Graduate student Kristin Brady’s research on the chemical composition of unusual stars in the old open stat cluster Messier 67 has been published in the Astronomical Journal. Congratulations, Kristin!
Kristin has reported on the compositions of five binary stars located either bluer or brighter (or both) than the main-sequence turnoff region in Messier 67. Messier 67 is a star cluster located at a distance of almost 3000 light years from Earth toward the constellation Cancer. Such unusual stars are commonly referred to as Blue Stragglers. To carry out her study, Kristin use high-resolution, near-infrared spectra obtained with the Immersion GRating INfrared Spectrograph (IGRINS) at the McDonald Observatory 2.7-m Harlan J. Smith Telescope in Texas. These spectra provided full coverage of the infrared bands in the 1.6 and 2.2 micron atmospheric windows.
Kristin was able to determine the abundances of nine elements, including C, Na, Mg, Al, Si, S, Ca, Fe, and Ni from neutral atomic absorption lines. Kristin was also able to measure projected rotation rates, finding that at least three of her stars rotate faster v sin i = 25 km s-1 . The abundances she measured are in good agreement with normal main sequence stars in Messier 67 and also in good agreement with other analyses of blue straggler stars from optical spectra.
The five stars are all binaries with faint companions, most likely white dwarfs. The relatively normal carbon abundances Kristin measured in her stars suggest that while the white dwarf companions may have previously transferred some of their mass onto the stars, detecting carbon enhancements or depletions resulting from mass transfer is difficult due both to the uncertainties in the carbon abundance and the relatively modest changes that may occur through red giant and asymptotic giant branch evolution of the companions.
Co-authors on the paper include Chris Sneden, G. N. Mace, D. T. Jaffe, and R. Sieffert from the University of Texas in Austin, Melike Afsar from Ege University in Turkey, N. M. Gosnell from Colorado College, and Catherine Pilachowski from Indiana University. The full paper can be found at
https://iopscience.iop.org/article/10.3847/1538-3881/acf2f3
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