the Solar System. In the disk the grains begin to collide. Whether the collision is at low or high velocity the outcome is quite different: accretion of the material in larger bodies or fragmentation. Since Jupiter grows very rapidly this causes orbital perturbations to excite the bodies in its neighborhood increasing their relative velocities. In the asteroidal region, therefore, the fragmentation regime starts and the accretion is terminated. By this process mass is removed from the region and what remains are the asteroids. Beyond Jupiter, on the other hand, the proto-planets grow nearly at the same rate and the particles form the outer planets. However, since the mass decreases outward in the disk only smaller and smaller objects are formed: the Trans-Neptunian objects. The scattered objects in all this process finally form the Oort cloud. From the above it emerges the importance in studying the astrophysical properties of the small bodies since they must retain much information about the early processes in the Solar System formation and can, thus, confirm as well as set constraints on the formation models. 2. Astrophysical Properties Among the physical properties of small Solar System objects which can be ob- tained from astrophysical observations, in conjuction or not with models, are the size, the form, the rotation state, the composition and the internal structure. Due to the large number of these bodies we can study these properties individ- ually or in a collective way trying to understand the formation and evolution of the diverse populations. The astrophysical properties can be obtained from diverse kind of observa- tions, the most traditional being broadband photometry in diverse wavelengths. In the visible, 0.4 0.9 µm, and near-infrared, 0.8 2.5 µm, the observation in one band can give− the magnitude and brightness− variations of an object, constraining its size, its rotational properties and, if present, its cometary activ- ity. Broadband photometry in several bands allows the determination of colors which give constraints on the surface composition. Broadband photometry in the mid-infrared, 8 23 µm, when adjusted to models, gives the thermal flux of an object and the− albedo. It is important to mention that the albedo is one of the most fundamental physical properties of an atmosphereless body since −0.2H it allows to obtain its size through the relation Dkm = (1329/√pV )10 V , Small Solar System Bodies: photometry and spectroscopy 317 where Dkm is the diameter in km, pV is the visual albedo and HV the visual absolute magnitude. The surface composition and mineralogy of small Solar System objects can be derived from low resolution spectroscopy in the visible and near-infrared (VNIR), through the comparison with spectra of minerals and meteorites ob- tained in laboratory. Low resolution spectroscopy, up to nearly 1 µm, has been used to derive taxonomies which give just an indication of the diverse surface properties of the bodies while the mineralogy is only obtained, and just in special cases, with spectra up to 2.5 µm. In what follows we will describe some recent results obtained by our group on individual and collective astrophysical properties of small Solar System bodies using broadband photometry and low resolution spectroscopy at diverse obser- vatories and telescopes. 2.1. Individual Cometary activity (2060) Chiron was discovered in 1987, at 17 AU, being the most distant asteroid known at that time. Staring in 1989, when the object was at 13 AU, it began to present cometary activity. Considering that an activity very distant from the Sun is somewhat unusual, we started a photometric monitoring of this object as it was approaching its first perihelion after discovery, to be reached 1996 (Lazzaro et al. 1996, Lazzaro et al. 1997, Duffard et al. 2002). CCD observation were performed using the following telescopes at the di- verse observatories: 0.6 m and 1.5 m at the Observat´orio do Pico dos Dias (Brazil), 1.54 m at the Estaci´on Astrof´ısica de Bosque Alegre (Argentina), 1.6 m at the Observatoire de Haute-Provence (France) and 2.15 m at the Observatorio El Leoncito (Argentina). Most of the broadband photometry was obtained using the Johnson R filter, although some observations were made also in the V filter. The analysis of the data showed that the brightness of (2060) Chiron reached a minimum value in 1999, just after perihelion and began increasing again in 2000. This data was the first to indicate that Chiron was starting a new out- burst of activity which would be compatible with sporadic cometary behavior, not related to heliocentric distance (Duffard et al. 2002). Surface composition Asteroids and comets have been considered as members of two distinct populations which could be dynamically separated by their Tis- serand invariant (T ). This is an integral of motion of the restricted problem Sun- 2 Jupiter-object, defined by the relation T = (aJ /a)+2cos(I)sqrt((a/aJ )(1 e )), − where aJ and a are the semi-major axis of the orbits of Jupiter and the object, respectively, while e and I are the eccentricity and inclination of the object’ s orbit. Kresak (1979) noted that most comets have T smaller than 3 while as- teroids have T > 3. In recent years, however, several asteroids have been found to have T < 3 without any sign of cometary activity. Among these is asteroid (5201) Ferraz-Mello which since its discovery has been suspected to be an extinct Jupiter family comet. In order to put constraints on the possible origin of this asteroid we decided to perform its spectrophotometric characterization. Photometric observations of the asteroid (5201) Ferraz-Mello with the g, r, i, and z filters of the SDSS system were acquired with the SOAR Optical Imager (SOI) mounted on the 4 m SOAR telescope on Serro Pachon (Chile), 318 D. Lazzaro during service mode observations. To obtain a taxonomic classification, the SDSS colors ((g r), (r i), and (r z) were converted to flux by removing the solar component− and the− resulting reflectance− was normalized on the r band.
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