The age of the Milky Way halo stars from the Sloan Digital Sky Survey

Abstract

We determined the age of the stellar content of the Galactic halo by considering main-sequence turn-off stars. From the large number of halo stars provided by Sloan Digital Sky Survey, we could accurately detect the turn-off as a function of metallicity of the youngest dominant population, which was done by looking at the hottest (bluest) stars of a population. Using the turn-off temperature of a population of a given metallicity, we looked for the isochrones with that turn-off temperature and metallicity and found no age gradient as a function of metallicity. This would mean that this dominating population of the Galactic halo formed rapidly, probably during the collapse of the proto-Galactic gas. Moreover, we could find a significant number of stars with hotter temperatures than the turn-off, which might be blue horizontal branch (BHB) stars, blue stragglers, or main sequence stars that are younger than the dominant population and were probably formed in external galaxies and accreted later on to our Milky Way. Motivated by the current debate about the efficiency of gravitational settling (atomic diffusion) in the interior of old solar-type stars, we used isochrones with and without settling to determine the ages. When ignoring diffusion in the isochrones we obtained ages of 14−16 Gyr. This result is a strong argument against inhibited diffusion in old halo field stars, since it results in a serious conflict with the age of the Universe of 13.7 Gyr. The age obtained including diffusion in the isochrones was 10−12 Gyr, which agrees with the absolute age of the old globular clusters in the inner halo.

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

“…We used a Sobel-Kernel edge detector algorithm to determine the temperature of the main sequence turnoff (for more details consult Jofré & Weiss 2011). The method assumes the temperature distribution of a stellar population will display a very sharp decline (i.e.…”

“…The Dartmouth isochrones produce larger ages compared to the Y 2 isochrones and in some cases have ages large than the accepted age of the universe (13.8 Gyr, Planck Collaboration et al 2013). This is likely a result of the prescription of atomic diffusion (Jofré & Weiss 2011).…”

“…In this context, the stellar halo is of paramount importance to reveal the early stages of the Milky Way because these stars are mostly very old and metal-poor (e.g. McWilliam 1997; Unavane, Wyse & Gilmore 1996;Helmi 2008;Jofré & Weiss 2011;Kalirai 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). This way of determining ages relies on using the colour (or temperature) of the main sequence turn-off of the population, which presents a sharp edge in the temperature distribution.…”

Are ancient dwarf satellites the building blocks of the Galactic halo?

“…It is worth reminding that Unavane, Wyse & Gilmore (1996) and Jofré & Weiss (2011) used the age distributions of stars in the halo and dSphs to test the origin of halo stars.…”