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The Stellar Population of the Chamaeleon I Star-Forming

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The Stellar Population of the Chamaeleon I Star-Forming The Stellar Population of the Chamaeleon I Star-Forming Association Ouge He Master of Photonics Bachelor of Science (Physics) School of Physical, Environmental and Mathematical Sciences The University of New South Wales Canberra, ACT, 2600, Australia March 2013 0 Declaration ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed ……………………………………………........................... Date ……………………………………………........................... 1 Abstract The objective of this thesis is to present a study of the disk-age relationship of stars in the Chamaeleon I star-forming region, to enhance our understanding of pre-main sequence stellar disk evolution with stellar age, and to further the understanding of the early-development of solar-like stars. Chamaeleon I (Cha I) is a star-forming region located about 160 pc from Earth, containing over 100 pre-main sequence stars in a compact group. This thesis presents an investigation and discussion of Cha I with respect to two fundamental stellar properties: the presence of a stellar disk and age. A study of the spatial distribution of entities in Cha I, in particular a comparison of the north and south parts of the region, was done to investigate the order of the whole cluster formation. This study also provides an analysis of solar-like stars pre-main sequence properties, which will enhance understanding of the early development of solar-like stars. In the disk study, Wide Field Infrared Survey Explorer (WISE) data were used in colour- colour diagrams to determine which stars have disks, the effects on the disks with respect to reddening in the colour-colour diagram will be discussed. The disk study also revealed the effect of the disks on the Spectral Energy Distribution (SED) of the Cha I stars. Finally, the spatial distribution of Cha I disk stars was studied, and the relationship between the north and the south Cha I populations was analysed using the disk distribution. The ages of the Cha I stars found from pre-main sequence stellar evolutionary models were studied, and a comparison between the north and the south Cha I populations made. The age comparison included comparisons of the median ages and age distributions, and hypothesis testing of the age distributions, using different mass ranges. Additional tests between different pre-main sequence stellar evolutionary models are also presented to show that the age analysis 2 is limited to relative comparisons rather than absolute comparisons between the Cha I north and south populations. Finally, the link between fundamental stellar properties i.e. disk and age was examined, which included mass data measurements from the same pre-main sequence stellar evolutionary models. The main results of this thesis can be summarized as follows: First, the Cha I cluster appears to be composed of a north region and the south region, with the disk percentage in, 62% for the south and 66% for the north. A statistical test was then carried out to see whether the two clusters could be related. The result shows it is not possible to say whether the two regions are from the same parent cluster. As the presence of a disk reflects the stars’ and cluster’s evolutionary stage, it is also reasonable to offer a hypothesis that there is no outer trigger for the Cha I star formation. Second, the stellar evolution models from Siess et al. (2000) and Baraffe, Chabrier & Allarid (1998) were used to determine ages of Cha I stars. Using the Siess model, the median ages of the north and south regions are 3.5 and 3.25 Myr respectively. This age difference suggests that the north region is older, but with less age difference than suggested by the Baraffe 1998 model, which has the northern region 1.3 Myr older than the southern region, with median ages of the southern and northern groups are 2.5 Myr and 3.8 Myr, respectively. 3 Acknowledgements One of the joys of completion is to look over the journey past and remember all the friends and family who have helped and supported me along this long but fulfilling road. First and foremost I offer my sincerest gratitude to my supervisors, Professor Warrick Lawson and Dr Robert Smith, who have supported me though out my study with their patience and knowledge whilst allowing me the room to work in my own way. Without them this thesis would not have been completed or written. I would not have contemplated this road if not for my parents. I would like to thank my parents for their unconditional support, both financially and emotionally throughout my degree. In particular, the patience and understanding that was shown by my mother during her chemotherapy. 4 Contents Declaration ........................................................................................................................................................................... 1 Abstract ................................................................................................................................................................................. 2 Acknowledgements .......................................................................................................................................................... 4 Chapter 1 Introduction ................................................................................................................................................... 7 1.1Brief introduction to Chamaeleon I ............................................................................................................... 7 1.2 Pre-main sequence stars, star formation and evolutionary models for low mass stars ........ 9 1.3 Wide-field Infrared Survey Explorer (WISE).......................................................................................... 14 1.4 Statistical tool: Wilcoxon-Mann-Whitney Rank Sum Test ................................................................ 15 1.5 Outline and goal of the thesis ........................................................................................................................ 16 Chapter 2 Distribution of disks in Cha I ................................................................................................................. 18 2.1 Introduction ......................................................................................................................................................... 18 2.2 Data from WISE ................................................................................................................................................... 19 2.3 Disk analysis ......................................................................................................................................................... 25 Colour-colour diagrams ..................................................................................................................... 25 The effect of disks on the SED ......................................................................................................... 33 Spatial distribution of disks ............................................................................................................. 36 Differences between the north and south regions .................................................................. 38 Data tests of the north and south regions .................................................................................. 40 2.4 Discussion of disk analysis ............................................................................................................................. 42 Chapter 3 Age analysis .................................................................................................................................................. 44 3.1 Introduction ......................................................................................................................................................... 44 3.2 Data .......................................................................................................................................................................... 44 3.3 Models ..................................................................................................................................................................... 46 Baraffe 1998 model ............................................................................................................................. 47 Siess model ............................................................................................................................................. 51 3.4 Data analysis ........................................................................................................................................................ 55 Inter model analysis ........................................................................................................................... 55 5 Same group
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