TOYS TIME-DOMAIN OBSERVATIONS OF YOUNG STARS Inna Nikolaeva Bozhinova A Thesis Submitted for the Degree of PhD at the University of St Andrews 2017 Full metadata for this item is available in St Andrews Research Repository at: http://research-repository.st-andrews.ac.uk/ Please use this identifier to cite or link to this item: http://hdl.handle.net/10023/12014 This item is protected by original copyright This item is licensed under a Creative Commons Licence TOYS Time-domain Observations of Young Stars by Inna Nikolaeva Bozhinova Accepted for the degree of Doctor of Philosophy in Astrophysics September 2017 Declaration I, Inna Bozhinova, hereby certify that this thesis, which is approximately 30,000 words in length, has been written by me, or principally by myself in collaboration with others as acknowledged, that it is the record of work carried out by me and that it has not been submitted in any previous application for a higher degree. Date Signature of candidate I was admitted as a research student in October 2013 and as a candidate for the degree of PhD in October 2013; the higher study for which this is a record was carried out in the University of St Andrews between 2013 and 2017. Date Signature of candidate I hereby certify that the candidate has fulfilled the conditions of the Resolution and Regula- tions appropriate for the degree of PhD in the University of St Andrews and that the candidate is qualified to submit this thesis in application for that degree. Date Signature of supervisor i Copyright Agreement In submitting this thesis to the University of St Andrews we understand that we are giving per- mission for it to be made available for use in accordance with the regulations of the University Library for the time being in force, subject to any copyright vested in the work not being affected thereby. We also understand that the title and the abstract will be published, and that a copy of the work may be made and supplied to any bona fide library or research worker, that my thesis will be electronically accessible for personal or research use unless exempt by award of an embargo as requested below, and that the library has the right to migrate my thesis into new electronic forms as required to ensure continued access to the thesis. We have obtained any third-party copyright permissions that may be required in order to allow such access and migration, or have requested the appropriate embargo below. The following is an agreed request by candidate and supervisor regarding the electronic pub- lication of this thesis: Access to Printed copy and electronic publication of thesis through the University of St Andrews. Date Signature of candidate Date Signature of supervisor iii Abstract Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star’s life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (.0.4 M ) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This v work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-righ, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks ( Ori). Acknowledgements I would like to mention a couple of names without whom I wouldn’t have made it this far in astronomy. Firstly, I want to thank my mum and dad for always supporting me no matter how unusual they may have found my career choice. A massive thank you to Dimitar Slavov for being the first to introduce me to the night sky, Dr Eva Bozhurova for making stellar physics fun and approachable and Valentin Velkov for always turning the long observing nights into fun memorable experiences. I am especially thankful to Dr Veselka Radeva for being my teacher and mentor, introducing me to the magic of the big telescope and the night sky on a mountain, and her endless care and support even to this day. As for my time in St Andrews, I would like to thank the Physics and Astronomy teaching body for providing me with an amazing environment to grow as an astronomer during both my undergraduate and postgraduate years. A very special thank you to my PhD supervisor, Dr Aleks Scholz, for guiding and teaching me, training me at the JGT and honing my observing skills, for always checking if I needed help while giving me the space and freedom to work on my own, and for giving me the occasional kick when I needed it. Thank you to all the friends I have made here over the years. Thanks to Ricky and the aikido club for turning me into a hardcore martial artist. Thanks to Ewa, Phillip and everyone at the tango society for giving me the dancing bug and memories for life. Thanks to my PhD colleagues for creating a tight little community where I felt like I belonged. Thanks to Graham and Alistair for Magic nights and thank you Gabi for games, aikido, silks, palenka, planetarium or just venting out to on a bad day. And finally thank you to my two most favourite people - for always being there for me, for boosting my confidence even when I feel stupid around you, for making me laugh or making me angry, for taking care of me when I break myself or get ill, for giving me hugs when I’m down vii and talking sense into me when I don’t make any. Thank you for being a part of my life. "Algebra!...But that’s far too difficult for seven-year-olds!" "Yes, but I didn’t tell them that and so far they haven’t found out." Miss Susan Thief of Time, Terry Pratchett Collaboration Statement This thesis is the result of my own work carried out at the University of St Andrews between October 2013 and March 2017. Parts of the work presented in this thesis have been published in refereed scientific journals. In all cases the text in the Chapters has been written entirely by me. All figures, unless explicitly stated in the text have been produced by me. Chapter 3 is based on: "The disappearing act: a dusty wind eclipsing RW Aur", Bozhinova, I.; Scholz, A.; Costigan, G.; Lux, O.; Davis, C. J.; Ray, T.; Boardman, N. F.; Hay, K. L.; Hewlett, T.; Hodosán, G.; Morton, B., 2016, Monthly Notices of the Royal Astronomy, 463, 4, 4459. I prepared and submitted the observational requests as well as wrote the codes and did the analysis for all the LCOGT data. The JGT data was taken by the JGT observing team: Aleks Scholz, Inna Bozhinova, Nicholas Boardman, Kistin Hay and Gabriella Hodosán.