Results and discussion

In Figure 7.1(b) we plot ages versus [Fe/H] together with the thin- and thick-disk samples from Bensby et al. (2003, 2005). Below [Fe/H] = 0 it appears that the Hercules stream divides into two (or maybe three) branches: one that follows the thick-disk age trend (the downward age-metallicity relation for the thick disk was seen in Bensby et al. (2004) and then also verified by Haywood (2006) and Schuster et al. (2006)); one that follows the thin-disk age trend; and a few stars (four or five) that tend to have high ages of ~15 Gyr in the interval -0.4 < [Fe/H] < 0. Although tempting, it seems premature to conclude that we see a thin-disk and a thick-disk branch in the Hercules stream.

Abundance trends for nine elements are shown in Figure 7.2. On considering the a-elements (Mg, Si, Ca, Ti), the first impression is that most stars follow the trends outlined by the thick-disk stars. At [Fe/H] > -0.1 the thin and thick disk are, however, too close to distinguish in the a-element trends, but the separation can be extended a few more tenths of a dex by looking at the Ba trend. Even here most Hercules stars follow the thick-disk trend. We note that the Al abundance also is a good criterion for distinguishing between the two disks; here we find that the trend for the Hercules stars again accompanies that of the thick-disk stars. The

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Figure 7.2. Plots of [X/Fe] versus [Fe/H] abundance trends, where X is the element indicated in each plot. Data for Hercules-stream stars are marked by gray circles. Data for the thin- and thick-disk samples from Bensby et al. (2003, 2005 are marked by open and filled circles, respectively.

iron-peak-element trends are all remarkably tight, but add no more than a consistency check for the Hercules-stream abundance trends.

Even though the Hercules stars do not split up as nicely in the abundance plots as they did in the age-metallicity plot, they do indeed mainly reproduce the trends outlined by the thin and thick disks, in fact, predominantly the thick disk. So what does this suggest about the origin of stars of the Hercules stream?

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