The quest for a classical bulge - a study of kinematics, abundances, and ages using microlensed dwarf stars and Gaia

Abstract: Context: We present an kinematical analysis of 90 F and G type dwarf, turn-off, and subgiant stars in the Galactic bulge. These stars were observed with high-resolution spectrographs during gravitational microlensing events This allowed for their ages, abundances, and radial velocities to be ascertained while they were significantly brighter. Plotting these stars as [C/O] vs [O/H], it is revealed that there appears to be a distinct sub-group of stars that is chemically different. Aims: The aims of the thesis was then to investigate whether these chemically distinct stars also exhibit distinct kinematics compared to the entire sample of stars. By undertaking a kinematical analysis on these Sun-like dwarf stars it will shed some light on the tentative nature of the formation and evolution of the Milky Way bulge. Method: Through the use of finding charts we will cross-match our sample with stellar catalogues such as Gaia Data Release 3, and VVV InfraRed Astrometric Catalogue in order to extract relevant kinematical parameters; we then use these parameters in tandem with Galpy to make orbital simulations of our sample of stars. Results: Orbital integrations were successfully completed for 63 out of 90 stars, resulting in four parameters that we could use for the kinematical analysis: apocentre, pericentre, eccentricity, and Zmax of the orbits. By plotting [C/O] vs [O/H] as a function of these parameters we found there is no kinematical distinctness between this chemically distinct sub-group of stars and the rest of the stars. Plotting eccentricity vs apocentre and Zmax showed a large degree of similarity to plots of the same parameters but with stars in the disk, which would indicate that the bulge and disk shared similar evolutions and history. Plotting the galactocentric radial velocity vs galactic longitude/latitude revealed characteristic trends of cylindrical rotation, indicative of a buckling instability scenario for the bulge. Conclusions: We conclude that the sub-group of chemically distinct stars are not kinematically unique or indicative of a remnant group of stars from a early merger event. The kinematics point more towards a secular origin of the Milky Way bulge rather than a classical bulge resulting from mergers early on in the universe.