Date of Award

12-14-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Todd Henry

Second Advisor

Douglas Gies

Third Advisor

Russel White

Fourth Advisor

Brian Thoms

Fifth Advisor

Suzanne Hawley

Abstract

This dissertation addresses the underlying causes for the observed widening of the main sequence for low mass stars and presents the results of three different studies using two different astrophysical observing methods to assess the properties of a sample of nearby, M dwarf stars and how these properties affect a star's position on an H-R diagram.

The first study is the assessment of the activity of the internal magnetic fields of 76 southern nearby, M dwarf stars through measurements of the relative changes in their V magnitudes over time periods of years. This long-term variability is then analyzed with respect to the vertical positions of these stars on the main sequence to determine the effects of stellar activity on these positions.

The second study uses a similar technique as the first study (relative photometry) only on short-term timescales of only hours. This study serves to assess the effects of magnetic activity and variability at the surfaces of 120 low mass stars on their main sequence positions.

The final study presented here analyzes the high-resolution spectra of 80 nearby, M dwarf stars to assess their compositions and activity indicators and how these properties affect the placement of these stars above, below, or on the central distribution of the main sequence. This study also includes the use of published metallicity values for many of the stars in our sample to better assess the importance of this property on main sequence position.

We find that variability plays an important role in the elevation of stars, particularly young stars, above the main sequence, likely due to an extension of their chromospheres caused by heightened levels of stellar activity. For stars below the main sequence we find a strong connection between their positions and abundances, as low metallicity results in a temperature displacement that ultimately places these stars below the main sequence. These results suggest that while each property is important, heightened activity is a stronger influence for stars above the main sequence while low metallicity is the primary property that places these stars below the main sequence.

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