Investigating chromospheric magnetic activity on young stars and the wide field CAMera for UKIRT

Abstract

Hertzprung-Russell diagrams are one of the most important tools for understanding pre-main sequence evolution when combined with theoretical evolutionary tracks. They are not only used to deduce the properties of the stars they are charting but to estimate the ages of clusters that house them and to investigate the age spreads for episodes of star formation. It is therefore vital that the determination of these dia­grams and tracks are built on solid theoretical and observational foundations. How­ever, work in recent years points to a potential problem. It has long been known that pre-main sequence stars exhibit regions of magnetic activity on their stellar surfaces similar to active regions observed on the Sun. What is not yet well known is the extent to which these active regions cover the stellar surfaces.

Most spectral classification relies on moderate resolution optical spectra which tend to be dominated by the non-active photosphere which is hotter than the active regions. Resultant effective temperatures are overestimated if a large portion of the pre-main sequence stellar surface is covered in active regions, which in turn can lead to substantial error in mass and age calculations. This thesis presents a novel approach to measuring the distribution of magnetic regions on T Tauri stars which aims to over­come limitations of other observing techniques such as Doppler imaging or Zeeman measurements. The central line emission from the strong visible Call H & K lines are a proxy indicator of surface magnetic fields and are known through observations of the Sun to be enhanced above active plage regions. Simultaneous optical spectroscopic and photometric observations of a significant sample of fast rotating T Tauri stars in the nearby clusters p Ophiuchus & Upper Scorpius have allowed us to ascertain a di­rect correlation between variations in the Call doublet emissions and light intensities. Computer simulations which model the surface conditions as understood on T Tauri stars and generate correlations which mimic those in the observational data offer a manipulable tool for estimating how much of the stellar surface is covered.

The Wide Field CAMera for the UKIRT Telescope on Mauna Kea is currently the most capable infrared imaging survey wide field camera in the world. The instrument focal plane consists of four Hawaii-II 2048 x 2048 IR detectors, to facilitate the best operating conditions and practises for the camera these detectors must be carefully characterised such that inherent qualities can either be corrected or accounted for. The second part of this thesis details the detector characterisation work carried out prior to the instrument delivery to the telescope. Obtaining a correct and stable operating temperature regardless of ambient temperature in the dome enclosure is key to the camera functioning optimally to carry out highly successful surveys. Presented here is a full model of the camera's thermal behaviour for the main instrument and the infrared detectors.