Origin of blue straggler stars in the Milky Way halo

Abstract: Blue straggler stars (BSSs) are exotic stellar objects that appear to be younger than the age of the population they come from suggests. They are observed as the extension of the main sequence stars, beyond the turn-off point. They are present in all types of stellar populations, such as open clusters, globular clusters, dwarf galaxies, and even in the open field. It is believed that they form mainly through binary star evolution. In this work, we are particularly interested in BSSs in the field of the Galactic halo as this is where the ancient dwarf galaxies, that were accreted by the Milky Way, deposited their debris. Studying the BSSs in this part of the Milky Way allows us to not only test the theories of binary star evolution, but it also gives us an insight into some of the oldest stellar populations, which probe the early history of our galaxy. We investigate how these BSSs form, how the theoretical formation channels vary between the accreted (formed ex-situ) and non-accreted (formed in-situ) samples, what the chemical abundance patterns are in the samples, and what that tells us about the populations they come from. Large datasets are needed to identify these rare stars and we use data from the spectroscopic surveys APOGEE and GALAH, cross-matched with Gaia. The datasets provide the necessary kinematic and chemical information about stars, which allows us to distinguish between stars that formed in the Milky Way and those that formed outside our galaxy, but were eventually accreted into the halo. Since BSSs are thought to form primarily through mass transfer in binary systems, we use the Binary Star Evolution code (BSE) to model possible formation channels for stars in the final sample. After applying our selection criteria, we end up with a small sample of possible BSSs, and study their chemical abundances and radial velocities. We use some chemical abundances (e.g. barium) to constrain the possible formation scenarios. We generate a large sample of model BSSs using BSE, look at their formation channels, and make synthetic radial velocity measurements, which we compare with the observed values. Lastly, we create detailed theoretical models of formation for two stars in the final sample, which have the radial velocity curve measured. We find that the vast majority of the stars in the final sample show radial velocity variations, which is consistent with our expectation that they form through binary star evolution. We show that the formation channel of a BSS correlates with its currently observed orbital period and mass. The modeling results show that mainly mass-transfer types B (mass transfer from a red giant), C (mass transfer from an asymptotic giant branch star), and D (accretion of mass from stellar winds) are able to reproduce the observed stars. We conclude that the accreted and non-accreted samples are not inconsistent with being the same. We are largely limited by the small size of the accreted sample of stars. The models also predict higher variation in radial velocity than is observed. Eccentric orbits of BSSs might be able to explain this offset, but our current understanding of binary star evolution cannot easily explain the higher eccentricity in post-mass-transfer systems. This proves to be the case even when modeling the two individual systems, which we are able to recreate accurately through type B or C mass transfer, except for the observed eccentricity.