Long-lived discs in T associations: Pre-main-sequence ages for low-mass stars
Rees, Jon Morgan
Thesis or dissertation
University of Exeter
In this thesis, ages have been derived for 4 young clusters by fitting the pre-main-sequence stars with semi-empirical models in colour-magnitude diagrams. Combining these ages with the (consistent) set presented in previous work, the first robust evidence of increased circumstellar disc lifetimes in low-mass, low-density regions is obtained. To obtain this result, the following steps were necessary: • Semi-empirical model isochrones have been constructed in a number of rizJHK photometric systems. These models overcome the issues typically seen in purely theoretical models in which the blue flux of low-mass stars is overestimated. These models are presented in a number of widely used filter sets for the first time, allowing for wider use with new clusters. Additionally the models constructed in previous filter sets have been refined using new observations. • To support the construction of these models, upper-main-sequence fitting is performed for 2 fiducial clusters, and it is demonstrated that the resulting age and distance measurements are consistent with other measures. • A new reduction process for data in the Blanco-DECam system is presented, and it is shown that the DECam photometric system is well characterised. • A photometric method for dereddening stars individually in regions of spatially variable extinction is presented, and applied to the young regions in this study. This method of photometric dereddening can be applied to large numbers of stars, greatly decreasing the time investment needed compared to spectroscopic methods. 2 The ages derived for the young clusters using the semi-empirical models are around a factor 2 older than typically assumed in the literature, which is in-line with that seen for the ages derived for other clusters using the same technique. By considering the disc fraction in these clusters as a function of age, it is shown that Taurus and Chamaeleon show a significant excess of discs compared to a set of massive, dense clusters of similar age. This is clear evidence that discs seem to survive longer in this low-mass, low-density region, giving crucial hints at different disc evolution in these regions. ρ-Oph is a low-mass region with a high stellar density, and so could be used to identify the dominant mechanism leading to these long-lived discs. However the presence of a similar disc excess in ρ-Oph is dependent on the assumed distance, which is currently poorly constrained, and so the dominant mechanism is still unclear.
PhD in Physics