| dc.description.abstract | Understanding stellar variability offers a wealth of information about the surfaces and nearby environments of stars.
In this Thesis I explore stellar variability in two principal contexts.
The first is a study of the photometric variability of young stars which still host their protoplanetary discs.
Photometric variability, especially aperiodic variability, is a widespread phenomenon within these systems, and is known to reflect changes on the stellar surface, in the protoplanetary disc, or with the interaction between the disc and the stellar magnetic field.\\
Using simultaneous optical and infrared light curves of disc-bearing young stars in NGC 2264, I perform the first multi-wavelength structure function study of YSOs.
I find that dippers are more variable than bursters and symmetric variables at all timescales longer than a few hours.
By analysing optical-infrared colour time-series, I also find that the variability in the bursters is systematically less chromatic at all timescales than the other variability types.
Finally, I propose a model of YSO variability in which symmetric, bursting, and dipping behaviour is observed in systems viewed at low, intermediate, and high inclinations, respectively.
I argue that the relatively short thermal timescale for the disc can explain the fact that the infrared light curves for bursters are more symmetric than their optical counterparts,
as the disc reprocesses the light from all rotational phases.
%more symmetric infrared light curves of bursters,
From this model, I find that the accretion variability onto these YSOs roughly follows a random-walk.\\
The second context in which I explore stellar variability is in the field of radial-velocity searches for exoplanets.
Exoplanet detection is a rapidly evolving field with a push to detect ever lower-mass planets.
I present an example planet search and mass confirmation of two low-mass transiting planets around the solar-type star TOI-5788.
The goal of the next generation of radial-velocity surveys is the robust detection of Earth-twins.
As instrumental precision has improved, it has become clear that intrinsic stellar variability poses the most significant challenge to detecting Earth-twins.
Although a range of techniques exist and are constantly being developed to mitigate the effects of isolated magnetically active regions, comparatively little attention is paid to the effect of the magnetically quiet stellar surface.\\
Using images from the Helioseismic and Magnetic Imager aboard the \textit{Solar Dynamics Observatory} (SDO/HMI), I extract the radial-velocity (RV) signal arising from the suppression of convective blue-shift and from bright faculae and dark sunspots transiting the rotating solar disc.
I remove these rotationally modulated magnetic-activity contributions from simultaneous radial velocities observed by the HARPS-N solar feed to produce a radial-velocity time series arising from the magnetically quiet solar surface (the \lq inactive-region radial velocities\rq).
I find that the level of variability in the inactive-region radial velocities remains constant over the almost 7 year baseline and shows no correlation with well-known activity indicators.
With an RMS of roughly 1 \mps, the inactive-region radial-velocity time series dominates the total RV variability budget during the decline of solar cycle 24.
% The roughly 1 \mps variability shown by the inactive-region RVs is, for large parts of solar cycle 24, significantly larger than the variability in the \lq magnetic activity\rq\ RVs we calculate from the SDO/HMI images.
Finally, I compare the variability amplitude and timescale of the inactive-region radial velocities with simulations of supergranulation.
I find consistency between the inactive-region radial-velocity and simulated time series, indicating that supergranulation is a significant contribution to the overall solar radial velocity variability, and may be the main source of variability towards solar minimum.
This work highlights supergranulation as a key barrier to detecting Earth twins. | en_GB |
