TOYS - time-domain observations of young stars

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 work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε 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-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori)

Variability in young very low mass stars : two surprises from spectrophotometric monitoring

The authors acknowledge support from the Science & Technology Facilities Council through grants no. ST/K502339/1 and ST/M001296/1.We present simultaneous photometric and spectroscopic observations of seven young and highly variable M dwarfs in star-forming regions in Orion, conducted in four observing nights with FOcal Reducer and low dispersion Spectrograph2 at European Southern Observatory/VLT. All seven targets show significant photometric variability in the I band, with amplitudes between 0.1–0.8 mag, The spectra, however, remain remarkably constant, with spectral type changes less than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that ‘fills in’ absorption features. Three objects in the σ Ori cluster (age ∼3 Myr) exhibit strong Hα emission and Hα variability, in addition to the continuum variations. Their behaviour is mostly consistent with the presence of spots with temperature of ∼300 K above the photosphere and filling factors between 0.2–0.4, in contrast to typical hotspots observed in more massive stars. The remaining targets near ϵ Ori, likely to be older, show eclipse-like light curves, no significant Hα activity and are better represented by variable extinction due to circumstellar material. Interestingly, two of them show no evidence of infrared excess emission. Our study shows that high-amplitude variability in young very low mass stars can be caused by different phenomena than in more massive T Tauri stars and can persist when the disc has disappeared and accretion has ceased.Publisher PDFPeer reviewe

A new introductory quantum mechanics curriculum

The Institute of Physics New Quantum Curriculum consists of freely available online learning and teaching materials (quantumphysics.iop.org) for a first course in university quantum mechanics starting from two-level systems. This approach immediately immerses students in inherently quantum mechanical aspects by focusing on experiments that have no classical explanation. It allows from the start a discussion of interpretive aspects of quantum mechanics and quantum information theory. This article gives an overview of the resources available at the IOP website. The core text is presented as around 80 articles co-authored by leading experts that are arranged in themes and can be used flexibly to provide a range of alternative approaches. Many of the articles include interactive simulations with accompanying activities and problem sets that can be explored by students to enhance their understanding. Much of the linear algebra needed for this approach is part of the resource. Solutions to activities are available to instructors. The resources can be used in a variety of ways from supplements to existing courses to a complete programme.Comment: 10 pages, 2 figures, 1 table; submitted to the European Journal of Physic

Clumpy dust rings around non-accreting young stars

This project was supported by STFC grant ST/R000824/1 to AS. MP acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907), and from the ERC under ERC-2011-ADG via the ECOGAL project (grant agreement number 291227).We investigate four young, but non-accreting, very low mass stars in Orion, which show irregular eclipses by circumstellar dust. The eclipses are not recurring periodically, are variable in depth, lack a flat bottom, and their duration is comparable to the typical time-scale between eclipses. The dimming is associated with reddening consistent with dust extinction. Taken together this implies the presence of rings around these four stars, with radii ranging from 0.01 to 40 au, comprised of optically thin dust clouds. The stars also show infrared excess indicating the presence of evolved circumstellar discs, with orders of magnitude more material than needed for the eclipses. However, the rings need to cover an opening angle of about 20 deg to explain how common these variable stars are in the coeval population in the same region, which is more extended than a typical disc. Thus, we propose that the rings may not be part of the discs, but instead separate structures with larger scale heights. To be sustained over years, the rings need to be replenished by dust from the disc or gravitationally bound to an object (e.g. planets or planetesimals). These four stars belong to a growing and diverse class of post-T Tauri stars with dips or eclipses in their light curves. Dusty rings with scale heights exceeding those of discs may be a common phenomenon at stellar ages between 5 and 10 Myr, in the transition from accretion discs to debris discs. These structures could be caused by migrating planets and may be signposts for the presence of young planetary systems.Publisher PDFPeer reviewe