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Photometric Colours of Centaurs

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Photometric Colours of Centaurs David Galvão Nunes Vaz de Mascarenhas PHOTOMETRIC COLOURS OF CENTAURS Dissertação no âmbito do Mestrado em Astrofísica e Instrumentação para o Espaço orientada pelo Doutor Nuno Vasco Munhoz Peixinho Miguel e apresentada ao Departamento de Física da Faculdade de Ciências e Tecnologia da Universidade de Coimbra. Fevereiro de 2020 Acknowledgements: This dissertation is, for me, the culmination of over two years of work and the result of a lifelong interest that grew into a passion early on which, ultimately, made me decide to alter my life’s trajectory after beginning my career as an IT engineer. I wished to dedicate myself to the frontier of space science fully. For me, there is no greater calling. There are countless people I would wish to thank for supporting me and enabling me to undertake this journey in many aspects; however, for the sake of brevity, I shall limit myself to those who had a direct impact on this dissertation. Firstly, I wish to thank my advisor, Nuno Peixinho, who has been nothing if not impeccable in his support throughout my time in Coimbra; his dedication to his students and his faith in their abilities should serve as an inspiration for many. His guidance in this dissertation has proven invaluable time and again and much of my introduction into the professional world of astronomical research wouldn’t have been possible without his infallible desire to include his students in all related activities and opportunities available. I also wish to thank the remaining members of the evaluation jury, for their time and availability. My parents and brother have been invaluable throughout my efforts and they deserve praise for the endless support and encouragement they each gave in their own way. My mother, in particular, seemed to have a limitless supply of patience and energy to help me achieve my goals. Andrey Solovov, Sérgio Gomes, Ana Vasconcelos and Cédric Pereira; classmates and friends – you have all not only helped with my work and my studies, but you have made the experience all that much more enjoyable. Ana Galvão. You have been an excellent friend since the day we met; you saw me through both the best and worst of these past two years, both academically and personally. It is something I shall never forget. Colm Dodd. Your friendship has been as important to me now as it has over this past decade. Your insights, suggestions and encouragement were invaluable not just for many aspects of this dissertation, but also for myself. Joana Marques, who has not only been a good, kind friend, but also the reason for which it was literally possible to hand in this dissertation on time, as well; if not for your keen memory and insight, the results of this work would have likely remained on hold for a year. Fernando Pinheiro, for the many insights into IRAF and the processes of data reduction, along with many hours of thoroughly enjoyable and fascinating conversation. Finally, I wish to thank the FCT (Fundação para a Ciência e Tecnologia) and CITEUC (Centro de Investigação da Terra e do Espaço da Universidade de Coimbra) for the research scholarship, “Lápitas – Bolsa de Licenciado”, which gave rise to this overall dissertation. iii iv Abstract: The objective of this dissertation is the observational, astronomical study of the surface reflectivity of three small bodies of the solar system of the Centaur family – 2010 FH92, 2011 MM4 and 2013 PH44 –, combined with other Centaurs available in the literature, through the photometric analysis of their colours, i.e. the measurement of the difference of the reflectivity in different wavelengths, and comparison with the previously-measured members of this population from the literature. This was achieved through the data reduction of astronomical optical observations taken from the 3.5 meter and 2.2 meter telescopes at the Hispano- Germanic Calar Alto Observatory Complex (CAHA) at Calar Alto, Spain. These observations were originally conducted in July and September of 2014 (for the 3.5 meter telescope) and January, February and October of 2014 (for the 2.2 meter telescope). This was the first astronomical photometric study performed of any of these three Centaurs. The measured B-R colours – i.e. the differences between the magnitudes measured on the B and R filters of the Johnson system – were 0.89±0.38 for 2010 FH92, 0.15±39.82 for 2011 MM4 and 1.68±0.42 for 2013 PH44. The signal of object 2011 MM4 was too low for photometric measurements. The colour error bar for 2010 FH92 was, unfortunately and evidently, too high to provide a useful value. 2013 PH44 presents an ultrared colour – i.e. a colour that is much higher than the solar B-R colour – even though it has observational errors in that colour’s determination that do not permit that assertion to be definitive. The Centaur population colour bimodality hypothesis was run through Hartigan’s statistical dip test and, contrary to the conclusions from 2012 and 2013, failed to reject the unimodality hypothesis with our sample, given that the confidence level obtained from the test was 86.41% (i.e. p-value = 0.01359). In spite of the lack of statistical confirmation for the bimodality with a confidence level over at least 95%, the Wilcoxon rank-sum test was used to test the non-existence of different median values of orbital inclinations for the redder Centaurs and the less red Centaurs, rejecting the null hypothesis with a 97.5% confidence level. Extra plot comparisons with other parameters, such as the objects’ perihelia, semimajor axes, Tisserand parameters, B-R colours, orbital inclinations and orbital eccentricities were conducted. No other new conclusions relative to the distributions of the Centaur population were obtained. Keywords: Centaurs; Photometry; Small Solar System Bodies; Tisserand Parameter; Colours. v vi Resumo: O objectivo desta dissertação é o estudo astronómico, observacional, da reflectividade das superfícies de três pequenos corpos do sistema solar da família dos Centauros – 2010 FH92, 2011 MM4 e 2013 PH44 –, em combinação com Centauros disponíveis na literatura, através da análise fotométrica das cores dos mesmos, i.e. da medição da diferença de reflectividades em diferentes comprimentos de onda, e comparações com outros membros desta população previamente estudados na literatura. Isto foi possível com a redução de dados de observações astronómicas ópticas efectuadas com os telescópios de 3.5 metros e 2.2 metros do Hispano-Germanic Calar Alto Observatory Complex (CAHA) em Calar Alto, na Espanha. Estas observações foram feitas em Julho e Setembro de 2014 (para o telescópio de 3.5 metros) e Janeiro, Fevereiro e Outubro de 2014 (para o telescópio de 2.2 metros). Este foi o primeiro estudo fotométrico astronómicos efectuado para qualquer um destes três Centauros. As cores B-R medidas – i.e. as diferenças entre as magnitudes medidas no filtro B e no filtro R do sistema Johnson – foram 0.89 ± 0.38 para o 2010 FH92, 0.15 ± 39.82 para o 2011 MM4 e 1.68 ± 0.42 para o 2013 PH44. O sinal do objecto 2011 MM4 foi demasiado baixo para permitir medições fotométricas. A barra de erros da cor do objecto 2010 FH92 foi, lamentavelmente e evidentemente, demasiado elevada para que nos fornecesse um valor útil. O objecto 2013 PH44 apresente uma cor ultravermelha – i.e. uma cor bastante mais alta que a cor B-R solar – embora possua erros observacionais na determinação dessa cor que não permitem que essa afirmação seja definitiva. A hipótese da bimodalidade das cores B-R da população dos Centauros foi testada através do teste estatístico dip test de Hartigan e, contrariamente às conclusões de 2012 e 2013, com a nossa amostra não se rejeita a hipótese de unimodalidade, dado o nível de confiança obtido com o nosso teste ser de apenas 86,41% (i.e. p.valor = 0.01359). Apesar da não confirmação estatística da bimodalidade com um nível de confiança superior a pelo menos 95%, foi testada, com o teste de rank-sum de Wilcoxon, a não existência de diferentes valores médios de inclinações orbitais dos Centauros mais vermelhos e dos menos vermelhos, rejeitando-se a hipótese nula com um nível de confiança de 97.5%. Gráficos para comparações suplementares foram criados com outros parâmetros, como os periélios, semi- eixos maiores, parâmetros de Tisserand, cores B-R, inclinações orbitais e excentricidades orbitais. Não se obtiveram novas conclusões relativamente às distribuições da população dos Centauros. Palavras-chave: Centauros; Fotometria; Pequenos Corpos do Sistema Solar; Parâmetro de Tisserand; Cores. vii viii Table of Contents 1 Introduction ...................................................................................................................... 1 1.1 Some Definitions and Concepts ................................................................................... 1 1.1.1 Some notes regarding terminology used in this dissertation .................................. 1 1.1.2 Colours ................................................................................................................. 2 1.1.3 Tisserand’s Parameter .......................................................................................... 2 1.2 Small Solar System Objects ........................................................................................ 3 1.2.1 Centaurs ............................................................................................................... 3 1.2.2 Comets ................................................................................................................. 5 1.2.3 Trojans .................................................................................................................
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