Variability in IC5070: two young stars with deep recurring eclipses

Investigating the structure and properties of the innermost parts of protoplanetary accretion disks on sub-AU scales is currently only possible via indirect methods. One option to map the planet-forming zone is to search for occultations of the central young stellar object (YSO) by circumstellar material, e.g., warps or clumps in the inner disks. Such disk eclipses typically last hours to days (Cody et al. 2014) and have been identified in massive HAeBe stars such as UX Ori (Herbst & Shevchenko 1999) and lower mass objects such as AA Tau (Bouvier et al. 1999). Of particular interest are quasi-periodic dimming events. They allow distance determinations of the occulting material from the central star. In such cases the actual azimuthal physical extent of the material can be determined from the duration of the dimming event relative to the period. Observations over several periods enable investigations into temporal changes in the line of sight column density distribution, and multi-wavelength data allows us to probe the dust scattering properties. Our citizen science project HOYS-CAPS (Froebrich et al. 2018) aims to identify such periodic dimming events around YSOs. We used this data-set to search for periodic signatures in light-curves from YSOs in the Pelican nebula (IC 5070). For this field we have ~200 individual observations in the V, R, and I-band filters, distributed over ~800 days. Hence, the average cadence is 4 days, but the most frequent gap (30%) between subsequent observations is 2 days. Observations are usually taken as 8 × 2 minutes integrations in all filters to achieve a consistent S/N

Measuring the Thermal Response of Small Asteroids via the YORP Effect

As part of the European Southern Observatory Large Programme (ESO LP), a selection of over 40 asteroids have been monitored photometrically since 2010, primarily, from the New Technology Telescope and Very Large Telescope facilities. These objects were chosen based on conditions which made them likely candidates to experience YORP accelerations. The principal aim of the ESO LP is to detect YORP-induced rotational period variations in order to further the understanding and development of YORP theories. Of these 40 asteroids, three were chosen for investigation in this thesis: (29075) 1950 DA, (68346) 2001 KZ66, and (89830) 2002 CE. Light curves were collected via the ESO LP for the asteroid (29075) 1950 DA and combined with published radar and optical data. Utilising the published shape models, it was not possible to fit our light curve dataset leading to attempts to remodel the asteroid. In the early stages of analysis, initial modelling indicated a deceleration in the asteroid's rotation rate; however, the cause of this deceleration could not be definitively attributed to YORP. In 2019, an additional set of light curves were obtained, with which the best-fit models indicated that the asteroid is experiencing negative YORP accelerations. This is a first-of-its-kind finding, which will have a profound significance for YORP theory. For asteroid (68346) 2001 KZ66, both optical and radar observations were obtained. The optical dataset allowed a comprehensive analysis of the asteroid's rotation period and pole orientation. While the radar observations were utilised to produce a detailed shape model of the object. The asteroid was determined to be a retrograde rotator with the pole located near the southern ecliptic pole and a rotation period of 4.985997 ± 0.000042 hours. By combining the radar-derived shape model with the optical light curves, a solution was developed which fit all available data by requiring a YORP strength of (8.50 ± 0.11) × 10−8 rad/day2. Moreover, possible mechanisms by which this asteroid could've been formed are explored in addition to the stability of its present shape. For the final object, a light-curve-only analysis was performed for the asteroid (89830) 2002 CE. The investigation of this object utilised 25 light curves obtained by the ESO LP and its auxiliary campaigns. These observations allowed the accurate determination of the asteroid's rotation period and pole orientation, which is located at (94◦ ±5◦, 47◦ ±5◦) in ecliptic coordinates. Constraints on the asteroid's shape were also developed using convex inversion techniques, producing two shape models capable of fitting the majority of the light curves. Attempts to define bounds for possible YORP accelerations produced several potential values equal in their ability to fit the data. This indicates that more data is required to determine meaningful constraints on YORP. With the number of direct YORP detections only in the single digit range, the work performed in this thesis adds an additional detection which is crucial in order to calibrate and further the field of YORP theory. Moreover, the indications of negative YORP acting on DA are the first of their kind. Before which, only positive accelerations were measured, which spurred the development of additional forms of YORP to explain the statistically unlikely non- binarity of detections hitherto. Beyond the scope of YORP, the shape models produced within this thesis can be used in further studies. For example, in studies aiming to determine the formation mechanisms capable of contact-binaries such as KZ66

Detection of the YORP Effect on the contact-binary (68346) 2001 KZ66 from combined radar and optical observations

The Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect is a small thermal-radiation torque experienced by small asteroids, and is considered to be crucial in their physical and dynamical evolution. It is important to understand this effect by providing measurements of YORP for a range of asteroid types to facilitate the development of a theoretical framework. We are conducting a long-term observational study on a selection of near-Earth asteroids to support this. We focus here on (68346) 2001 KZ66, for which we obtained both optical and radar observations spanning a decade. This allowed us to perform a comprehensive analysis of the asteroid’s rotational evolution. Furthermore, radar observations from the Arecibo Observatory enabled us to generate a detailed shape model. We determined that (68346) is a retrograde rotator with its pole near the southern ecliptic pole, within a 15○ radius of longitude 170○ and latitude −85○. By combining our radar-derived shape model with the optical light curves we developed a refined solution to fit all available data, which required a YORP strength of (8.43±0.69)×10−8 rad d−2 (68346) has a distinct bifurcated shape comprising a large ellipsoidal component joined by a sharp neckline to a smaller non-ellipsoidal component. This object likely formed from either the gentle merging of a binary system, or from the deformation of a rubble pile due to YORP spin-up. The shape exists in a stable configuration close to its minimum in topographic variation, where regolith is unlikely to migrate from areas of higher potential

A survey for variable young stars with small telescopes - IV. Rotation periods of YSOs in IC 5070

Studying rotational variability of young stars is enabling us to investigate a multitude of properties of young star-disc systems. We utilize high cadence, multiwavelength optical time series data from the Hunting Outbursting Young Stars citizen science project to identify periodic variables in the Pelican Nebula (IC 5070). A double blind study using nine different period-finding algorithms was conducted and a sample of 59 periodic variables was identified. We find that a combination of four period finding algorithms can achieve a completeness of 85 per cent and a contamination of 30 per cent in identifying periods in inhomogeneous data sets. The best performing methods are periodograms that rely on fitting a sine curve. Utilizing Gaia EDR3 data, we have identified an unbiased sample of 40 periodic young stellar objects (YSOs), without using any colour or magnitude selections. With a 98.9 per cent probability, we can exclude a homogeneous YSO period distribution. Instead, we find a bi-modal distribution with peaks at 3 and 8 d. The sample has a disc fraction of 50 per cent, and its statistical properties are in agreement with other similarly aged YSOs populations. In particular, we confirm that the presence of the disc is linked to predominantly slow rotation and find a probability of 4.8 × 10−3 that the observed relation between period and presence of a disc has occurred by chance. In our sample of periodic variables, we also find pulsating giants, an eclipsing binary, and potential YSOs in the foreground of IC 5070