Sample Plot Caption
The entire APO-K2 sample shown as their positions relative to the plane of the Milky Way Galaxy (Gaia). The x- (y-) axis shows Galactic latitude (longitude) in degrees. The colors represent the metallicities of the stars (APOGEE). The actual range of metallicities are both more metal-rich and metal-poor that those represented by this plot, and have been scaled to show the relative metallicities between the campaigns. Each of the utilized K2 campaigns is shown and labelled as C followed by the number of the campaign. The Kepler field is shown for reference, with metallicities taken from APOKASC.
Sample Plot Takeaways
The K2 sample, is chosen using a well-understood selection function and is almost complete in it's survey of evolved stars. This plot represents ~8,000 evolved stars with some stars observed in multiple, overlapping, campaigns. The distinction between the average metallicity of the Kepler sample and the K2 samples that fall above and below the plane is stark, with the Kepler sample being generally more metal-rich than the APO-K2 sample. This is due to the selection of the Kepler sample, and is not a physical phenomenon. This difference in metallicity helps to represent the benefit of our sample, both in completeness of the selection function and the abundance of metal-poor stars. Metal-poor stars are particularly interesting for the new parameter spaces this sample opens up.
H-R Diagram Caption
H-R diagrams representing the red clump stars on the right and the red giant branch stars on the left. In order to show these two samples distinctly, and in relation to one another, crosses represent the sample on the opposing plot, i.e. the red giant branch stars are shown by crosses on the left and the red clump stars are shown by the circular markers. The markers are sized using the asteroseismic mass of the star (K2) and colored by the metallicity of the star (APOGEE). The x- (y-) axis show effective temperature (luminosity) in units of Kelvin (solar luminosity).
H-R Diagram Takeaways
The red clump stars on the left show some particular interesting sub-sets of stars. The crosses in this plot, that represent the red giant branch stars from the right hand side of the plot show smaller densities well, for instance representing the red bump stars just below the red clump stars. The secondary red clump is also visible, slightly below the main red clump and to the left of the red bump. The size of stars increases with increasing luminosity and decreasing effective temperature. The left and plot shows a gradient in metallicity to the right hand side of the red giant branch. The branch also increases in width towards the upper-right hand corner of the plot. This may represent possibly mischaracterized AGB stars that have been mistaken for fight-ascent red giants.
Alpha Bimodality Plot Caption
The circular points on this plot represent the stars in the sample, where the size of the point represents the Galactic eccentricity of the star (Gaia) and the color of the point indicates the mass of the star in solar masses. The x- (y-) axis represent the iron α abundances (APOGEE). Representative error bars are shown. The black line delineating the high- and low-alpha samples is drawn by eye and well separates the sample. The mass has been scaled to make the differences clear in the plot, with the real maximum and minimum masses shown in the title of the plot.
Alpha Bimodality Plot Takeaways
The clear separation between the high- and low-alpha abundances is evident in this plot, with the sample stretching down to low metallicities not seen in the previous Kepler sample. The high-alpha abundance stars are likely thick disk stars, these form quickly at the beginning of the formation of the Galaxy, aided by core collapse supernova. When these supernova slow down, the Type 1a supernova take over in relative abundance, and the low-alpha sample is created, with higher iron content but less alpha-rich elements (not made in Type 1a). Our plot supports this formation theory.