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.