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Long-Lived Discs in T Associations: Pre-Main-Sequence Ages for Low-Mass Stars

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Long-Lived Discs in T Associations: Pre-Main-Sequence Ages for Low-Mass Stars Long-lived discs in T associations: Pre-main-sequence ages for low-mass stars Jon Morgan Rees Submitted by Jon Morgan Rees to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Physics, August, 2016. This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signed: . Jon Morgan Rees Date: . Abstract In this thesis, ages have been derived for 4 young clusters by fitting the pre-main-sequence stars with semi-empirical models in colour-magnitude diagrams. Combining these ages with the (consistent) set presented in previous work, the first robust evidence of increased circumstellar disc lifetimes in low-mass, low-density regions is obtained. To obtain this result, the following steps were necessary: • Semi-empirical model isochrones have been constructed in a number of rizJHK photomet- ric systems. These models overcome the issues typically seen in purely theoretical models in which the blue flux of low-mass stars is overestimated. These models are presented in a number of widely used filter sets for the first time, allowing for wider use with new clus- ters. Additionally the models constructed in previous filter sets have been refined using new observations. • To support the construction of these models, upper-main-sequence fitting is performed for 2 fiducial clusters, and it is demonstrated that the resulting age and distance measurements are consistent with other measures. • A new reduction process for data in the Blanco-DECam system is presented, and it is shown that the DECam photometric system is well characterised. • A photometric method for dereddening stars individually in regions of spatially variable extinction is presented, and applied to the young regions in this study. This method of photometric dereddening can be applied to large numbers of stars, greatly decreasing the time investment needed compared to spectroscopic methods. 2 The ages derived for the young clusters using the semi-empirical models are around a factor 2 older than typically assumed in the literature, which is in-line with that seen for the ages derived for other clusters using the same technique. By considering the disc fraction in these clusters as a function of age, it is shown that Taurus and Chamaeleon show a significant excess of discs compared to a set of massive, dense clusters of similar age. This is clear evidence that discs seem to survive longer in this low-mass, low-density region, giving crucial hints at different disc evolution in these regions. ρ-Oph is a low-mass region with a high stellar density, and so could be used to identify the dominant mechanism leading to these long-lived discs. However the presence of a similar disc excess in ρ-Oph is dependent on the assumed distance, which is currently poorly constrained, and so the dominant mechanism is still unclear. Contents 1 Introduction 14 1.1 The Birth of Low-Mass Stars . 14 1.1.1 The Interstellar Medium and Molecular Clouds . 14 1.1.2 Protostellar Evolution . 17 1.1.3 YSO Lifetimes . 19 1.2 The Pre-Main-Sequence . 21 1.2.1 Pre-main-sequence Evolution in the Hertzsprung-Russell Diagram . 22 1.2.2 Stellar Evolutionary Models . 25 1.2.3 Creating Bolometric Corrections From the Model Atmospheres . 27 1.3 The Colour-Magnitude Diagram . 29 1.3.1 τ2 Fitting . 32 1.3.2 The Lithium Depletion Boundary . 33 1.4 Identifying Pre-Main-Sequence stars . 34 1.5 Previous Work . 37 1.6 Summary . 37 2 Photometry in the Isaac Newton Telescope Wide-Field Camera System 39 2.1 Motivation . 39 2.2 Observations . 40 2.3 Data Reduction . 40 2.3.1 Deriving Linearity Coefficients . 42 2.3.2 Flat Fields . 42 2.3.3 Fringe Frames . 45 33 CONTENTS 4 2.3.4 Reduction Process . 46 2.3.5 Optimal Photometry . 49 2.3.6 Photometric Calibration . 50 2.3.7 Astrometric Calibration . 53 3 Constructing and Using Semi-empirical Models in the INT-WFC System 55 3.1 Motivation . 55 3.2 The Data . 56 3.3 The Models . 57 3.3.1 Reddening vectors . 58 3.4 Constructing the Semi-Empirical Models . 58 3.5 Determining the Age of NGC 2264 . 60 3.5.1 Introduction to NGC 2264 . 60 3.5.2 The Data . 64 3.5.3 Membership, Extinction and Distance of NGC 2264 . 64 3.5.4 Determining an Age . 64 3.5.5 Disc Fraction . 65 4 Semi-empirical Models in the SDSS, 2MASS and UKIDSS Systems 68 4.1 Motivation . 68 4.2 The Data . 69 4.2.1 SDSS . 69 4.2.2 2MASS . 69 4.2.3 UKIDSS . 70 4.2.4 INT-WFC Observations of Praesepe . 70 4.3 Praesepe - An Alternative Fiducial Cluster . 70 4.3.1 The Age and Distance of Praesepe . 71 4.3.2 The Models . 72 4.3.3 τ2 Fitting in Praesepe . 73 4.3.4 τ2 Fitting in the Pleiades . 75 4.4 Tuning the Models in the SDSS Photometric System . 75 4.5 Factors affecting the use of Praesepe in place of the Pleiades . 77 CONTENTS 5 4.5.1 Effects of enhanced magnetic activity . 80 4.5.2 CMD comparison of the Pleiades and Praesepe . 80 4.5.3 Effects of metallicity on semi-empirical BC-Teff relations . 82 4.5.4 Effects of metallicity on stellar interior models . 84 4.5.5 Effects of metallicity on ∆BC ....................... 84 4.6 Semi-Empirical Models in the UKIDSS System . 87 4.7 Semi-Empirical Models in the 2MASS System . 87 5 A Method of Photometric Dereddening from iZJH Data 91 5.1 Dereddenning - The i-Z, J-H diagram . 91 5.1.1 Reddening vectors . 92 5.1.2 iZJH Dereddening Methodology . 93 5.1.3 Bayesian fitting . 93 5.1.4 The Bayesian method vs χ2 ........................ 98 5.1.5 Robustness . 98 5.2 Testing Derived Reddenings Using IC348 . 102 6 The Age and Disc Fraction of Taurus 103 6.1 Motivation . 103 6.2 New Data . 104 6.2.1 Membership and Distance for Taurus . 104 6.3 Dereddedning the Taurus Members . 106 6.3.1 Comparison with Literature Extinctions . 106 6.4 Determining the Age of Taurus . 115 6.4.1 Age variations across the field . 116 6.4.2 Consistency . 117 6.5 Implications . 117 6.5.1 Disc fraction . 117 6.5.2 Mass function . 118 6.6 Conclusions . 121 7 Photometry in the Blanco DECam system 123 7.1 Motivation . 123 CONTENTS 6 7.2 Observations . 124 7.3 Reduction Process . 126 7.3.1 Bias, Flats and Fringing . 126 7.3.2 Searching for Objects . 127 7.3.3 Optimal Photometry . 128 7.3.4 Photometric Calibration . 128 7.4 Summary . 137 8 Blanco 1 - The Southern Pleiades 138 8.1 Motivation . 138 8.2 Blanco 1 . 139 8.2.1 The Age, Extinction and Distance of Blanco 1 . 139 8.2.2 Blanco 1 Memberships . 140 8.3 Constructing Semi-Empirical Models in the DECam System . 142 8.3.1 The Blanco DECam System vs the INT-WFC System . 142 9 The Age and Disc Lifetimes in Chamaeleon I and ρ-Ophiuchus 151 9.1 Motivation . 151 9.2 The Data, Memberships and Distance . 152 9.2.1 Chamaeleon I Membership and Distance . 153 9.2.2 ρ-Oph Membership and Distance . 154 9.3 Extinction Fitting for the DECam Clusters . 155 9.3.1 Extinction Fitting in Chamaeleon I . ..
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