AbstractWith the push towards finding true Earth-analogs, instrumental radial velocity (RV) precision no longer presents a limiting factor. The effects of astrophysical noise, which can act to mask or mimic planetary RV motions, represents the fundamental barrier to achieving this goal. For inactive stars like the Sun, plage/faculae are thought to be the major contributors to this noise, but its effects have so far proven difficult to trace.
In this thesis, I investigate the effect activity has on stellar spectra, using HARPS data of the well observed K1 V star α Centauri B. I compare spectra from high- and low-activity periods (as traced by log R′HK) of α Cen B. I produce ‘relative spectra’ by dividing the high-activity spectra by a low-activity template, and find a forest of pseudo-emission features. The features show a strong correlation with logR′HK, modulation on the stellar rotation period, and a peak-to-peak RV variation of 300m s−1, all of which suggest that the features are activity driven. The features also show two distinct morphologies that, when modelled, show evidence that the ‘immaculate photosphere’ during the more active phase is fundamentally different to the inactive case. This may be due to enhanced contributions of e.g. magnetic bright points and plage, which act as a source of additional line broadening. An extension of this work looked at HARPS-N Solar Telescope data, where similar ‘relative spectra’ were created for the Sun, making use of the unique Sun-as-a-star observing capability of the instrument. Here the relative spectra were compared not only to logR′HK but to spot and faculae filling factors calculated using SDO images. Equivalent features to those found in α Cen B were found, further confirming that the features are activity driven. With the solar data, the features were found to more strongly correlated with changes in unassociated faculae than with spots. This suggests that the features seen in the relative spectra are driven by facular regions, and may provide a way of tracking plage/faculae on the surface of stars by measuring their influence directly from stellar spectra.
Finally, I constructed a tool called UnEarth that combines the techniques of wavelet analysis, GLS, and DCF to search for stellar rotation. This tool was specifically de- signed to be used with NGTS light-curves. I apply the tool to a number of interesting NGTS targets with the results from each target highlighted in the relevant sections. The combined methods were able to provide more robust detections of stellar rotation periods and a potential correlation between the likelihood of a flare event and the concentration of active regions on the stellar surface.
|Date of Award||2019|
|Supervisor||Christopher Watson (Supervisor)|