Testing Stellar Models for M Dwarfs
hdl:1811/60400
Published:
Abstract:
M dwarfs have expected lifetimes of at least 15 Gyr for the main phase of their lives, which is longer than the current age of the Universe. The chemical composition of the surface of an M dwarf, which is nearly constant during this main phase, is the same as the nearby gas of the galaxy in which it formed and at the time that it formed, so M dwarfs create a “fossil record” with which to examine the history and evolution of their host galaxies. This makes M dwarfs extremely important for study, but we do not see enough M dwarfs with few heavy elements, which are the oldest of the M dwarfs, to match predictions of compositions of stars for the local stellar neighborhood. Distances for these M dwarfs are important to accurately determine the extent of this deficiency, but these are difficult to determine accurately for the older M dwarfs that are of the most interest. M dwarfs, and especially older M dwarfs, are also observationally difficult to study in general because they are very dim compared to other stars. In this thesis, we test two different stellar isochrone models, the one by the Dartmouth group and the one by the Padova group, which we will later use to calculate M dwarf distances and investigate the observed discrepancy further. We find that the Padova group’s model ts better with spectroscopic and photometric data taken from two stellar surveys of the Galaxy, APOGEE and SDSS. We then suggest improvements on the tests we have completed and detail the next steps we hope to take in our investigation. We hope that this deep study of M dwarfs will provide more insights into the chemical evolution of the Milky Way, and allow models of stellar formation and Galactic chemical evolution to be improved upon for future use.
Summary
We studied the relation between color, temperatur, and metallicity for 37 stars selected as M dwarfs based on temperature and surface gravity with clean Sloan Digital Sky Survey (SDSS) photometry and spectra from the year 1 data of the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We compared the colors and temperatures of these stars to two sets of isochrones, with the hope of finding an isochrone model that could be used to predict the absolute magnitudes of a larger set of M dwarfs with less accurate temperatures from the Sloan Extension for Galactic Understanding and Exploration (SEGUE). This would be used with the SEGUE colors to get distances, and combined with metallicities to construct an volume-complete M dwarf metallicity distribution function. The paper found that the sample was not large enough to make any strong conclusions, but that the two isochrones explored did not fit the available data well.
Contribution
This was my undergraduate thesis project, so the majority of this work was completed by me with help from my advisors, Dr. Jennifer Johnson and Dr. Sarah Schmidt. I downloaded catalogs from the SDSS server query system, ran isochrone models at different metallicities, and made plots of the color vs. temperature for the stars and isochrones. I also used a $\chi^2$ analysis to determine how well the isochrones were able to fit the color-temperature relation in the data.