Study of trans-neptunian objects using photometric techniques and numerical simulations

Resumen

In the eighties, our Solar System was composed by nine planets with their satellites, an asteroid belt between the orbit of Mars and Jupiter, the Trojan asteroids at the Jupiter's Lagrange points, and the comets. Comets and asteroids were classified as "minor planets". Nevertheless, the existence of a belt composed by planetesimals beyond Neptune's orbit was suspected. The observational confirmation of the existence of a reservoir of small bodies (the Trans-Neptunian Objects (TNOs)) at the edges of the Solar System needed more than sixty years after the discovery of Pluto in 1930. In around twenty years since the discovery of 1992 QB1 by Jewitt and Luu (1993), the Trans-Neptunian belt moved from a speculation or a theoretical postulate (Leonard, 1930; Edgeworth, 1943; Edgeworth, 1949; Kuiper, 1951; Whipple, 1964; Fernandez, 1980) to be the most populated region of the Solar System. Currently, it is estimated that the Trans-Neptunian belt within the distance 30-50 AU from the Sun comprises approximately 100,000 objects with diameters of around 100 km (Trujillo, Jewitt and Luu, 2001). The discovery of a multitude of objects with very similar orbits to Pluto's orbit and also the fact that a mixture of ice/rock seems common to the majority of TNOs, implied that Pluto appeared to be not unique but one object of many more. Then, with the discovery of some TNOs with similar size to that of Pluto (Brown et al., 2006a; Sicardy et al., 2011), the definition of the term "planet" needed to be reviewed. The General Assembly of the International Astronomical Union (IAU) in 2006 changed the definition of planet and introduced a new type of bodies: the dwarf planets. According to the IAU definition: - A planet is a celestial body that: - is in orbit around the Sun. -has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape. - has cleared the neighborhood around its orbit. - A dwarf planet is a celestial body that: - is in orbit around the Sun. -has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape. -has not cleared the neighborhood around its orbit. -and is not a satellite. - All other objects, except satellites, orbiting the Sun shall be referred to collectively as small Solar System bodies. In conclusion, currently Pluto is not a planet but a dwarf planet and there are already three more bodies in the Trans-Neptunian region that have been officially recognized as dwarf planets by the IAU. To date, around 1400 TNOs have been discovered and can be classified in different dynamical groups: the classical objects, the scattered disk objects, the detached disk objects, and the resonant objects. The main dynamical classes in the Trans-Neptunian belt are well known but their definitions can vary. To date, mostly two main classifications are used: i) the Deep Ecliptic Survey (DES) classification from Elliot et al. (2005), and ii) the Gladman, Marsden and Vanlaerhoven (2008) classification. There are several populations that do not qualify as trans-Neptunian objects mainly because their orbits are not beyond Neptune's orbit, but they are associated with the TNOs. This includes the centaurs, the short-period comets, and even the irregular satellites of the giant planets. However, our knowledge about the Trans-Neptunian belt is very limited. The general idea about TNOs is that they are composed by a mixture of rock and ice very similar to the comets composition. Spectroscopically, ices of water, methane, nitrogen, carbon monoxide, etc have been detected (Licandro et al., 2006a; Licandro et al., 2006b; Licandro et al., 2006c; Trujillo et al., 2005; Trujillo et al., 2007; Carry et al., 2011; Barucci et al., 2011; Brown, Burgasser and Fraser, 2011). Most of the TNOs are inactive; in other words, the ice on their surface is not sublimated, mainly because of their distances to the Sun. Some objects have been sent between the orbits of Jupiter and Neptune because of collisions, close encounters in the belt, or planetary encounters. Such objects, that are dynamically evolved and on unstable orbits, are called centaurs (Gladman, Marsden and Vanlaerhoven, 2008). Therefore, the centaurs are not Trans-Neptunian Objects because their orbits are not beyond Neptune¿s orbit, but they are an associated population to the TNOs. Centaurs can be ejected to very large perihelia or sent to the inner parts of the Solar System as short-period comets. Due to their distances from the Sun, the TNOs are considered the least evolved bodies of the Solar System and therefore, their studies provide us with information about the composition and properties of the primitive solar nebula. The study of these bodies provide us clues about the origin and the evolution of the early Solar System. In addition, the Trans-Neptunian belt provides a natural connection to the study of the protoplanetary disks observed around some stars. The main objective of this thesis was to determine and analyze, for a large sample of objects, the ranges of variability, their rotational periods, as well as other physical parameters that can be derived from short-term variability. The aim was to derive physical parameters such as axis ratios, phase coefficients, albedos, density, porosity, etc., for a good sample of TNOs and centaurs because only few studies were published prior to this thesis. Short-term variability studies allow us to determine the rotational, dynamical and physical evolution of these objects. But a lot of observing time is required to provide reliable short-term variability studies. In addition, it is thought that large objects are less colisionally evolved, so they probably retain the distribution of the primitive angular momentum of the early stages of the Solar System (Davis and Farinella, 1997). At the beginning of this PhD, the sample of objects with measured rotational periods and lightcurve amplitudes was very limited. Only around 50 objects with short-term variability were reported and many published rotational periods were uncertain or erroneous. In addition, Sheppard, Lacerda and Ortiz (2008) noticed an observational bias towards large amplitudes and short rotational periods. Increasing the sample size, improving rotational periods, lightcurves, and trying to overcome some observational biases were some of the objectives of this study. On the other hand, binary objects required a special treatment, with the objective to derive relevant physical parameters, some of them from the tidal effects in such systems. Another motivation to carry out photometry observations was the support to the Herschel Space Observatory (HSO) key project "TNOs are cool!". HSO is a mission of the European Space Agency (ESA) and of the National Aeronautics and Space Administration (NASA). "TNOs are cool!" is a key-project of HSO dedicated to the observations of thermal emission from 130 TNOs and centaurs in around 400h of observing time (Müller et al., 2009). This key project was the largest key-project of HSO and required a large international effort with more than 40 team members, which include several researchers from IAA's solar system department. For the analysis and interpretation of the thermal data from HSO, thermal models or thermophysical models (Müller et al., 2009; Vilenius et al., 2012; Mommert et al., 2012) are required. To derive diameters and albedos, all these models require input parameters such as absolute magnitudes and spin periods or constraints on them, all of which require ground based photometry. As a result of early findings during the project, a new model from a numerical point of view to explain the formation of the Haumea system is developed. By extension, this model is also able to explain the formation of some binary/multiple systems, and even the formation of unbound pairs of TNOs that was not considered as a possibility in the Trans-Neptunian belt. Haumea is a large object with very peculiar characteristics. Several models have been proposed by different authors to explain the formation of this object and its "family" as well as the peculiar characteristics of Haumea, but all of them have some inconsistencies. This PhD thesis is divided into six parts: - Part I provides general background to the reader and discusses some basic issues. The Trans-Neptunians Objects and the associated populations are described (Chapters I and II). - Part II is dedicated to data calibration and reduction, the lightcurve physics and the description of the observational runs and instrumentation used (Chapters III, IV, and V). - In part III, an exhaustive summary about short-term variability for all objects studied during this investigation is presented. For each object, lightcurve, rotational period and amplitude variation are reported. Some physical properties are derived from the lightcurves and exhaustive statistical studies are reported (Chapters VI and VII). - In part IV, several physical properties of binary systems based on their short-term variability studies are presented. An exhaustive study based on a search of correlations/anti-correlations between orbital and physical parameters is proposed. (Chapter VIII). - Part V presents a new formation model of Haumea system. A presentation of Haumea and its family characteristics, as well as a description of all formation models proposed to date, is presented. A summary about numerical simulations of a new model is reported. Such numerical simulations give us alternatives to explain the creation of the Haumea system (Chapter IX). - Finally, in part VI, the conclusions of this thesis are summarized (Chapter X). Most of the initial objectives have been achieved during this thesis and even some unforeseen discoveries resulted in other interesting scientific results. Most of the work presented in this manuscript has been published in international scientific journals. Here is reported some of the research of these papers as well as unpublished material.