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Parameters of evolved stars in nearby star-forming dwarf galaxies
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
The resolved stellar populations of a sample of 13 nearby ($<1.5$ Mpc) dwarf irregular galaxies were investigated. Spectral energy distributions were produced using multi-wavelength photometry from literature data. These were used to calculate the temperature and luminosity of evolved stars in each galaxy, which were statistically separated from foreground and background objects using Hertzsprung--Russell diagrams. \\Systematic uncertainties in temperature and luminosity were calculated for typical objects in each galaxy. A comparison was carried out between the SED-derived temperatures from this study and the spectroscopically-determined temperatures of a limited sample of supergiants. Comparison with stellar evolution models allowed for new estimates of distance and [Fe/H] to be calculated for several of these galaxies, with distance estimates differing up to $\sim10$\% compared with literature values. However, these parameters and the systematic uncertainties are correlated. Mid-IR photometry was used to measure the IR excess of evolved stars in the 3.6 and 4.5 $\mu$m bands, being tracers of carbon-rich dust. It was found that the IR excess can be used to separate the AGB from the RGB on an H--R diagram, although no correlation was found with the limited available variability data. \\The luminosity distribution of known carbon stars in these galaxies was investigated, and used to produce tentative limits on the range of initial masses at which carbon stars can form. The minimum mass of 1.4 M$_{\odot}$ is greater than theoretically calculated values at the range of metallicities found in dwarf irregular galaxies, although this may be in part attributable to systematic uncertainties in the conversion of luminosity to initial mass. The upper limit of $3.5-5.0$ M$_{\odot}$ was more difficult to define, as relatively few stars are found in this mass range, but it remains comparable to literature values (\citealt{Marigo07,Fishlock14,Ventura16}). The mass distribution of carbon stars was found to peak at $\sim2$ M$_{\odot}$. Further studies using $JWST$ will likely constrain these values further, and allow far more-precise spectra and photometry to be obtained.