The APO-K2 Catalog: ~8,000 Red Giants with Fundamental Stellar Parameters from APOGEE  Spectroscopy and K2 Asteroseismology.

Authors and Affiliations: Jessica Schonhut-Stasik (Vanderbilt University and the Frist Center for Autism and Innovation), Joel Zinn (NSF Postdoctoral Fellow and the American Museum of Natural History), Jack Warfield (University of Virginia), Keivan Stassun (Vanderbilt University) and Mark Pinsonneault (Ohio State University).

The APO-K2 catalog combines spectroscopic parameters from APOGEE DR17 (Abdurro'uf et al., 2022), asteroseismic parameters from the K2 GAP program (Stello et al., 2017) and Gaia eDR3 (Gaia Collaboration et al., 2021) astrometry for ~8,000 evolved stars, with the motivation to improve our understanding of the formation and evolution of the Milky Way through the application of Galactic archaeology.

Spatial Overview of Sample: K2 provides more extensive sky coverage than Kepler, by covering multiple campaigns over the ecliptic plane and has a program dedicated to the evolved stars, with a well-understood selection function (Sharma et al., 2021) resulting in ~8,000 evolved stars in this catalog; ideal for pursuing Galactic archaeology studies. 

Figure 1 Takeaway: K2 covers large areas of the Galaxy at varying distances from the Galactic plane. The average metallicities of the K2 

campaigns are all more metal-poor than the APOKASC sample (Pinsonneault et al., 2018). The selection of the K2 stars prioritized the 

completeness of the evolved sample versus the planet-search criteria of Kepler. By and large, the campaigns are evenly populated.

Figure 1 Caption: Each star in the sample, including those observed in multiple campaigns, plotted according to their position relative to the Galactic plane. Color represents metallicity [APOGEE]. Kepler stars shown for reference [APOKASC]. Colorbar scaled to show difference in campaigns, real metallicity range is -2.5 < [Fe/H] < 0.5.

Figure 1 Alt Text: This plot has an x- and y-axis corresponding the galactic longitude and latitude, respectively, in units of degrees. The axis ticks show these values from roughly -200 to +200 in the x and -100 to +100 in the y. The background of the plot is an image of the Milky Way Galaxy, with the Galactic Center centered on the 0, 0, point. This image is for illustration purposes only and shows the Milky Way in optical color with the darkness of space to the top and bottom of the plot and the Milky Way in the middle, the brightness of the Milky Way is largest in the middle, where the Galactic center resides.  Over the top of this background image is the data from this paper, this looks like 18 Kepler field of view shaped areas, made up of the points in the sample, so that some of the areas are more populated than others. The areas covered by K2 start on the left hand side and follow a sinusoidal pattern, first down below the Galactic plane and then up through the Galactic center, before curving upward and back down again at the other end. The order of the areas are: C13, C4, C8, C12, C3, C7, C11, C2, C15, C6, C17, C10, C1, C14, where C means Campaign. C16, C5, and C18 are clustered together on the right hand side of the plot, just above the plane, with C16 at the highest latitude, followed by C5, and C18. They overlap quite a bit. All of these areas are labelled with white text. The Kepler field is also shown for comparison, it falls just above the galactic plane on the left hand side of the plot. It is the most populated of all the areas, due to the abundance of data for the Kepler mission. This is labelled with white text. The Kepler data was taken from the APOKASC catalog. Each of the data points is colored using a colormap that moves from purple to yellow through blue and green. The colors correspond to metallicity, with the yellow end showing more metal rich stars and the purple end showing more metal poor stars. A representative color bar, is shown on the right hand side of the plot and is labelled '[Fe/H]'. The bar is scaled between -1.0 and just above 0.5, to show the relative differences between the campaigns, although this is not the full range of metallicities in the study. The Kepler field and campaigns 11, 2 and 15, can be seen to be more metal rich than the rest, although the campaigns toward the Galactic center are less populated than those above or below. 

Full Sample in H-R Space: The APO-K2 catalog allows for new levels of detail on the giant branch. Accurately characterizing 

evolved stars into their appropriate evolutionary states is vital for understanding the complex nature of this area of the H-R diagram. Figure 2 Takeaways: RC Stars (LHS): We see mass (proxy for age) increasing with increased temperature and luminosity; younger stars being more luminous. At L > 40LSUN, there are fewer hot stars, indicating their evolution to the AGB. The RC stars are (on average) hotter than the RGB stars. The RGB bump and secondary red clump are visible in this plot. RGB Stars (RHS): There is an increase in width of the branch at higher luminosities, indicating a greater temperature range for more luminous stars. Some of these may be mischaracterized AGB stars 

that look similar to first ascent RGB stars. 

Figure 2: H-R diagram of Red Clump (RC) stars (LHS) and Red Giant Branch (RGB) stars (RHS). Marker colors represent the metallicity [APOGEE] of the stars and the size of the points indicate the asteroseismic mass [K2]. Cross symbols show the points from the other sample plotted under the data (for example, the crosses in the left-hand plot show the position of the RGB stars in the right-hand plot). 

Figure 2 Alt Text: This plot has two scatter graphs, one on the left hand side and one on the right. The left hand side plot corresponds to the Red Clump Stars in the sample, and the title states this followed by the number of red clump stars (2446). These plots are roughly square with the y-axis and x-axis roughly the same dimensions and scalings. Luminosity in units of solar luminosities is on the y-axis and are plotted in log space and the effective temperature in Kelvin is on the x-axis, plotted in linear space. These axes are labelled as such. Each plot has both 'x' markers and circular markers. The x markers on the left hand side show the positions of the stars on the right hand side and vice versa. The circular markers show the points that are labeled for the specific plot. The colors of the circular markers correspond to the metallicity of the star and the size of the circular markers corresponds to the stellar mass (from asteroseismology) of the stars the circles represent. The colorscale of the metallicity ranges from white and low metallicity to blue at higher metallicity. The red clump stars on the left hand side look like a filled in elliptical blob with streams of less dense marks around them. The larger points, symbolizing more massive stars are to the upper left of the plot.  There are more relatively metal rich stars towards the red giant branch. The crosses on this plot, that symbolize the red giant branch stars, show a clump just below the red clump that symbolizes the red bump. Some stars can also be seen that symbolize the red clump. On the poster version of this plot the red bump is circled with a red circle and the label Red Giant Bump. The right hand side of the plot shows crosses where the red clump stars are and circle markers for the red giant stars, the title of this plot states that it shows red giant branch stars and the number of stars in the plot (4053). The circular markers create a patter that looks like a wide line of stars from the bottom left of the plot (low luminosities and high temperatures) to the top right of the plot (high luminosities and low temperatures). The right hand side of the red branch has more dark blue circle markers, showing more relatively metal rich stars. There is a color bar on the right hand side of the plot labelled with '[M/H]'. 

The Alpha Bimodality: The alpha bimodality is the apparent division of stars in iron abundance versus alpha abundance creating two distinct sets; the low-alpha and high-alpha sample. The high-alpha stars are thought to belong to the thick disk, 

be generally older and have formed quickly, predominatly through the gas enriched by Type II supernova. The low-alpha stars are 

thought to be younger, have formed more slowly, and belong to the thin disk; most likely the result of gas enriched by Type Ia 

supernova. As the the Galaxy ages, less Type II supernova occur relative to Type Ia, and the abundance of alpha elements will 

decrease relative to the abundance of iron. The alpha bimodality informs our understanding of the chemical evolution of the Galaxy, 

this data set helps to constrain this history. 

Figure 3 Takeaways: We see a vast increase in the number of stars available at low metallicities, when compared to previous samples. More massive stars appear in the low-alpha sequence and their masses are similar. Eccentricity increases with increased alpha abundance and decreased iron abundance which indicates older, low mass stars in our Galaxy or even stars that have been accreted from the Gaia-Enceladus-Sausage, the remnants of a dwarf galaxy that merged with the Milky Way ~10 Gyr ago (Helmi et al., 2018). 

Figure 3: Alpha-abundance (y-axis) and metallicity (x-axis) from APOGEE DR17. Colors correspond to asteroseismic mass and are derived from the asteroseismic scaling relations, using a correction term on ∆ν. Point size corresponds to the Galactic eccentricity of the star, which were derived from the Gaia eDR3 kinematics and the Python module gala. Line deliniates the samples, and was drawn by eye.

This plot shows the stars in the sample in the alpha-bimodailty space. The y-axis shows the alpha abundance between -0.2 at the bottom of the y-axis and 0.5 at the top of the y-axis. It is labelled as '[alpha/Fe]'. The x-axis (which is about twice as long in aspect ratio as the y-axis) shows the iron abundance between ~-2.0 on the left and ~0.5 on the right. It is labelled with '[Fe/H]'. The points in the sample are shown as circular markers with black edges. The colors of he markers correspond to the asteroseismic mass of the stars in units of solar mass, with the colormap ranging from dark blue for lower mass stars and white for higher mass stars. This colorer has been scaled between ~0.5 and 2.0 to show the relative masses of the stars on the plot but the actual range in masses is 0.32 - 4.21, which is written as the title of the plot. The size of the points correspond to the Galactic eccentricity of the stars, with bigger points corresponding to bigger eccentricity. There sample itself looks like a bimodal distribution with one long sausage shape of stars on the bottom, leading from the bottom right hand corner (low alpha abundance and relatively high iron abundance) to toward the top left, stopping roughly in the middle of the plot, with relatively low iron abundance (around -1.0) and relatively high alpha abundance (around 0.1). The top clump of stars starts at the same place as the previous sample, and lies above it. It reaches toward the top left of the plot, similar to the lower sample, but peaking at a larger alpha abundance. There is a black line on the plot that delineates the high alpha sample from the low alpha sample. The high alpha sample has more, spread out stars, reaching almost completely to the left hand side of the plot. There are two representative error bars on the plot, one at low values of iron abundance and one at relatively higher values of iron abundance (0.0). The bottom sample of stars have more high mass stars than the higher alpha stars. The higher alpha stars have larger eccentricities for the stars at lower iron abundance.  

Sample in Kinematic Space: The kinematic space of stars in the Milky Way (when combined with asteroseimic masses and 

ages) acts as a fossil record for the formation  and evolution of our Galaxy. The precision astronomy provided by Gaia eDR3 allows

for the discovery of stars that were the result of mergers in the Galaxy’s youth. 

Figure 4 Takeaways: The grey areas correspond to stars that are kinematically consistent with membership to Gaia-Enceladus.

All but one of these stars demonstrate high alpha abundance. The spread in the high-alpha stars is greater than the low-alpha stars, 

indicating thick disk or halo kinematics. 

Figure 4: Angular momentum and energy retrieved using the Python module gala, and Gaia eDR3 kinematics. Grey box corresponds to the kinematic space represented by the Gaia-Enceladus-Sausage. Energy lines are from Koppelman et al., (2020) and angular momemtum lines are from Helmi et al., (2018).

This figure shows almost two square plots of equal size, with the plot on the left hand side showing red clump stars and the plot on the right hand side showing red giant branch stars, both are titled to state this. These plots are both showing the kinematic space for the stars in the sample, with the total energy, E_tot, on the y-axis in units of kilometers squared per seconds squared, with a multiplier of ten to the power of five for scalng. This y-axis for the red clump stars range between -1.7 at the bottom left hand corner of the plot and ~-0.8 at the top left hand corner of the plot. The y-axis for the red giant stars ranges between ~-2.1 and -0.8. The x-axis shows the angular moment of the sample L_z, and are in units of kilo parsecs per kilometers per seconds squared. These stars have a multiplier of ten to the power of 3 for scaling. The x-axis for the red clump stars ranges between ~-4 and 2. The x-axis for the red giant branch stars ranges between ~-4 to ~2. The points on both of these plots are separated into two colors, green to depict the low alpha stars and purple to depict the high alpha stars. In both plots the stars look like a diagonal line, reaching from the bottom middle of the plot (low total energy and roughly -1 in angular momentum) to the upper left (relatively higher total energy around -1 and low angular momentum of -3). Both plots show the green stars on the left hand side of the sequence and the purple stars on the right. Stars are depicted by circular markers. There is no variation in the color or opacity of inidividual stars. The purple stars are more spread out and reach out, with decreasing number density, from the original sequence towards the right hand side of the plot. There are two vertical dotted lines toward the right hand side of each plot and two dashed lines that separate the plots into almost thirds. Where these four lines cross the area is shaded grey with grey cross-hatching. The circular markers that fall into this space are those that represented stars from Gaia-Enceldus.