Characterising the Young Sco-Cen Association By Aaron Rizzuto A thesis submitted to Macquarie University for the degree of Doctor of Philosophy Department of Physics and Astronomy March 2015 ii c Aaron Rizzuto, 2015. Typeset in LATEX 2". iii Except where acknowledged in the customary manner, the material presented in this thesis is, to the best of my knowledge, original and has not been submitted in whole or part for a degree in any university. Aaron Rizzuto iv Acknowledgements Firstly, I would like to express enormous gratitude to my PhD supervisor Mike Ireland, for providing just the right balance of direction and freedom to allow me to develop my own ideas while not losing track of the goals we had planned, for providing me with the opportunity to be involved in international collaborations, and for always treating me like a friend and colleague rather than a student. I would also like to thank Gordon Robertson, for providing carefully thought-out comments to reports and papers I had written and for his encouraging words, and Dan Zucker, for lots of proof reading. Thanks to the SUSI group members for assistance in observing and engineering for SUSI. A particular mention to Yitping Kok, for our interesting nights tandem observing. I also thank our Adam Kraus for our various discussions which helped improve this thesis. Thanks to fellow PhD students and postdocs, including Niyas Alikutty, Dimitri Douchin, Izabella Spelaniak, Joao Bento, and Carlos Bacigalupo for providing good conversation and company, and thanks to Bjorn Sturmberg for the climbing excursions. Thank you to my family, including my mother who set me on this path that I have taken and supported me along the way, and to my brothers David and Michael who always provided encouragement and showed pride in what I was doing. Finally I would like to thank my wife Vaness, who has provided tremendous support over the past two years. Thank you for always putting a smile on my face. - Aaron C. Rizzuto v vi Acknowledgements Publications • Rizzuto, A. C., Ireland, M. J., Zucker, D. B.,Wise Circumstellar Disks in the Young Sco-Cen Association. Monthly Notices of the Royal Astronomical Society: Letters, Volume 421, Issue 1, pp. L97-L101. (2012) • Rizzuto, A. C., Ireland, M. J., Robertson, J. G., Kok, Y.; Tuthill, P. G., Warrington, B. A., Haubois, X., Tango, W. J.; Norris, B., ten Brummelaar, T., Kraus, A. L., Jacob, A., Laliberte-Houdeville, C., Long-Baseline Interferometric Multiplicity Survey of the Sco-Cen OB Association. Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 2, p.1694-1707. (2013) • Kok, Y., Ireland, M. J., Tuthill, P. G.; Robertson, J. G., Warrington, B. A., Rizzuto, A. C., Tango, W. J., Phase-Referenced Interferometry and Narrow- Angle Astrometry with SUSI Journal of Astronomical Instrumentation, Volume 2, Issue 2, id. 1340011. (2013) • Kok, Y., Ireland, M. J., Rizzuto, A. C., Tuthill, P. G.; Robertson, J. G., War- rington, B. A., Tango, W. J., Alternative approach to precision narrow-angle as- trometry for Antarctic long baseline interferometry, SPIE Astronomical Tele- scopes and Instrumentation conference, June 2014, Paper ID 9146-103. • Rizzuto, A. C., Ireland, M. J., Kraus, A. L., New Pre-main-Sequence Stars in the Upper Scorpius Subgroup of Sco-Cen, Monthly Notices of the Royal Astronomical Society, in press. (2015) vii viii Publications Abstract The young Sco-Cen association provides a unique astrophysical laboratory for the study of many different stellar properties. In this thesis we present the results of our charac- terisation of the young OB association Scorpius-Centaurus via four different avenues; the stellar membership of the association, the multiplicity of the high-mass stars, the prevalence of circumstellar disks among Sco-Cen members, and age-dating the associ- ation with close binary systems. These are presented in the form of four chapters, two of which are journal publications. In the first section we present an analysis of the WISE photometric data for 829 B, A and F-type stars in the Sco-Cen association, using the latest high-mass membership probabilities. We detect debris disks associated with 134 Sco-Cen stars, with a clear increase in IR excess fraction with membership probability. We determine that 41±5% of Sco-Cen BAF stars have IR excesses, compared to 1±4% of field stars, and do not see any change in excess fraction between the Sco-Cen subgroups. Within our sample, we have observed that B-type association members have a significantly smaller excess fraction than A and F-type association members. In the second section we present a search for new low-mass members of the Sco- Cen association, focussing on the Upper Scorpius subgroup. We developed a Bayesian kinematic selection method to prioritise candidate members, and spectroscopically con- firmed 232 new Upper-Scorpius G to M-type members via their Li absorption with the WiFeS IFU. Among these new members we also identify eight companions in H-α emis- sion using spectro-astrometric techniques, four of which are candidate wide gas-giant ix x Abstract planets. Additionally, we observed the wide gas-giant planet host GSC-6214-0210 to have a significantly reduced H-α equivalent width of −0:63 A,˚ compared to the previ- ous observation of −1:51 A˚ , suggesting that the rate of accretion onto the planetary companion has slowed or stopped. In the third section we present the first multiplicity-dedicated long baseline optical interferometric survey of Sco-Cen. We have surveyed 58 Sco-Cen B-type stars with the Sydney University Stellar Interferometer and detected 23 companions at separations ranging from 7-130 mas, 13 of which are new detections. We then apply a Bayesian analysis to all available information in the literature to determine the multiplicity distribution of the 58 stars in our sample, showing that the companion frequency is +0:27 γ F = 1:35−0:20 and the mass ratio distribution is best described by q with γ = −0:46, agreeing with previous Sco-Cen high-mass star work and differing significantly from lower-mass stars in Tau-Aur. Based on our analysis, we estimate that among young B-type stars in moving groups, up to 27% are apparently single. In the final section we present the results of a Keck NIRC2 aperture-masking pro- gram of 7 G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB association. We present orbital solutions for the binary systems we have monitored, and also determine the age, component masses, distance and reddening for each system using the orbital solutions and multi-band photometry using a Bayesian fitting pro- cedure. We find that the age of the Upper Scorpius subgroup is 7±2 Myr, with some members as old as ∼ 10 Myr. This is younger than the previous estimate of Pecaut et al. 2012, but supports the hypothesis that there is an age distribution among stars kinematically consistent with Upper-Scorpius membership stretching from < 5 Myr up to ∼10 Myr. We propose that the current evidence for the age of Upper-Scorpius is consistent with the existence of two populations of stars in this part of Sco-Cen; one of ∼15 Myr, which formed with the rest of greater Sco-Cen, and a younger population of age ∼5 Myr, including the clearly young stars τ-Sco and !-Sco, which formed through supernova triggered star formation from a separate molecular cloud. Contents Acknowledgementsv Publications vii Abstract ix List of Figures xv List of Tables xix 1 Introduction and Background1 1.1 The Scorpius-Centaurus-Lupus-Crux Association . .2 1.1.1 Upper Scorpius . .5 1.1.2 Upper-Centaurus-Lupus and Lower-Centaurus-Crux . .8 1.1.3 Low-Mass Stars in UCL and LCC . 10 1.1.4 Binary Systems in Sco-Cen . 12 1.1.5 Interferometry . 14 1.2 Youth Indicators for Low-Mass Stars . 17 1.2.1 X-Ray Emission . 18 1.2.2 The Calcium HK Doublet . 20 1.2.3 The Relation Between Chromospheric Calcium II and X-Ray Ac- tivity . 22 1.2.4 Lithium Depletion . 22 xi xii Contents 1.3 Bayesian Statistics . 24 1.3.1 Hypothesis Testing and Model Likelihood Ratios . 26 2 WISE Debris Disks in the Young Sco-Cen Association 29 2.1 Introduction . 30 2.2 Data Sample . 31 2.3 WISE Excesses . 31 2.4 Discussion . 34 2.5 Summary and Conclusions . 39 3 New Low-Mass Sco-Cen Members 41 3.1 Bayesian Membership Selection . 43 3.1.1 Kinematic Models . 44 3.1.2 The Bayesian Algorithm . 47 3.1.3 Results of the Selection Algorithm . 55 3.2 Spectroscopic Membership Confirmation of Sco-Cen Low-Mass Stars . 57 3.2.1 Young Stars and Gas Giants with WiFeS . 58 3.2.2 Sample Construction and Observations . 60 3.2.3 Data Reduction . 65 3.2.4 Survey Results . 69 3.2.5 Spectro-astrometric Companion Detection . 79 4 Long-Baseline Interferometric Multiplicity Survey of Sco-Cen 93 4.1 Introduction . 94 4.2 Observations and Data Reduction . 95 4.2.1 Target Sample . 95 4.2.2 Observations . 97 4.2.3 Data Reduction and Calibration . 99 4.3 Companion Detections . 101 4.4 Detected Companions and Detection Limits . 104 4.5 Wide Companions with All-Sky Data . 108 Contents xiii 4.6 The Multiplicity Distribution of the Sco-Cen High-Mass Stars . 109 4.6.1 Compilation of the Sample Data . 112 4.6.2 Bayesian Analysis . 116 4.6.3 Single Stars . 122 4.6.4 The Effects of Multiplicity on Kinematics . 124 4.7 Conclusion . 124 5 Age Dating the Upper-Scorpius Association using Close Binary Sys- tems 127 5.1 Introduction .