Two distinct halo populations in the solar neighborhood

Abstract

Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between "high-alpha" and "low-alpha" components using the Results. The "high-alpha" halo stars have ages 2-3 Gyr larger than the "low-alpha" ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the "high-alpha" and "low-alpha" halo stars. The "low-alpha" ones have r max 's to 30-40 kpc, z max 's to ≈18 kpc, and e max 's clumped at values greater than 0.85, while the "high-alpha" ones, r max 's to about 16 kpc, z max 's to 6-8 kpc, and e max values more or less uniformly distributed over 0.4-1.0. Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the r max 's obtained for the in situ "high-alpha" component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the "low-alpha" component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the "low-alpha" component.

On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

“…This confirms the notion that the Galactic halo was formed and assembled very early on as suggested by both theory (Robertson et al 2005;Font et al 2006;Zolotov et al 2009Zolotov et al , 2010Font et al 2011) and observations (e.g. Schuster et al 2012). The theoretical work of Robertson et al (2005) and Font et al (2006) lays out what Schuster et al (2012) refer to as a 'dualaccretion' scenario.…”

“…Yet, the recent simulations of Font et al (2011) found that in situ stars can be as much as 3-4 Gyr younger than the accreted population. Schuster et al (2012) argued that their α-rich and α-poor populations can be explained by a scenario where an initial disk/bulge formed in a monolithic collapse producing the α-rich population. At later times, the α-rich stars in the primeval disk were scattered into the halo via merging events that subsequently populated the α-poor component of the halo.…”

“…At later times, the α-rich stars in the primeval disk were scattered into the halo via merging events that subsequently populated the α-poor component of the halo. However, the conclusions of Schuster et al (2012) were drawn from only 9 stars lying in the metallicity range of -0.4 < [Fe/H] < -1.40 dex. This made us wonder whether a difference in age between α-poor and α-rich can also be found using the data from the Sloan Digital Sky Survey (SDSS, York et al 2000), which contains thousands of halo stars extending the metallicity domain towards much lower metallicities compared to Schuster et al (2012).…”

“…However, the conclusions of Schuster et al (2012) were drawn from only 9 stars lying in the metallicity range of -0.4 < [Fe/H] < -1.40 dex. This made us wonder whether a difference in age between α-poor and α-rich can also be found using the data from the Sloan Digital Sky Survey (SDSS, York et al 2000), which contains thousands of halo stars extending the metallicity domain towards much lower metallicities compared to Schuster et al (2012). Although the spectra from SDSS have much lower resolution, it is still possible to rank the metal-poor stars in α-abundance space.…”

“…Due to poor distance estimates, we did not attempt to determine ages with the standard isochrone fitting technique (for a discussion on the method consult, Soderblom 2010) which was also employed by Schuster et al (2012). The large number of stars in our sample allows us to group stars in metallicity and determine the age of that population very accurately if there is a coeval dominant population (Unavane, Wyse & Gilmore 1996;Schuster et al 2006;Jofré & Weiss 2011).…”