Disentangling the stellar components of the metal-poor Milky Way Matthew Shetrone University of Texas, McDonald Observatory Giuseppina Battaglia Instituto de Astrofisica De Canarias Jennifer Johnson, Joel Zinn Ohio State University Dennis Stello University of New South Wales, University of Sydney Sanjib Sharma University of Sydney

APOGEE DATA Similar to Hayden et al. 2015 but extending to larger Z heights. Blue line to guide the eye based on the “local” alpha-poor population. Red line to guide the eye based on “local” alpha-rich population. Magenta line to guide the eye based on the “outer halo” population.

Simple Model inspired by Trilegal & Bovy et al. 2016 Uses simple exponential R and Z terms with flared disk from Bovy et al. 2016. Blue is the point at which 50% of the metal-poor stars are in the halo. Red is the point at which 90% of the metal-poor stars are in the halo. Green is the point at which the thick disk model dominates the halo by a factor of 4. Model might not be right but gives a good guide for expectations.

GAIA+APOGEE data The halo sample fills the parameter space with little net rotation. The disk is slightly asymmetric towards a secondary component having a slower net rotation.

GAIA+APOGEE data The halo sample fills the parameter space with little net rotation. We select the targets in red which have 2-sigma errors consistent with not being in the rotating disk sample. We select in blue all stars with 2-sigma errors within the 3-sigma dispersion of the disk rotation.

GAIA+APOGEE data

Definition of a Halo Sample All stars above the 90% contour (red line) of our simple model. Stars with Halo kinematics (red points) which are below the 90% contour (red line). Removed clusters and Sagittarius core stars.

The halo sample This is the DR13 halo sample defined in the previous slide. Errors here are the DR13 errors. We will concentrate on the metal-rich alpha-rich sample for this talk.

Metal-rich Alpha-rich Halo Sample The Metal-rich Alpha-rich Halo Sample The blue region is the region that I will pull the metal-rich alpha-rich halo sample. The same region will be used for a comparison disk sample.

Metal-rich Alpha-rich The Metal-rich Alpha-rich Halo Sample Blue is alpha-rich disk Red are the metal-rich alpha-rich halo stars Each point is a median of 20 or 60 stars for the halo and disk samples. The bars represent the scatter in median. Similar slopes. Different MDF.

Metal-rich Alpha-rich The Metal-rich Alpha-rich Halo Sample Inspired by Martig et al. 2016 and Ness et al. 2017 Pairs of stars selected between the alpha-rich disk and metal-rich alpha-rich halo samples with minimized Teff, logg and [Fe/H] differences. Weighted average difference is -0.02 dex or ~-0.04 Msolar

Metal-rich Alpha-rich The Metal-rich Alpha-rich Halo Sample Red are the metal-rich alpha-rich halo stars The black line is the line of 90% likelihood of being in the halo from a simple model.

Conclusions We expand on the results of Hawkins et al. 2016 that the metal-rich alpha-rich halo is nearly impossible to chemically separate from the thick disk. [Although we have not looked at the heavy elements, Z>30] The ages/masses implied by the [C/N] ratios suggest that the metal-rich alpha-rich halo is the same age/mass as the alpha-rich disk. The alpha-rich metal-rich halo is not very “spheroidal”, in fact it has similar radial distribution to the thick disk, although it has a larger Z height. One might speculate that the early alpha-rich disk was partially disrupted resulting in a “thick disk” and a small oblate “halo”.

The halo sample

APOKASC RGB sample Divided by Alpha element ratio The alpha rich stars have lower masses/older ages than the alpha poor sample. Where the [C/N] ratios for the alpha-rich and alpha-poor samples overlap the agreement with mass is very good. Thus no large alpha-term in the conversion to mass.

[C/N] for the full alpha-rich sample